system
Given the following post, respond as if you're commenting on Reddit:
human
Why doesn't the flu vaccine contain more than four strains?

So I get that the flu vaccine is a quadrivalent vaccine and it's made up of what the doctors hypothesise will be the top 4 variants/strains.

But why only 4? Why not the top 5 most likely? Why not 10?

Is there a logistical issue for the flu vaccine to contain more strains? Is there concern for greater potential impact on the body if the vaccine had more variants/strains in the same vaccine?
gpt
There are 4 strains because that's what you need to cover all the circulating viruses.

Humans are infected by two subtypes of influenza A (H1N1 and H3N2), and 2 families of influenza B (Victoria and Yamagata). If you add those up, they equal 4. Seasonal vaccines contain one example of each.

There's essentially no cross-reactivity between H1N1 and H3N2, and very little between B-Vic and B-Yam strains, so you need one of each. Within each subtype/family, you have strains that change moderately every few years (3-5 years on average for A, longer for B).  It's those strain differences that need to be matched within the vaccines, but even if you miss the strain a little bit you'll still get some protection. (Even though it takes several years for each strain to drift, because there are the 4 subtypes in the vaccine the overall vaccine often needs annual updates.)

Flu vaccines used to be trivalent, only containing one influenza B example. Originally there was only one fluB type, and when Yamagata and Victoria diverged in the 1970s it was usually possible to guess whether B-Vic or B-Yam would be most common. But that was always a little tricky, so it was easier to just cover both, in a quadrivalent vaccine.

Why not include additional strains? Mostly, flu scientists are pretty good about predicting the most abundant strains. You only see a miss -- in one of four strains -- maybe once a decade or so, so if you included backups for all four components then 39 of 40 times they'd be a waste of time and money.

(There are rumors that B-Yam may have gone extinct because of the lockdowns/masking associated with COVID, which have suppressed all flu cases very dramatically. If that turns out to be true, in a few years manufacturers may be able to go back to trivalent vaccines.)