Guided by argumentation schema theory, we conducted five psychological studies in the United States and China on arguments about vaccination. Study 1 replicated research about arguments on several topics, finding that agreement judgments are weighted toward claims, whereas quality judgments are weighted toward reasons. However, consistent with recent research, when this paradigm was extended to arguments about vaccination (Study 2), claims received more weight than reasons in judgments about agreement and quality. Studies 3 and 4 were conducted in the United States and China on how people process counterarguments against anti-vaccination assertions. Rebuttals did not influence agreement but played a role in argument quality judgments. Both political position (in the United States) and medical education (in China) predicted differences in argument evaluation. Bad reasons lowered agreement (Study 5), especially among participants studying health care. Political polarization apparently heightens the impact of claim side in the argumentation schema, likely to the detriment of public discourse.