In the background of the worldwide fight against covid-19, other respiratory disease has been struck a heavy blow. Incidence of seasonal flu has plummeted more than 99% globally: in the peak week of the 2020-2021 flu season, only 412 cases of the flu were laboratory-confirmed worldwide, with many countries not detecting the flu in the whole season. (Note that the WHO report has the data for northern and southern hemispheres reversed.)
This shows that the death and disease brought by the flu virus is preventable. With the average US flu victim having 16 years of life left, we should not resign ourselves to the inevitability of this illness. I believe in several decades we will regard deaths from the seasonal flu the way we today regard deaths from measles or cholera. Let us push to achieve that future now instead of waiting for it passively.
Although very few countries, such as New Zealand and China, have successfully been able to contain covid, this widespread accidental suppression of the flu is possible because the flu is much less contagious than covid. Evidently the global anti-covid efforts are commensurate with the effort needed to convincingly defeat the flu, but grossly insufficient for fighting covid. (Much as the mediocre efforts to mitigate climate change have incidentally led to modest improvements in air quality while being orders of magnitude less than needed for climate change.)
As the flu is less contagious than covid, intuitively one might suppose less effort should be expended in the fight: after this logic has been filtered through policy makers we end up with the present level of effort, to wit, middling vaccination campaigns, and some companies let people with the flu stay home. This is the wrong intuition. Rather, the flu being less contagious implies that there is more benefit from the same degree of effort and thus we should be looking for any useful avenue by which to fight the flu.
I propose that we can use the exact tools that we’ve been using to half-heartedly fight covid to defeat the flu. I believe this is possible without the invasive strategies needed to defeat covid, although such strategies should be held in reserve if necessary. And if we start right now, during the covid pandemic, we can make full use of the massive head start on defeating the flu before it slips away.
There are two components of this fight: specific government interventions against the flu, and a change in public perception of the flu as an accepted and inevitable component of daily life. I will focus on the former below.
|Incidence||estimated 800 million annually||145 million official cases as of 2021 April|
|Mortality||estimated 500 thousand annually||3 million official deaths as of 2021 April|
|Mortality rate||estimated 0.05%||estimated 1% (counting only official cases gives 2%)|
|Incubation period||2 days (1 to 4)||5 days (2 to 14)|
|Start of infectious period||0 to 1 days before symptoms||often 2 to 3 days before symptoms|
|Seasonality||winter in temperate climes; year round / complicated in tropics||year-round|
|1.19 to 1.37||2.0 to 4.0|
I do not trust most scientific work I have read for estimating , and would discourage the lay use of values; I only included it in the table to motivate writing this disclaimer. It is sufficient to observe that covid is much more contagious than the flu without trying to exactly quantify the relationship. Note that covid has become steadily more contagious as the pandemic has continued.
(If you were to use to quantifiably compare disease, the appropriate measure would be the ratio of the logarithms: this calculation comes out to covid being 2 to 8 times more contagious than seasonal flu.)
A significant fraction of infections of both flu and covid are asymptomatic. Research suggests that transmission of the flu from asymptomatic or presymptomatic people is rare. In contrast, it appears that asymptomatic or presymptomatic transmission of covid is responsible for about 50% of covid cases, and there are many known instances of superspreading events initiated by people without symptoms at the time.
Lockdowns are the gold standard for eliminating covid. Very few countries enforced any lockdowns, and to my knowledge all of them now have almost zero community transmission of the disease. (Although many used the word “lockdown”, it turns out actual lockdowns are much more effective than saying the word.)
The facile solution to all contagious respiratory disease is simply, what if we all stayed home for two-ish weeks? Covid, the flu, colds, and other disease – all gone. The cost would be high, but the benefit far higher.
Of course, there is no half-lockdown. If one small town somewhere failed to fully commit, it could wholly ruin the project. If we can’t coordinate 8 billion people well enough to give everyone drinking water (according to the CDC, 700 million people lack an adequate source of drinking water and almost a billion have zero sanitation), then I rather suspect it might be somewhat beyond our ability to coordinate 8 billion people to lockdown for a few weeks.
