The weekly edition of New Scientist contains questions in the last few pages, and one this week obviously pinged me. I doubt my answer will be printed, as it’s quite long, so here it is!
I had my flu vaccination this year, then promptly came down with a really bad case of flu three weeks later. How can that happen?
There are a few different things at play here. The first option is that perhaps it wasn’t the flu. You haven’t described your symptoms, so it’s impossible to judge, but a lot of people think of a really bad cold as the flu — but the flu vaccine only covers you against illnesses caused by an influenza virus. There’s a whole menu of options when it comes to diseases with flu-like symptoms.
Secondly, even if you did have influenza, there’s a chance you caught a strain that wasn’t covered by the flu vaccine you received. There are many different strains of flu, and several can be circulating in the population at once, but the flu vaccine doesn’t cover every possible strain in the world. New ones can arise or existing ones can become unexpectedly prevalent after the seasonal vaccine is formulated.
Finally, perhaps you didn’t have complete immunity yet. Still, that sounds unlikely because the usual time for that is 10-14 days from when you were vaccinated.