Millions of Covid-19 vaccines are being administered worldwide every day, and for the vast majority of people, the vaccines are safe and effective against severe disease and death caused by the virus.
In some cases, however, serious – but rare – side effects have emerged after vaccination. This is known as an adverse event following immunisation (AEFI). Examples include allergic reactions (anaphylaxis); myocarditis and pericarditis; and blood clots with low platelets.
Since so many people are being vaccinated, it makes sense that such very rare events will start to appear in safety reports, Hilda Bastian, an independent scientist who studies evidence-based medicine in Victoria, Australia, told Nature.