Lesser lockdowns, limited to geographic regions suffering from an outbreak of the flu, may still be useful; certainly these have been successful against covid. While I think it would be wise to keep in reserve the option of local lockdowns, I doubt they would ever be necessary – I believe the flu could be defeated globally without the use of lockdowns or other large-scale interruptions to daily life for the general public.
I present the general structure of one strategy for defeating seasonal flu; it is meant as a starting point for discussion and comparison. Specialized expertise and research would be needed to develop a refined, actionable plan.
The components of the strategy are:
This should be very familiar as they are lifted directly from interventions against covid, with lockdowns and distancing omitted. I’ll elaborate some details about each intervention below.
The steps of the plan are:
Eventually the only noticeable burden for most people will be annual vaccination and regular travel screening; those developing any respiratory illness will use a home testing kit, and any rare cases of the flu will be followed by isolation and extensive testing of their contacts. Ill people will wear masks regardless of test results. If measures are loosened too far and the flu escapes containment, the outbreak is naturally reset in the summer and a fresh start can begin next flu season (for countries with flu seasons).
Defeating the flu is not possible unless people are onboard with the project. People need to know what the goal of the campaign is, the steps being taken, and their role. Most of all, there needs to be a change in the public attitude that the flu is merely an accepted, inevitable part of daily life, which just happens to kill a large fraction of elderly people each year – an attitude I held until very recently.
I expect such a change in attitude to largely precede any government action, so at least some of this messaging needs to be done by non-government actors, which is why I am writing this.
In many individuals, the flu creates symptoms easily confused with the common cold or other respiratory illnesses. Unless we plan on taking on all contagious respiratory illnesses at once (a strategy I did not analyze), testing will be necessary to identify cases of the flu against a background of lesser illnesses.
As will be a common theme in the following subsections, the appropriate level of testing depends on the contemporary incidence of the flu. Flu tests are readily administered at most hospitals, and this practice should be continued, but it is not sufficient to only test people who are seeking medical attention. In some cases it may be appropriate to screen individuals in advance of coming to some gathering, as discussed in the travel screening subsection. On demand testing at doctor’s offices, pharmacies, or specialized testing centers should be made available to the public.
The one essential change that would greatly increase flu testing is to make testing at home cheap and standard practice – this is also a big missed opportunity in covid testing. It takes initiative and investment to get in a car and drive to a testing site, wait in line, provide some identification and fill out a form, and then have to wait to speak with someone on the phone about your test results (or wait at the site, or dig up your login credentials for their website). If you are sick, fatigued, and not looking presentable that can be an additional obstacle. With home testing you don’t have to leave your bed, which is especially relevant to someone too ill to stand up.
Furthermore, home testing is faster than testing-on-site, which is necessary for successful contact tracing, as discussed below.
There are no technical obstacles to adapting existing flu tests for the home, as they have multi-year shelf life, require no lab equipment, and can be performed by unskilled and untrained users: if you can follow a 3-step Ikea pictographic instruction, you can perform a flu test. However no such test is approved or packaged for home use, and tests suitable for consumer use are still in development.
For covid, certain universities and companies practice population-scale screening by requiring biweekly tests for their community. This is key because of the long incubation period of covid and widespread asymptomatic transmission; I don’t expect such measures to be necessary or helpful for defeating the flu, and such screening would pose a significant burden to the public.
Sick people should wear a mask: this applies equally for any contagious, respiratory illness, and regardless of whether there is a larger fight against the flu. I hope that the creation of such an expectation in the West is a lasting consequence of the covid pandemic – I had not given it any consideration before 2020, but now it seems like it should have been obvious. A similar cultural change would be replacing handshakes with nodding or bowing the head, which is already common in the US and should be made the default practice.
