What Are Reasons for "Breakthrough Infections" of COVID?

[kyphysics]

I know even the best vaccines aren't 100% effective at preventing infections and there are lots of people who take immunosuppressants whose immune systems don't mount the proper response to the COVID vaccines. Other than these specific folks, I'm wondering what other reasons there might be for vaccines not protecting against infection?

Might it be that a fully vaccinated person is taking some meds that (unbeknownst to them) interfere with the intended immune response from the vaccine? Might some medical condition, such as cancer or HIV do the same? Does very old age just lead to much "slower" and/or malfunctioning immune responses that don't let the vaccines work optimally in such people?

Just a curiosity I have and haven't read anything on it, so wondering if others may have and don't mind sharing.

 

[russ_watters]

The mRNA vaccines are unusually effective. Flu vaccines vary between like 30% and 80% and I don't think I'd ever heard the term "breakthrough infection" before COVID. Vaccines just aren't ever 100% effective and that's a totally normal thing.

Ultimately it is surely a numbers game: how many initial virus particles vs how many antibodies determines which wins and by how much.