Pro-Vaxxers are people who have strong believes over vaccines than its dire consequences towards immunization. They perceive these vaccines to be protective measures over some disease, and they don't look at the packet insert since they don't mind about the resultant effects. Therefore, they are not the people who give us the vaccines but rather the people who strongly encourage others to take these vaccines on the belief that it cures or instead protect diseases.
Hence, Pro-Vaxxers are unreliable because they fail to look at the packet insert simply because of their scientific beliefs of a specific vaccine over the resultant effects it may cause to those being immunized. Typically immunization over a particular disease is essential if an only if the vaccines are tested to have no resultant severe effects.