The use of masks among people with no known illness can be a cheap though only modestly effective way to reduce transmissions in situations involving lots of disparate contact. There is already a precedent for certain occupations, notably medical workers, to have frequent mask usage. Occupations that involve lots of public contact are also good candidates for masks: ushers, waiters, cashiers, barbers, etc. Having a mask-by-default rule, but allowing the removal or lowering of a mask when inconvenient or away from the public, allows for minimal burden while retaining most of the benefit.
Requiring mask usage among the general public for certain large gatherings or while on public transit would also cut incidence of the flu, and may become necessary to address transmission events that cannot be reasonably traced. While such mask usage is cheap and not especially odious, it may be too much to expect the public to sustain for an extended period of time. I have tried to restrict myself to interventions that do not require daily action from a significant fraction of the public, which rules out requiring masks in public transit. I would only push for such measures if mandatory sick leave combined with aggressive vaccination failed to produce sufficient results.
Obviously letting sick people stay home from work reduces the spread of illness. Among developed countries this is largely a US- and Japan-centric problem. I don’t think more needs to be said here, but did you know that workers on the ancient Egyptian pyramids got paid sick leave?
Sick leave is especially pertinent to the flu as almost all transmission is from symptomatic individuals. It is estimated that during local flu outbreaks, 80% of otherwise healthy people with a cough or sore throat have the flu virus. These symptomatic people should stay home even if they feel well enough to work!
If we can invasively screen every airplane traveler’s luggage at great expense in search of the remotest chance of someone carrying a bomb, then we can handle flu tests and fever checks as well. Fever checks with infrared cameras are already standard practice in many countries. Existing rapid flu tests cost $5 to $10 and produce results in 15 minutes; all people entering the secure region of the airport should be screened, and arrivals from locations not performing screening should also be screened. Since you have to wait more than 15 minutes at your gate before boarding anyhow, in theory this could be designed so that it takes no more time than the actual administration of the test. While these tests have a noticeable false negative rate, particularly for people who are not contagious, they have a low false positive rate.
With economies of scale and/or adjusting the design of flu tests for this use case, the cost can likely be driven down relative to using existing off-the-shelf rapid flu tests. It may make sense to pool the samples from everyone on the same flight if interventions don’t depend on which passenger tests positive.
If we wanted to actually protect other people at the airport from catching the flu, we would need some kind of layered or staged system so that travelers are not admitted into the general airport population until their flu test has come back negative. Not coincidentally, these are the exact same changes to airport layout that would be necessary to protect against physical security threats, in case we wanted to upgrade from just security theater. However I don’t expect such changes to be worth the investment: a cheaper alternative would be to require travelers to wear a mask until they have confirmed negative, and mask policies can be easily changed to adapt to changing circumstances.
Possible responses to positive tests among travelers include:
The appropriate reaction to the receipt of positive tests depends on the existing incidence of flu in the people being tested, and in particular might vary with time of year. As community incidence of the flu decreases, a higher fraction of positive tests are false positives and follow-up interventions will be more capable of handling secondary cases, so less intervention is necessary. However, as the flu gets close to eradication (either locally or globally), the potential benefit of getting the rest of the way there greatly increases, and stronger interventions become favored again.
In the extreme, mandatory isolation of all people crossing an international border may be taken: probably 24 hours would be sufficient. However this would pose a much greater burden than the other measures I have proposed here, and I hope that nothing of the sort would be necessary.
Screening may also be appropriate for long-distance train, bus, and car travel, or before large gatherings.
Vaccination is the one measure that developed countries already expend a roughly commensurate effort on. In the US, almost 50% of adults were vaccinated for the 2019-2020 flu season, the highest rate of the decade:
Given that vaccination is on the order of 50% effective, if a randomly distributed 50% of Americans were vaccinated, then naively using an value of 1.33 for the flu suggests that flu outbreaks would be self-limiting in the US.
Observably this is not true, and the current level of vaccination is insufficient to defeat the flu. However there is a lot of room for improvement on 50% of adult Americans receiving vaccines each year. I believe that achieving 90% flu vaccination alongside technological improvements in vaccine effectiveness would nearly eliminate the flu in the US, with relatively little effort on other interventions needed.
