In December 2020, the Food and Drug Administration granted emergency use authorization for the first two COVID-19 vaccines—one from Pfizer and BioNTech, the other from Moderna and the National Institutes of Health.

Most people can safely receive the vaccines. Although many people experienced side effects, such as injection site reactions, fatigue and headache, in clinical trials—especially after the second dose—these were usually mild to moderate and lasted only a couple of days.

A small number of people who have received the Pfizer/BioNTech vaccine experienced severe allergic reactions. But these rare reactions should not derail the rollout of the vaccines, experts say. The Centers for Disease Control and Prevention recommends that a clinician with access to emergency supplies should be available to manage anaphylactic reactions. People with a prior history of such reactions should be observed for 30 minutes post-vaccination; others should be observed for 15 minutes.

Outcomes have not yet been analyzed separately for the HIV-positive participants in the Pfizer/BioNTech and Moderna trials. Experts encourage people living with HIV to get vaccinated and say there’s no reason to think the vaccines won’t be safe.

Vaccine safety and effectiveness also have not yet been determined for people with cancer. The vaccines are not contraindicated for such individuals—and experts do not foresee problems—but they should talk to their care providers about their specific situation.

People with more advanced immune suppression may not respond as well to the new vaccines; this requires further study. But unlike vaccines that contain live virus, these two mRNA vaccines pose no known risk and potentially offer some benefit.

The vaccines have not yet been studied in children or adolescents. The Pfizer/BioNTech trial did include a small number of 16- and 17-year olds, and no safety concerns were reported. This vaccine is authorized for people ages 16 and up, and the Moderna vaccine is indicated for those 18 and older. Trials for teens and younger children are now getting underway or will start soon.

The vaccines also have not yet been studied in pregnant people. But again, experts say this group should not be excluded from vaccination if they are otherwise eligible.

Finally, people who have previously had the new coronavirus—many of whom are unaware of it because they were asymptomatic or weren’t tested—can safely receive the vaccines, and vaccination may offer additional protection beyond natural immunity.