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 Global Vaccine Data Network cohort study included 99 million vaccinated people from 10 sites across eight countries. Researchers compared the observed with expected rate for 13 neurological, blood, and heart related medical conditions. [...] 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.