After the successful completion of phase III trials and after the product is licensed, phase IV studies, also called post-marketing surveillance studies, are used to continue monitoring the safety and effectiveness of the vaccine once applied to the population. Phase IV constitutes the expansion of knowledge about the efficacy of the vaccine once it has been approved for commercialization and begins to be applied systematically in the population. In addition to the very rare adverse reactions that could occur with its use and that had not been detected in the previous phases, the effectiveness is also evaluated through continuous epidemiological surveillance.
Billions of doses of COVID-19 vaccines have been administered globally in 2021. Thanks to robust surveillance systems, some very rare, previously unidentified adverse effects have been reported following widespread use of COVID-19 vaccines. Very rare cases of anaphylaxis have been reported with the most COVID-19 vaccines – anaphylaxis is treatable if recognized early and treated promptly.
Very rare cases of myocarditis and pericarditis have been observed following vaccination with mRNA vaccines, mostly after the second dose of vaccination, and more commonly in younger men. Myocarditis and pericarditis are mild in most cases and resolve with treatment and rest.
A small number of very rare thromboembolic events, in combination with thrombocytopenia, have been reported following vaccination with AstraZeneca and Johnson & Johnson vaccines.
Rare cases of Guillain-Barré Syndrome (GBS) have also been reported following vaccination with these two vaccines. WHO has reviewed all evidence on these rare events and concluded that the benefits of these vaccines in preventing severe illness and deaths far outweigh the small risks. In addition, WHO and countries are conducting research and implementing actions to mitigate further those small risks.
All COVID-19 vaccines will continue to be monitored closely and WHO will review all robust evidence related to their safety and effectiveness to ensure COVID-19 vaccination programmes are as safe as possible.