Currently available data suggest that the side effects from the COVID-19 vaccine booster doses are similar to side effects from the initial set of vaccine doses. Approved booster shots are considered safe and are currently recommended in some countries for people who are immunocompromised, people who are older than age 65, and people who are at higher risk of exposure due to their work (e.g. medical and other frontline jobs). 

Side effects from booster shots were mostly mild to moderate, according to clinical trial results that Pfizer and BioNTech submitted to the U.S. Food and Drug Administration (FDA) for approval. The most common side effects reported in the clinical trial recipients from ages 16-55 were: pain at the injection site (88.6%), fatigue (70.1%), headache (64.9%), muscle pain (45.5%), chills (41.5%), joint pain (27.5%), fever (17.8%), and injection site swelling (10.6%). The most common side effects reported in the clinical trial recipients older than age 56 were: pain at the injection site (78.2%), fatigue (56.9%), headache, (45.9%), muscle pain (32.5%), chills (24.8%), joint pain (21.5%), injection site swelling (11.8%), fever (11.5%), and injection site redness (10.4%). These booster trial results are similar to the clinical trial data on side effects from the second dose of the Pfizer vaccine.

Moderna also reported that their studies of booster vaccine doses showed high levels of safety and tolerability, similar to what was observed after the second dose in their Phase 2 and Phase 3 clinical trials. The side effects from the Moderna booster shots were also mostly mild to moderate and the most commonly reported include: injection site pain, fatigue, headache, myalgia (muscle pain), and arthralgia (joint pain). 

The scientific and medical community is monitoring side effects as more people become eligible to receive a booster vaccine dose. Additionally, studies are continuing to provide more data on side effects from booster shots by various manufacturers.

Although there are vast inequities in COVID-19 vaccine access around the world, multiple countries have started, or are considering, COVID-19 vaccine booster shots, particularly for populations who are at higher risk for severe COVID-19 (e.g. immunocompromised people, people over age 65, and frontline workers). For example, the Philippines announced that the government's 2022 budget will include 45.3 billion pesos (equivalent to $899 million in U.S. dollars) for COVID-19 booster shots. Given this context, there is growing interest in learning about the potential side effects of a COVID-19 vaccine booster dose.