evidence: The 2012 report by the Institute of Medicine (IOM) 8, now called the National Academy of Medicine (NAM), described three studies with sufficient validity and precision that reported null associations between MMR vaccine and meningitis 9-11. The report also described several studies assessing meningitis, encephalitis or encephalopathy and MMR 9,12,13, DTaP 14,15 or meningococcal 13 vaccines, but these studies did not provide convincing evidence due to a lack of validity and precision. The IOM found no relevant studies of quality in the literature assessing encephalitis or encephalopathy and varicella, influenza or hepatitis B vaccines, since the only applicable studies available either had serious methodological limitations or used passive surveillance systems and therefore lacked an unvaccinated comparison group 8. [...] The IOM also concluded that there was no mechanistic evidence of quality showing an association between encephalitis or encephalopathy and varicella, hepatitis b and meningococcal vaccines, nor for an association between meningitis and measles or rubella vaccines, as the publications reviewed provided no evidence beyond a temporal association 8. The 2012 IOM report described several cases of encephalitis or encephalopathy after MMR 32-34, influenza 35 and DTaP 36 vaccines, and four cases of meningitis after mumps vaccine 32,37,38, but when considering knowledge about the natural infection the IOM concluded this mechanistic evidence was weak 8. However, there is one well documented case of measles vaccine virus isolated from the cerebrospinal fluid of a patient with encephalitis in Canada 39, as well as documented cases of meningitis following reactivation of vaccine-type varicella zoster virus 40-42.