 In 2021, millions of people will be receiving COVID-19 vaccine doses around the world. Thanks to rigorous testing, we know that they are effective and safe. But as millions are vaccinated, we may see some more rare adverse events related to vaccination. So what are adverse events? When vaccines are tested in clinical trials, researchers look for potential side effects or unintended medical outcomes. During clinical trials, volunteers are monitored very closely to see if something happens, and if there is any link to the vaccine. Researchers collect information on any side effects after the injection, from more common side effects like a sore arm or a headache, or if something unintended happens. In particular, they are looking for any severe effects like hospitalization. In these cases, trials can even be paused whilst they investigate and figure out if it's linked to taking the vaccine or just a coincidence. So far, the vaccines have been found to be very safe, and all the minor side effects like fatigue have been measured. But as you can't spot a million-to-one effect until a million people have taken a vaccine, we may see some during 2021. But, for example, the chance of having a serious allergic reaction with any vaccine is around 2 million-to-one. You are hundreds of thousands of times more likely to seriously injure yourself putting on your trousers. We know it's likely we will only see some very rare side effects as the rollout continues. But we can all be assured that there is a rigorous process for managing and responding to any of these rare events, as the vaccines help us control the spread of COVID-19.