[reissbaker]

Not rendered ineffective per se, but it's part of why you need a flu shot every year. There are a lot of strains and they mutate a lot, so we do a best guess of which current strains will be the most infectious in a given year and develop vaccines targeting those strains.

Generally the vaccines are moderately effective, but it's because we update them every year. If we didn't, they wouldn't be.

That being said there are efforts underway to make vaccines that would be more broadly effective against the flu and will be less susceptible to variants escaping ("universal flu vaccines"), and so this may not always be true in the future.

In the meantime, mRNA vaccines will likely be a huge aid for fighting the flu, because they can be manufactured rapidly for specific strains. Currently there's a six-month or longer lead time for inactivated- or attenuated-virus vaccines — which is what flu shots generally are — and sometimes new variants pop up or are discovered to be more infectious in between the manufacturing start date and that year's flu season.

Moderna apparently plans to make COVID booster shots that are also flu shots, although they may not be ready this year.

[Spare_account]

>but it's because we update them every year.

OK, this is the part I wasn't aware of. I assumed (dangerous, I know) that each year's 'new' flu vaccine was really just a new 'blend' of existing proven vaccines, for the most prevalent strains that year.

Edited to remove badly conceived question. New Question: How often do we have to develop novel vaccines for new Flu variants?

[reissbaker]

From this [1] Wikipedia article it looks like there aren't new strains every year, but generally every few years there will be a new variant that replaces an older variant in the mix. The oldest variant currently in flu vaccines is from 2013, and the newest is from 2019.

That being said, the process generally doesn't change — you still grow and kill (or grow and weaken) viruses the same way, you just start from a different base strain. For already-approved inactivated-virus flu vaccines, the FDA does not require new clinical trials for strain updates, and for already-approved attenuated virus vaccines, they only require very minimal trials (300 adults to prove adequate attenuation) [2]. So they're not novel in the sense of the COVID vaccines, which needed large scale clinical trials to prove efficacy — the flu vaccines have generally already proved efficacy, so for strain updates the most they need to prove is safety.

1: https://en.m.wikipedia.org/wiki/Influenza_vaccine

2: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4947948/

[Spare_account]

Thanks for this

[Taniwha]

yes - they make a prediction about which strains will be prevalent every Northern and Southern winters - it's a bit of a crap shoot, they're not always right though usually they are.

As I understand it predicting which flu strains to vaccinate this year is particularly hard because mask wearing/isolation/social distancing has reduced flu levels enough that they don't have a lot of data to base their predictions on