Before any vaccine can be approved for use it must go through rigorous testing, to make sure it is safe, as well as effective.
Around 43,500 people were enrolled in the Pfizer-BioNTech clinical trials, and 30,000 people in the Moderna clinical trials. Safety has been assessed throughout, and no major side-effects have been reported during the phase I, II and III trials.
Once the trials are complete and the full data has been analysed, regulators around the world review it and decide whether the vaccines can be approved for use in their countries. They look at all the preclinical, clinical and manufacturing process data, including the safety and efficacy data.
Once approved, the vaccines will be monitored as they are given to prioritised high-risk groups, to understand how they perform in different population groups over time, and to look for very rare side-effects or long-term safety issues. Of the millions who have received the vaccines so far, a few people have had a very rare allergic reaction.
Nothing in medicine is 100% safe, and very rare side-effects may emerge as millions of people are vaccinated. This is the same for all vaccines.