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.

Vaccines trigger the production of antibodies against SARS-CoV-2, but these typically wane after a few months. However, they also stimulate memory B-cell and T-cell responses, which provide longer-lasting protection against severe disease.

Four COVID-19 vaccines have been authorized in the Unites States:

The original Pfizer-BioNTech and Moderna messenger RNA (mRNA) vaccines contained blueprints for the spike protein from the original (Wuhan) SARS-CoV-2 variant. Newer bivalent versions contain mRNA from both the original strain and more recent omicron variants. The bivalent vaccines are the only authorized mRNA options.

Previously, the original Pfizer-BioNTech and Moderna monovalent vaccines were used for an initial two-dose series, and the bivalent version was originally used as a booster. Now, a single dose of the Pfizer-BioNTech or Moderna bivalent vaccine is considered sufficient as an initial regimen for adults and children ages 5 and older. Different regimens are recommended for children ages 6 months to 5 years.

Most people, regardless of their initial vaccine regimen, are eligible for a single bivalent booster. People ages 65 and older may receive a second bivalent booster, and immunocompromised people can receive two or more boosters at the discretion of their healthcare providers. 

The J&J vaccine, authorized only for adults, was initially touted as a “one and done” option, but now a bivalent mRNA booster is recommended two months later. The availability of this vaccine is limited.

The Novavax vaccine, authorized for people ages 12 and older, may be used as an initial two-dose series or as a first booster, but not for people who have already received a prior booster.

The Centers for Disease Control and Prevention updates its vaccine and booster recommendations as SARS-CoV-2 variants evolve and as new evidence becomes available. Click here for the current recommendations.

Vaccine Safety and Effectiveness

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 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 (not authorized in the United States) uses a chimpanzee adenovirus vector. The Novavax vaccine uses SARS-CoV-2 spike proteins grown in insect cells.

All of the authorized vaccines were shown to be highly effective at preventing severe illness, hospitalization and death. They also appear to reduce the risk of developing long COVID. However, they do not reliably prevent infection or transmission, and they do not work as well against more recent SARS-CoV-2 variants. Immunocompromised people, including cancer patients on active treatment, people with advanced or untreated HIV and organ transplant recipients, may not respond as well to vaccines and may need additional doses.

All of the vaccines are generally safe and well tolerated. The most common side effect is temporary soreness, redness or swelling at the injection site. Some recipients experience flu-like symptoms, such as fever or fatigue. Severe allergic reactions (anaphylaxis) are rare and usually can be managed with medical care. The J&J and AstraZeneca vaccines can rarely cause an unusual blood clotting disorder. The Pfizer-BioNTech and Moderna vaccines rarely cause myocarditis, or heart inflammation, especially in young men. 

Last Reviewed: April 19, 2023