People believe this because most vaccines are sterilizing. If you get the measles vaccine you can't get measles again ever. The Covid and flu vaccines don't work that way.

Re: measles, this isn’t necessarily the case. Breakthrough infections have been known and discussed before.[0] The measles vaccine efficacy is estimated at 97% implying 3% chance of a breakthrough.

However, Covid-19 vaccine efficacy against infection was originally 93-95% but has dropped to 60% or less against the Delta variant. A lot of discussion seems to ignore this. To get back to the 90% plus efficacy we’ll need the vaccine to be re-tuned against delta, which no one seems in a rush to do for some reason…

[0] https://pubmed.ncbi.nlm.nih.gov/31039835/ “In an outbreak of measles in Gothenburg, Sweden, breakthrough infections (i.e. infections in individuals with a history of vaccination) were common. [. . .] Sixteen of 28 confirmed cases of measles in this outbreak were breakthrough infections.”

Can I interest you in

Katherine J. Wu (The Atlantic) - We’re Asking the Impossible of Vaccines // Complete protection against infection has long been hailed as the holy grail of vaccination. It might simply be unachievable.

https://www.theatlantic.com/science/archive/2021/09/steriliz...