Comment by ashwinpp

We know that influenza is different from other viruses because 1) There are many strains of influenza, it has a high mutation rate 2) Flu vaccines are based on a forecast of likely strains and typically cover only at most 50% of the strains in a given year [1]

Now let us make the following assumption - Actual flu infection boosts immunity after recovery from the infection compared to vaccination (perhaps an analogy would be how muscle recovery happens after microtears after a strength training workout with heavier weights).

In which case, one might conclude - Since a stronger immune response is great against a fast mutating virus and around 50% of strains are not covered by vaccines anyways, that the benefit of boosting immune response against future infections (beyond the current year) outweighs cost of getting infected and is a better choice than getting immunized against 50% of the strains with increased chances of getting flu due to the other 50% anyways

Assumption 2 - Getting exposed, but not infected by flu, increases the number of strains one's body is immune to. Something sort of like vaccination, but without taking the vaccines themselves.

For the purpose of modelling, let us also make the assumptions that all strains arise and propagate independently. That is the strains not covered by vaccination expose the same number of people unaffected by herd immunity, while the strains covered by the vaccination face herd immunity.

Now consider the strains covered by the vaccine, whose spread is reduced due to herd immunity. It is great for everyone vaccinated against the strains, but for people not vaccinated against it, it prevents exposure from and hence immunity. This might be a case where the utility of vaccination decreases for non-vaccinated individuals instead of increasing (herd immunity) as a function of the fraction of vaccinated population. In other words, the more people vaccinated for flu, the less likely an non-vaccinated person is to get exposed, the less likely is he/she to develop immunity towards those strains naturally. Thus this would prevent population level immunity from arising, where it would have arisen naturally.

Note that in this scenario, one might question that in the natural course of events, a certain percentage of population will develop immunity anyways and hence become equivalent to the scenario where some part of the population is vaccinated. However there are three major differences. First is that there is a certain delay between getting infected, spreading to others and gaining immunity. Thus for the same percentage of immune population, there might be larger population exposed in the natural course of events. Second is that many individuals might get infected, transmit and immunized to a strain without getting infected, thus such an individual gets all the benefits of vaccination, but might help others get exposed and immunized to a strain. And third that vaccination is more aligned with socio-economic factors while infection less so.

To conclude, if any of the two assumptions I state are true, then we should perhaps not blindly apply the same logic that applies to other viruses for which we have vaccinations.

[1] https://news.ycombinator.com/item?id=15562258