As Barbara indicates, pro-vaxxer means "someone who promotes immunizations" (e.g. a public health official or doctor). It's a term that's often used pejoratively in political arguments by pro-vaxxers' opponents, anti-vaxxers, who fear the risks of vaccines and underestimate the public health risks of widespread vaccine refusal. Compare these to terms like "pro-abortion" and "anti-choice" used to frame opponents' views on reproductive choice: they're not necessarily labels people use to identify themselves, but terms they use to label those they disagree with.