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.
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