To achieve a goal of 90% flu vaccination,
Currently two thirds of flu vaccines are administered in doctor offices. Scheduling an appointment with a doctor can be inconvenient, a financial burden, and carry negative mental associations; flu drives would address these issues, and incidentally raise public awareness of the necessity of flu vaccinations.
Children are a major vector of flu illness and must be vaccinated as well.
While 90% vaccination makes a simple public-facing goal, really the goal is that each community separately reaches an acceptable vaccination rate, say 60% or so. Without this, flu can remain endemic in such communities and will constantly resurface elsewhere. I expect most of the cost of vaccinations to go into outreach to communities with little engagement with the formal medical system. Poverty, religious fundamentalism, and science opposition are a hindrance here, along with everywhere else.
Vaccine technology has been advancing rapidly in the last decade, as anyone who has heard of mRNA is aware.
Chicken eggs are no longer used in the US since 2019 (I have seen conflicting information: the CDC says 80% of flu vaccines for 2021 were made with chicken eggs). A variety of non-egg flu vaccine technologies have recently begun use in the US. The mRNA platform is not yet used for the flu but might lead to more efficacious and better targeted flu vaccines in the future.
Note that if the global number of flu cases is brought down significantly, the mutation rate declines, and vaccination becomes substantially more effective. This compounds with any technological improvements to the vaccine.
Most of the interventions discussed, such as travel screening and vaccination, are non-targeted population-scale efforts. These are cheap ways to greatly reduce infection among the large majority of people, but each intervention by itself will miss a significant fraction of cases: very highly effective travel screening would pose a tremendous burden, as all travelers would need to quarantine for several days, and existing vaccines simply aren’t highly effective, even if we could reach 100% vaccination rates. The purpose of contact tracing is to clean up the outbreaks that escape the cheap, population-scale interventions.
Similarly, the cost of contact tracing scales approximately with the number of cases, whereas the cost of population-scale interventions scales with the total population. As incidence of the flu goes down, contact tracing becomes cheaper and more beneficial, while other interventions cost the same for less benefit.
With existing incidence of the flu contact tracing is nearly meaningless: during flu season, a large fraction of working adults will become possible-contacts of someone with the flu, so identifying which people are possible-contacts doesn’t add any new information. However, after the incidence of the flu is driven down by other interventions (or by it not being local flu season anymore), contact tracing is essential for identifying who is at risk and stopping further transmission. It is also essential for identifying existing outbreaks of the flu: health agencies only find out about a small fraction of flu cases through hospital reporting, and other cases must be found by working backwards from known cases. This is especially important for communities with little engagement with the formal medical system. Note that increasing reporting of the flu has a substantial impact on reducing the number of outbreaks that escape detection from contact tracing.
Of course, simply identifying who has the flu and their contacts does not serve a purpose unless some action is taken in response to that information. A variety of responses are possible:
The appropriate degree of intervention depends on the incidence of flu in the community, and the fraction of flu clusters being successfully identified. If most everyone is exposed to the flu anyways, what’s the harm if some cases escape from one of the known clusters? Conversely, if very few cases remain, then there is a very large benefit to stopping clusters from getting out of control into the general community.
A key component of contact tracing identified in the WHO report on China’s actions against covid is speed. China hired 10000 contact tracers to address an epidemic with only 80000 confirmed cases in total, and heavily emphasized rapid testing and tracing. Contact tracing must be very fast – symptoms of the flu begins an average of two days after exposure, usually 1 to 4. A delay of 24 hours could be the difference of a whole level of indirection of contacts that need to be identified, and the number of people involved grows exponentially with the number of levels of indirection of contact. Therefore successful contact tracing of flu cases requires
As contact tracing is the lynchpin for the closing fight to defeat the flu, each of the above are essential components of the larger project.
Some interventions often discussed in context of covid are absent from my flu plan because I don’t think they have a significant effect, either for the flu or covid.
