Vaccination gives lifelong protection against some viruses, against others not, why?

Answer

Why some vaccines only protect for a few weeks while others can protect for a lifetime is the key question of vaccinology. So the best answer to your question is: people don’t know yet.

Of course, the mutation rate of the virus plays an important role here. And viruses are very diverse in that regard. In general, RNA viruses e.g. faster than DNA viruses. But double-stranded DNA viruses mutate less quickly than single-stranded DNA viruses, coronaviruses less quickly than retroviruses, etc. This has to do with the specific enzyme used by the virus to multiply its genome. The enzyme of coronaviruses makes fewer mistakes (mutations) than e.g. the enzyme of HIV. Due to the mutations, the protein against which a vaccine and the immune system are directed may or may not change quickly so that it is no longer recognized by the immune system.

The structure of the virus, more specifically the outer coat, may also play a role. Immunologists have, among other things, a hypothesis according to which the density of proteins, and their repetitive properties, would play an important role. Coat proteins that occur in a high density and highly repetitive manner on the virus are said to elicit a better antibody response from the immune system.

But the main uncertainty is in what immunologists call “immune memory.” In fact, too little is known about it. Immune memory is determined by the lifetime of certain antibodies, memory B lymphocytes, plasma cells, T lymphocytes. Monitoring this in the long term (decades!), is not obvious for researchers.