evidence: I can only find one study that looks at the relationship between vaccine side effects and MTHFR mutations. This study was looking at side effects of giving small pox vaccination in a population of ~125 patients. Those that had MTHFR mutations did show an increase in adverse events. The adverse events listed were both local (larger mark at the site) and systemic (fever, rashes and lymph node swelling). They did not report any further or long-term side effects. [...] I think it is very possible that, over time, we will discover that children may have differing responses to vaccines based on their genetics. The differences may be in side effects or effectiveness of the vaccines and will likely only affect an extremely small percentage of children. The problem is that we currently don’t have any information to inform those decisions. Further research is necessary and anyone who claims to know the secret link should have their claims evaluated with extreme caution. In addition, because differences in MTHFR genes are so common, it is hard to imagine that it will play a significant role in vaccine reactions. If they were, I would expect reactions to be much more common than they are. [...] So my experiences influence me to believe that MTHFR is an issue.