You could ask more politely by showing you did the bare minimum of independent research before asking for a citation.
'That's an interesting assertion but I couldn't find anything backing it up searching for "SARS immunity 20 years" or "long lasting SARS immunity", do you have a cite I could read?'
Even showing you've put in even the teensiest bit of effort makes people much more charitable about helping you.
I Googled "SARS immunity 20 years" and the first result is a research paper abstract[1] stating "SARS patients might be susceptible to reinfection >3 years after initial exposure".
How much effort do I spend assessing whether I believe this research paper? It's probably not what the OP had in mind since it conflicts with what they were saying. Do I make arguments from this paper (either for or against the OP's claim)? Again, it's probably not even what they were referring too.
I know just asking for a source is low effort, but it often seems like the best starting point.
'That's an interesting assertion but I couldn't find anything backing it up searching for "SARS immunity 20 years" or "long lasting SARS immunity", do you have a cite I could read?'
Even showing you've put in even the teensiest bit of effort makes people much more charitable about helping you.