From a sore arm to anaphylaxis, a wide range of adverse events have been reported after people have received a COVID-19 vaccine. And yet it is unclear how many of these events are actually caused by the vaccine. In the vast majority of cases, reactions are mild and can be explained by the body's own immune response. But monitoring systems designed to track adverse events are catching much rarer but more serious events. Now scientists need to work out if they are causally liked to the vaccine, or are just statistical anomalies - and that is not an easy task. News: Why is it so hard to investigate the rare side effects of COVID vaccines? Subscribe to Nature Briefing, an unmissable daily round-up of science news, opinion and analysis free in your inbox every weekday. Never miss an episode: Subscribe to the Nature Podcast on Apple Podcasts, Google Podcasts, Spotify or your favourite podcast app. Head here for the Nature Podcast RSS feed.   See acast.com/privacy for privacy and opt-out information.