Hand washing with soap was a great invention, and something more people should take advantage of. It is very effective for digestive ailments, diseases found on raw foods, and disease with fecal-oral transmission, among others; for respiratory illnesses, direct inhalation of droplets is generally the greater danger. Certainly hand washing should be promoted but it will not have a measurable impact on the flu. Furthermore, a public health campaign can only influence the behavior of so many people, and it is difficult to enforce hand washing mandates outside of very narrow contexts.
Whatever “deep cleaning” really is, it doesn’t help reduce spread of respiratory disease.
“Social distancing” refers to reducing the number of social contacts you have, and how frequently you interact with them, to reduce the connectivity of the graph of close contacts. This can be effective if a small number of people enter a mutual agreement to form a “quarantine circle” that carefully restricts contacts outside the group. However, I imagine unilateral spontaneous cuts in contacts to be totally useless:
“Physical distancing” refers to maintaining a physical separation from others, especially from members of the general public who are not social contacts. Under laboratory conditions, the quantity of respiratory drops from someone rapidly declines with increasing distance. However, I am skeptical about the effectiveness of physical distance in practice, as rooms with closed circulation that do not rapidly exchange air with the outside seem to be the main vector for respiratory transmission, and after an extended period of time in a small such room the location of the people within it becomes irrelevant. I still practice physical distancing for whatever marginal benefit it may provide. Regardless, what I’ve observed is that people do not practice physical distancing no matter how many signs demanding it are posted or how often it is repeated in the media. The only place I have seen it successfully enforced in the general public is at large gatherings, but if you are faced with a disease so dire that physical distancing becomes worth the effort, then large gatherings should be banned long before.
The cost of the interventions described above varies greatly depending on how far along the fight against the flu has progressed: with local eradication, most of these interventions are unnecessary or can be scaled back, and with global eradication the cost becomes zero. A full cost-benefit analysis would require far more work and domain-specific knowledge than I can contribute. However, the costs of each of the interventions separately can reasonably be estimated, and can be compared with the existing costs of the flu to see what level of intervention is break even. Near our present level of intervention, I believe the benefits of intervention should scale roughly linearly with the level of intervention; as the amount of flu cases are driven down, non-linearities will develop, until eventually at eradication the ongoing cost becomes zero.
Extensive scientific work on the burden of seasonal flu has been conducted, and it is infeasible for me to do a critical analysis of this body of knowledge, so I will uncritically cite some studies selected at random.
How many is 36000 deaths? The average American has a 1.3% chance of eventually dying of the flu. Like many diseases, deaths occur disproportionately among older people, but that is not to suggest victims of the flu are already on death’s door: a 2007 study finds that the flu is responsible for 611 thousand life-years lost annually. As this study also estimated 36000 deaths annually, the average victim of the flu had 16 years more life left; and overall, the average American loses more than two months of life to the flu.
Lacking better data, let us imagine the typical person with the flu spends four days convalescing before they have sufficiently recovered to be productive or engage in enjoyable activities (which is a form of productivity, in that it increases human welfare, just not in a way that you can exchange for money: e.g., you can’t make someone happy by selling them your happiness, but you can sell them your chocolate, which is close enough).
In Canada, the average hospitalization lasts 11 days and costs about $10000 USD, has a 14% chance of going to the ICU; it appears US numbers are comparable. In the US, direct medical expenses come to around $10 billion annually. When including “indirect” expenses, studies give “total” costs of the flu at around $90 billion annually, mostly from loss of wages and productivity; however I would be wary of taking such a number too literally, as it will be very sensitive to how it is calculated.
From 2010 to 2020, the CDC estimates that the existing level of flu vaccinations have annually averted 5.1 million symptomatic illnesses, 2.5 million medical visits, 72 thousand hospitalizations, and 6000 deaths. This is about one sixth of the non-averted harm. Note that these benefits are disproportionately felt by those who get vaccinated.
