evidence: BMJ 2024; 384 doi: https://doi.org/10.1136/bmj.q488 (Published 26 February 2024) Cite this as: BMJ 2024;384:q488The largest vaccine safety study to date has identified two new, but very rare, side effects associated with covid-19 vaccines—transverse myelitis and acute disseminated encephalomyelitis. [...] The researchers said the findings translate to "an extremely small absolute risk" of acute disseminated encephalomyelitis (0.78 per million vaccine doses) and of transverse myelitis (1.82 per million vaccine doses). "Any potential risk of acute disseminated encephalomyelitis or transverse myelitis should be weighed against the well established protective benefits of vaccination against covid-19 and its complications," they wrote. [...] The Global Covid Vaccine Safety Project is funded by the US Centers for Disease Control and Prevention to allow the comparison of the safety of vaccines across diverse global populations.