COVID-19 vaccine development is occurring at an unprecedented pace, with numerous candidates in mid- or late-stage clinical trials. The first vaccines were authorized for emergency use in December 2020.

Several different vaccine approaches are being studied. Messenger RNA (mRNA) vaccines use lipid nanoparticles, or fat bubbles, to deliver bits of genetic material that encode instructions for making the spike protein that the SARS-CoV-2 coronavirus uses to enter human cells. When injected into a muscle, the cells produce the viral protein, triggering an immune response.

Other vaccines in late-stage studies use weakened adenoviruses—similar to viruses that cause the common cold—as a vector to deliver genes that encode the coronvirus spike protein. Vaccines further back in the development pipeline use recombinant viral proteins, virus-like particles or inactivated SARS-CoV-2.

Early studies 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.

An mRNA vaccine from Pfizer and BioNTech received emergency use authorization from the Food and Drug Administration (FDA) on December 11, 2020. A Phase III clinical trial showed that the two-dose vaccine is 95% effective at reducing the risk of symptomatic COVID-19. It is not yet known whether it will prevent asymptomatic SARS-CoV-2 infection or transmission of the virus, nor is it clear how long immunity will last. The vaccine is 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.

Another mRNA vaccine from Moderna and the National Institutes of Health was shown to be 94% effective after two doses in a Phase III study. It was also safe and generally well tolerated. An FDA advisory panel will discuss authorization of this vaccine on December 17, 2020.

A vaccine from AstraZeneca and the University of Oxford uses a chimpanzee adenovirus vector. It was found to be 70% effective overall in a Phase III trial, but unexpectedly, people who initially received a half dose followed by a full-dose booster had a better response rate (90%) than those who received two full doses (62%).

Chinese and Russian researchers have also developed vaccines that have demonstrated good effectiveness.

COVID-19 vaccines will initially be in very short supply. The first vaccines will go to health care workers and residents of long-term care facilities. Other priority groups include frontline essential workers who are at high risk for coronavirus exposure and people over age 65 and those with underlying health conditions that put them at risk for severe COVID-19. States and local jurisdictions will make final decisions about vaccine allocation.

Last Reviewed: December 15, 2020