Combining the above figures, and linearly extrapolating the averted harm, each year the average American:
The above costs, including those already being averted by existing interventions, are approximately the budget for defeating the flu while unambiguously being net beneficial. Additional benefits, such as those discussed below in the section on preventing pandemics, may be far larger but are harder to quantify without sophisticated analysis.
Let us begin with the cost of vaccinations. Currently, half of all Americans are vaccinated each year, at an administrative cost of $20 per shot; vaccinated adults below 65 spend an additional $14 in time spent receiving the vaccine – this compares favorably to the averted cost of $90 per vaccinated person. While there may be some cost savings to be found in vaccinating all Americans or in technological improvements in vaccines, more realistically I expect the difficulty of reaching the most hesitant or inaccessible Americans to increase administrative costs, say to $30 per person plus time loss of one hour.
Existing off-the-shelf rapid flu tests cost about $5 to $10 each. As there are about 700 million security screenings at airports per year, if all passengers are tested for the flu then the average American will receive such tests twice a year. This additional expense would be included in ticket prices, as other screening expenses already are: most tickets include a $5.60 security fee, which offsets the $10 the TSA spends on each screening. I suspect most travelers will not notice the price increase (did you already know how much the security fee is?), and as the people paying are those who most directly stand to benefit from the screening, this would increase the palatability of such testing to the public.
While in theory the 15 minutes of waiting for test results could be parallelized with other waiting time in the airport, due to inevitable inefficiencies I will suppose travelers lose 10 minutes to the testing process. Home tests perhaps take 20 minutes due to lack of familiarity with the procedure, although if performed by the sick person that time would probably be lost anyhow.
In addition to general airport screening, we will strongly encourage the use of home testing kits. The average adult has 1 to 2 colds a year along with the flu 0.1 times; ideally all of these illnesses would be tested. Let us suppose the average adult usefully uses 1 home testing kit a year and wastes another 1.
Finally, if one additional test is performed each year for miscellaneous purposes, such as screening at large events or other modes of transport, this makes 5 tests per year for the average American. About 1 of those would be due to air travel outside of flu season, which could be omitted. If the flu were locally eradicated it would only be necessary to test international arrivals, which is less than 0.5 per American per year. Most of those international arrivals are air travelers.
If economies of scale allow the TSA to screen people at only $10 per person, it seems only fair to suppose similar savings would occur with flu screenings as well. As flu tests currently cost $5 to $10, I will imagine that economies of scale will conservatively bring the cost to at most $5 per test. The cost of adding infrared cameras to identify fevers among travelers is negligible.
In all, the average American will be tested 5 times at a cost of $25 and one hour per year.
Disposable masks are currently available to end consumers at 20 cents each. The cost of using one per day of illness and per flight is negligible, but let us add a time loss of one hour to cover the inconvenience of getting and dealing with masks. During the covid pandemic, the US planned to distribute 5 re-usable masks to every household, in total 650 million masks, at a cost of $1 per mask; this would only be a one-time expense rather than repeating every year and only modestly more expensive than disposable masks, but still negligible.
Contact tracing would not be performed, and thus have no cost, until the incidence of flu is reduced by several orders of magnitude. For the eradication of the last, intransigent cases, contact tracing will become greatly expensive per case due to the effort to reliably identify almost every transmission event, although the total cost would be manageable. Precedent for contact tracing of respiratory diseases include tuberculosis, measles, and the handful of countries that were able to eliminate covid. I have no data on the cost of these contact tracing precedents and would find it difficult to extrapolate to the flu.
In total, the estimated annual cost of the above interventions works out to about $60 per person, plus three hours of time, plus the cost of contact tracing. Of that, about $10 and half an hour is already being spent on flu vaccinations. These costs would go down if the flu were locally or globally eradicated.
The total cost of the seasonal flu each year is about 26 hours sick at home or dead and $30 in direct medical expenses, plus 5 more hours and $5 averted by current flu vaccinations. (There is an additional $240 (and $40 averted) in social / productivity costs, but “social costs” includes time spent sick or dead converted to money.)
