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.

Researchers have taken several different approaches. Messenger RNA (mRNA) vaccines use lipid nanoparticles, or fat bubbles, to deliver bits of genetic material 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. Some vaccines use weakened adenoviruses—similar to viruses that cause the common cold—as a vector to deliver genes that encode the coronavirus spike protein. Other strategies are currently under study.

Early research showed that experimental vaccines stimulate the production of neutralizing antibodies that inactivate the coronavirus. They also induce T-cell immune responses that may persist even if antibody levels decline over time.

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

  • 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.

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. These vaccines are not yet authorized in the United States.

All of these 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.

All of the vaccines are safe and generally well tolerated. Some recipients experience mild to moderate side effects including injection site reactions, fatigue and headache. 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.

COVID-19 vaccines are in short supply, but production and distribution is ramping up rapidly in the United Stated. Health care workers and residents of long-term care facilities, people over 65, those with underlying health conditions that put them at risk for severe COVID-19 and certain frontline essential workers have been prioritized for vaccination. States and local jurisdictions make the final decisions about vaccine allocation.

Last Reviewed: March 2, 2021