Hopefully title covers the question. All of the above are viruses and I understand that the latter two mutate. Wouldn’t the first ones I listed also mutate over time? Is the difference more a function of type of vaccines, or the viruses themselves? Why does this difference in preventative care exist? I think even the new RSV vaccine is a single does. Looking forward to understanding this better. TIA
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Measles, Varicella (chicken pox/shingles) and polio are human specific diseases, they don’t infect other species (which is why it would be possible for them to be eradicated if antivaxxers didn’t exist).
All viruses mutate to some extent but Influenza mutates rapidly. It has a segmented genome, meaning instead of having one big piece of DNA or RNA, it has 8 pieces. If two different strains of flu infect someone at the same time they can swap entire pieces of genome allowing a totally new strain to form essentially overnight. Even worse, flu infects a lot of different animals, so a human flu virus can infect pigs or birds, etc and swap RNA with animal flu viruses and then return to humans in a totally different form. This is likely how 1918, 1957, 1968, 1977 and 2009 pandemic flu strains formed. Coronaviruses act in a similar way, infecting different species and then jumping to humans and rapidly mutating as they go.
So each year new flu strains circulate and new vaccines are needed. Coronaviruses mutate rapidly and so boosters are needed, although they are different from flu in that new coronavirus strains now are progressive variants of SARS-CoV-2 rather than the stepwise differences we see in influenza (thanks to its segmented genome and seasonality).
New coronavirus outbreaks tend to happen every decade now: OC43 likely caused the 1889 pandemic, HKU1 crossed to humans in ~1995, SARS caused the [2002 pandemic](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3329048/) (halted by the WHO), MERS caused a [2012 epidemic](https://www.who.int/health-topics/middle-east-respiratory-syndrome-coronavirus-mers#tab=tab_1), SARS-CoV-2 (COVID) caused the 2019 pandemic, in all likelihood we will see more coronaviruses jump to humans and will need new vaccines for them. (Most people had never heard of coronaviruses until now but this is at least the 3rd major outbreak caused by a coronavirus)
The exception to this is HIV, which is a human specific virus but it mutates rapidly due to the nature of [how it replicates](https://en.m.wikipedia.org/wiki/Retrovirus). Which has made HIV vaccination incredibly difficult (among other things).
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