COVID-19 vaccine development has occurred at an unprecedented pace. The first vaccines were authorized for emergency use in the United States in December 2020—less than a year after the genetic sequence of the SARS-CoV-2 coronavirus was discovered.

Three COVID-19 vaccines have received emergency use authorization from the Food and Drug Administration (FDA).

  • Pfizer-BioNTech mRNA vaccine: This two-dose vaccine was 95% effective at reducing symptomatic COVID-19 in a Phase III clinical trial. The mRNA is fragile, and the vaccine must be stored at an ultracold temperature. 
  • Moderna mRNA vaccine: This two-dose vaccine was 94% effective at reducing symptomatic COVID-19 in a Phase III trial. It also requires special cold storage. 
  • Johnson & Johnson (Janssen) adenovirus vaccine: This single-dose vaccine was 66% effective at reducing moderate or severe COVID-19 in a Phase III trial, rising to 72% for participants in the United States. This vaccine can be stored in a standard refrigerator.

All U.S. residents ages 16 and older are eligible to receive a COVID-19 vaccine. However, only the Pfizer-BioNTech vaccine is authorized for those ages 16 or 17.

Vaccine researchers have taken several different approaches. The Pfizer-BioNTech and Moderna vaccines use lipid nanoparticles, or fat bubbles, to deliver bits of genetic material (messenger RNA) that encode instructions for making the spike protein the coronavirus uses to enter cells. When injected, the cells produce the viral protein, triggering an immune response. The Johnson & Johnson (J&J) vaccine uses a weakened adenovirus—similar to viruses that cause the common cold—as a vector to deliver genes that encode the coronavirus spike protein. A vaccine from AstraZeneca and the University of Oxford uses a chimpanzee adenovirus vector. Another vaccine from Novavax uses SARS-CoV-2 spike proteins grown in insect cells. (The letter two vaccines are not yet authorized in the United States.)

The vaccines stimulate the production of antibodies that inactivate SARS-CoV-2. They also induce T-cell immune responses that may persist even as antibody levels decline over time.

All of the authorized vaccines are highly effective at reducing the risk of severe illness. No one who received any of these vaccines in clinical trials was hospitalized or died from COVID-19. Studies show that they also greatly reduce the risk of asymptomatic infection and SARS-CoV-2 transmission. People with serious immune suppression, such as those with advanced HIV, organ transplant recipients and people on certain types of cancer treatment, may not respond as well. But the vaccines still offer partial protection and are recommended for these groups.

The vaccines are safe and generally well tolerated. Some recipients experience mild to moderate side effects including injection site reactions, fatigue and headaches. Flu-like symptoms are not unusual after receiving vaccines and are an indication that the immune system is working. Severe allergic reactions (anaphylaxis) are rare and can be managed with medical care.

A small number of people who received the J&J and AstraZeneca vaccines developed an unusual blood clotting disorder. This occurred in approximately one in a million J&J vaccine recipients, most of whom were women under age 50. The FDA briefly paused use of the J&J vaccine for further investigation, but determined that its benefits outweigh its risks and allowed vaccination to resume for all groups.

Last Reviewed: April 29, 2021