The COVID-19 pandemic

[ac3r]

This is why seniors can now receive an extra-strength dose.  I believe todays vaccines also offer protection against more than one strain.  I do recall that, a few years ago, they got the strain wrong and the vaccine that year was not very effective.

They generally attempt to target a few types each year and it also depends on where you live in the world. For example (pulling this from Wikipedia), the EU states:

The composition of virus vaccines for use in the European Union for the 2021–2022 Northern Hemisphere influenza season recommended by the European Medicines Agency on March 30, 2021, is:

Egg-based or live attenuated trivalent vaccines should contain:

an A/Victoria/2570/2019 (H1N1)pdm09-like virus;
an A/Cambodia/e0826360/2020 (H3N2)-like virus;
a B/Washington/02/2019 (B/Victoria lineage)-like virus.

Cell-based trivalent vaccines should contain:

an A/Wisconsin/588/2019 (H1N1)pdm09-like virus;
an A/Cambodia/e0826360/2020 (H3N2)-like virus;
a B/Washington/02/2019 (B/Victoria lineage)-like virus.
A B/Phuket/3073/2013-like virus is recommended in addition to the strains mentioned above for the quadrivalent vaccines.

So, in this case, EU vaccines are attempting to target both influenza A and B types and multiple subtypes and lineages of those two main types. I don't know enough about virology to say whether or not these target specific clades and subclades, though I imagine they don't go that far, similar to how we don't have specific vaccines for SARS-CoV-2 variants, just SARS-CoV-2 itself.

It definitely makes it hard to predict what to target and mistakes do occur. At the same time, the fact we can still develop a vaccine a priori each influenza season is an incredible feat. The idea of a universal flu vaccine is nice, but scientifically quite challenging - although there has been some progress over the years. It'd be nice to see that one day assuming it's possible. Likewise, the NIAID in the USA is hoping to develop a universal coronavirus vaccine.