Additionally, the costs of the flu are not equally borne by all people. Most people will never suffer from a serious case of the flu: only an unlucky few flu victims will be hospitalized, be permanently injured, or die. Flu interventions are a form of insurance, with a guaranteed small loss protecting against a small chance of a very large loss. The risk-averse option is generally preferred even if average costs were equal.
Furthermore, the unequal costs of the flu are correlated with other inequalities, as poverty increases one’s exposure to illness and susceptibility to illness if exposed, and decreases access to medical care, including very basic care like staying home from work when ill.
Finally, there are indirect benefits to defeating the flu, as elaborated in the section on pandemics. I expect there to be many other ancillary benefits beyond those specifically mentioned.
In balance I find interventions against the flu to be overwhelmingly preferred.
I have left ambiguous what exactly it means to “defeat” the flu. It makes little difference to human welfare whether the flu is merely defeated, by which I mean its incidence is reduced many orders of magnitude like polio, or whether the flu is eradicated, like smallpox. However, different strategies are suitable for these goals, and it is not immediately apparent which goal is more appropriate.
Likewise, a strategy needs to be designed around either the goal of local eradication (formally the term is “elimination”) or global eradication (formally, just “eradication”).
Is eradication of the flu plausible? Based on our track record of disease eradication so far, the safe bet is that the flu will not be eradicated in the coming decades – although measles, which is much more contagious than the flu or covid, is on the shortlist for eradication. None-the-less I believe global eradication of the flu is within our capability (though see animal-to-human transmission, below), and should be explicitly taken as a primary or secondary goal.
Understanding in what way seasonal flu is seasonal is essential to deciding whether to pursue local or global eradication. Scientists still do not fully know why the flu is seasonal or where it “goes” in the off-season.
In temperature zones, the flu nearly disappears in summer, and consistently resurges each winter. It is often suggested that warm weather causes conditions to be less favorable to the flu, but this immediately appears implausible as the flu is endemic year-round in parts of the tropics.
This contradiction remains unresolved. However, it appears that the key factor is humidity, which is closely correlated to temperature: the flu spreads best either when it is cold and dry, with an increase in cases observed after dry spells, or warm and humid, or possibly warm and rainy in specific. For example in India, the primary flu season begins in spring / summer, concurrent with the monsoon season.
In tropical regions, the flu is often observed to have two distinct seasons, or to otherwise be significant throughout the year.
So where does the flu come from in temperate regions? It appears most likely that the flu regularly spreads from south and east Asia, where it is endemic, to other parts of the world; such seed cases only grow to form large outbreaks in the local flu season, when conditions are favorable enough for the flu to be self-sustaining. The obvious question then is what flu seasons were like before the industrial era, but I could not find relevant information. Alternative hypotheses are that the flu alternates between northern and southern hemispheres, spreading directly from one temperate region to the opposite temperate region, or that it remains endemic at low levels within each region even within the off-season; this last hypothesis seems self-evidently improbable.
For a temperate country seeking to defeat the flu, this suggests that it would be viable to forego all but the most cost-effective of internal measures and focus on preventing flu from crossing the border. This is more feasible than with covid, as there is very little non-symptomatic transmission of flu, and flu cases tend to have a short incubation period of only two days, limiting the period of surveillance necessary. However only a few cases of flu escaping detection at the border would render the strategy useless without arduous barriers to international travel.
Alternatively, as part of a global campaign to defeat the flu, one could similarly forego expensive interventions in parts of the world without endemic flu, and focus everything on south and east Asia. India and China each have flu vaccination rates below 5%; it is hard to imagine much forwards progress against the flu without improvement there. If we were able to break the cycle of flu transmission in Asia, perhaps we would get the rest of the world for free. However this is not the huge savings one might hope for: half the world population lives in these endemic regions, and obstacles to universal vaccination are high there.
I think neither of these risky strategies are a good gamble. Instead, either a country can unilaterally fight the flu locally, employing both strong internal interventions and robust border protections from the flu, or the world can aim to globally defeat the flu, with strong internal interventions in all countries whether temperate or not, and only weak border protections in use.
