The COVID-19 pandemic

[tomh009]

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.

Nostradamus allegedly said, “Prediction is difficult, especially about the future."

And that's what they do with flu vaccines: they try to anticipate which flu strains will be predominant over the coming season. There is no way to know the correct answer at that time, all you can do is try to use your knowledge of prior patterns.