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 are available in the Unites States:

  • Pfizer-BioNTech mRNA vaccine: This two-dose vaccine received full Food and Drug Administration (FDA) approval for people ages 16 or older in August 2021, and it has emergency use authorization for teens ages 12 to 15. This vaccine was 95% effective at reducing symptomatic COVID-19 in a Phase III clinical trial. 
  • Moderna mRNA vaccine: This two-dose vaccine has FDA emergency use authorization for adults ages 18 or older. It was 94% effective at reducing symptomatic COVID-19 in a Phase III trial. 
  • Johnson & Johnson (Janssen) vaccine: This single-dose vaccine has emergency use authorization for adults ages 18 or older. It was 72% effective at preventing moderate or severe COVID-19 among U.S. participants in a Phase III trial. Unlike the mRNA vaccines, which require special cold storage, this vaccine can be stored in a standard refrigerator.

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 latter two vaccines are not yet authorized in the United States.)

The vaccines stimulate the production of antibodies that prevent SARS-CoV-2 from entering cells. They also induce memory B-cell and T-cell immune responses that may persist even as antibody levels decline over time.

All of the authorized vaccines are highly effective at preventing severe illness, hospitalization and death. However, they are less effective at preventing asymptomatic infection or mild illness caused by the delta variant of SARS-CoV-2.

People with immune suppression, such as those receiving certain types of cancer treatment, people with advanced or untreated HIV and organ transplant recipients, may not respond as well. But the vaccines still offer partial protection and are recommended for these groups.

All of 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 very small number of people who received the J&J or AstraZeneca vaccines develop an unusual blood clotting disorder. A small number of people who received the Pfizer-BioNTech or Moderna vaccines develop myocarditis, or heart inflammation; this appears to occur more often in young men.

Last Reviewed: September 7, 2021