The number of animal-to-human transmissions of the flu is totally negligible compared with human-to-human transmissions, but is large enough to pose an obstacle to eradication of influenza A. (Influenza B only spreads significantly among humans.)
Active measures are taken in the US to reduce incidence of flu in domestic pig herds. Pigs are believed to pose a specific danger of pandemic flu because of the ability for avian flu, swine flu, and human flu to co-infect pigs.
Avian flu is common in domestic bird flocks. Certain types of avian flu are naturally found in wild waterfowl, from which they can spread to domestic birds and humans unmonitored, making the eradication of those strains essentially impossible for current techniques.
Horse flu, dog flu, bat flu, and others are not thought to pose a particular risk to humans.
I can’t usefully speculate on a strategy for addressing animal-to-human transmission. Perhaps the only answer is to concede eradication of certain strains as infeasible and accept low levels of flu among those who directly work with animals, with active measures to prevent further spread.
With influenza eliminated or eradicated from the human population, human-to-animal transmission of the flu would likewise end, and human-specific adaptions to the flu virus circulating among animal hosts would gradually die out. This decreases the danger that future animal-to-human flu transmissions carry the mutations necessary to be contagious among humans, and particularly reduces the rare chance of a flu pandemic event where such a virus is especially well adapted to humans.
Besides reducing the risk of such events, reducing incidence of the flu makes it much easier to spot incipient pandemics in the early stages when they can be most easily stopped. That place where 100 people suddenly have the flu stands out a lot more when noone else has the flu. This applies for other pandemics as well: covid-19 was first identified as a novel disease at the end of December 2019, by which time 174 cases of covid were already known. This early virus was far less contagious than the later variants, and genetic analysis suggests that it circulated for perhaps one to four months among human hosts prior to its identification – this delay being necessary for it adapt to human hosts. During this same time interval, Wuhan was suffering from a particularly severe flu season, with one surveillance hospital alone detecting about 300 cases of the flu each day. It is remarkable that against this background of approximately one million people becoming ill with the flu, it was possible to detect covid at all!
But what if there were no flu? Covid could have been identified much earlier, and testing kits and vaccines similarly designed earlier. No specialized infrastructure for contact tracing would need to be rushed to completion, as contact tracing is much easier when there isn’t a similar but much more common disease circulating at the same time – and the infrastructure would already have been built and tested for dealing with the flu. Even if the initial outbreak wasn’t stopped and covid began to slip out of China, the head start on making test kits and lack of an ongoing flu season would make identification of incoming cases more reliable. It’s hard to imagine covid growing into a pandemic in such a environment.
It is possible that in response to a plan such as the one I have laid out, the flu will evolve to defeat the interventions under taken; for example, it could evolve to become more asymptomatic, as covid appears to have done. If only one country undertook such an effort, maybe they produce a local variant adapted to the circumstances that escapes to other countries before the local eradication program can be successful. However, I expect this locally adapted variant to be outcompeted by the wild type in other countries, and so not contribute to the flu gene pool.
While I don’t foresee flu mutations posing a serious problem to the above plan, a well-designed plan should properly consider such an issue and prepare contingencies before the plan is begun.
Now! As with most progressive causes, the earlier we make progress the sooner we begin to collect benefits. But with defeating the flu, we have an absolutely golden opportunity during the covid pandemic. Interventions against the flu and against covid largely overlap, and working on one benefits the other. Cases of flu have plummeted to almost non-detectable levels worldwide, and defeating the flu has never been cheaper than right now. New Zealand has already locally eliminated the flu by accident. Eradicating the flu almost comes for free with eradicating covid, it’d be an agonizing failure to do the latter without taking up the “two for the price of one” offer on sale right now. In a few years the flu could be right back to where it was before the covid pandemic, and we’re back to paying full price – though still a price worth taking.
Fighting the flu is a fight we can win, as the last year has ably demonstrated. Currently the flu is on its last legs, we just need to finish the job.
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