Can donating blood just before coronavirus vaccination prevent blood clots?

There are no facts to support this. In any case, it’s important to drink plenty of fluids before and after donating blood to prevent dehydration and the associated nausea. In theory, dehydration may slightly increase the risk of blood clots. Donating blood just before vaccination is not harmful in general – as long as the donor is not too nervous about the vaccination and does not remain standing for long periods of time.