Now that a majority of Americans have received their initial COVID-19 vaccines, the question becomes who needs additional doses to maintain or increase protection against SARS-CoV-2 (the coronavirus that causes COVID-19). Two main factors determine if boosters are needed:

  • Do the original vaccines still work against current SARS-CoV-2 variants?
  • Does immunity wane over time? 

The three vaccines authorized in the United States, from Pfizer-BioNTech, Moderna and Johnson & Johnson, remain active against the predominant Delta variant. However, while they still provide strong protection against severe disease, hospitalization and death, they are less effective at preventing infection.

Immunity against SARS-CoV-2, either after infection or vaccination, does wane over time. But while antibody levels begin to decline after a few months, memory B cells are left behind to produce more antibodies if the virus is encountered again. T cells also play a role in fighting SARS-CoV-2. However, B cell and T cell responses take a few days; they may not prevent initial infection, but they can stop the virus from taking hold in the body and causing serious illness.

In August 2021, the Food and Drug Administration (FDA) authorized and the Centers for Disease Control and Prevention (CDC) recommended an additional Pfizer-BioNTech or Moderna vaccine dose for moderately to severely immunocompromised people, including organ transplant recipients, people undergoing treatment for cancer and those with advanced or untreated HIV. People with suppressed immune function may not produce enough antibodies after the first two doses. For some, a third dose does the trick, but others are still not fully protected.

The FDA and CDC later went further, recommending Pfizer-BioNTech and Moderna boosters after six months for all people ages 65 and older, younger adults with underlying health conditions—including cancer and HIV—and those at high risk for exposure due to their work or living situation. All adults who received the J&J vaccine can get another dose two months after their first shot. (Many experts think the J&J vaccine should be considered a two-dose regimen.) People do not need to get the same vaccine brand they originally received for a booster.

The FDA is considering whether to authorize boosters for everyone, but experts disagree about whether most healthy younger adults need them. Studies have shown that an additional vaccine dose raises antibody levels and reduces hospitalization in older people, but protection against severe disease appears to be holding up well for younger people after two doses. On a public health level, boosters can help curb transmission. However, the increase in antibodies after a booster may also last only a few months.

It is unclear whether people will need COVID-19 boosters every year or even every six months. Some experts think that because the initial Pfizer-BioNTech and Moderna shots are given so close together (three or four weeks, respectively), a third dose with longer spacing could lead to a stronger immune response, and frequent boosters won’t be necessary. This suggests the best schedule may be similar to vaccines for hepatitis B and human papillomavirus (HPV), which involve three doses, the second a month after the first and the third around six months later.

New types of COVID-19 vaccines may also be an option in the future. Currently, boosters are additional doses of the same vaccine, but researchers are working on vaccines tailored to the Delta variant and others that would work more broadly across multiple variants or even against different coronaviruses. Scientists are also testing nasal vaccines, which produce mucosal antibodies in the nose, in the hope that they will better prevent initial infection and transmission.