COVID-19 Vaccines: What We Know About Side Effects Five Years Later

COVID-19 vaccines were developed and introduced at unprecedented speed during the global pandemic. Since then, billions of doses have been administered, giving researchers and public-health agencies a much larger body of evidence to evaluate vaccine safety.

Most vaccine side effects are mild and temporary, such as soreness at the injection site, fatigue, headache, muscle aches, chills, or fever. However, researchers have also identified several rare but important adverse events associated with particular COVID-19 vaccines.

A Brief Look Back at the Pandemic

COVID-19 emerged in late 2019 and spread rapidly around the world. In March 2020, the World Health Organization characterized the outbreak as a pandemic.

Countries introduced measures such as physical distancing, masks, travel restrictions, and lockdowns while scientists worked to develop effective vaccines.

Several vaccines were subsequently authorized or approved, including vaccines developed by Pfizer-BioNTech, Moderna, AstraZeneca, and Johnson & Johnson/Janssen.

What Side Effects Have Been Confirmed?

After hundreds of millions of doses were administered and safety monitoring continued, health authorities identified several rare adverse reactions.

1. Myocarditis and Pericarditis

Myocarditis is inflammation of the heart muscle, while pericarditis involves inflammation of the tissue surrounding the heart.

These conditions have been reported rarely after mRNA COVID-19 vaccination, particularly in adolescent and young adult males and more often after a second dose. Most reported patients recovered with appropriate medical care.

2. Severe Allergic Reactions

Severe allergic reactions, including anaphylaxis, can occur after vaccination, although they are rare.

For this reason, vaccination providers generally monitor people for a short period after vaccination and have procedures in place to treat severe allergic reactions.

3. Menstrual Changes

Some people reported temporary changes in menstrual bleeding or cycle patterns following COVID-19 vaccination.

Research has investigated these reports, and evidence suggests that some vaccinated people can experience short-term menstrual changes. These observations should not be confused with evidence that COVID-19 vaccines cause permanent reproductive problems.