To be fair, doing the exact opposite of what Trump supports is a good rule of thumb if you have no other info or expertise to go on.

Though in this particular case, I don't really remember him being all that positive about vaccines for Covid or in general.

Here's a publication worrying about him undermining vaccines before Covid was a thing: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5678397/

Which is another data point to support the general principle.