Forty-one percent of the U.S. population is fully vaccinated against COVID-19, which translates to 296 million doses. Researchers have been monitoring how COVID-19 vaccinations perform to determine whether booster shots will be needed, as variants of the novel coronavirus continue to mutate and spread. Now, the National Institutes of Health (NIH) announced they are beginning a Phase I/II clinical trial to test the safety and efficacy of different booster schedules.

“Although the vaccines currently authorized by the U.S. Food and Drug Administration offer strong protection against COVID-19, we need to prepare for the possibility of needing booster shots to counter waning immunity and to keep pace with an evolving virus,” explained Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases. “The results of this trial are intended to inform public health policy decisions on the potential use of mixed vaccine schedules should booster doses be indicated.”

Booster shots are not new to Americans. They are, however, more associated with childhood vaccines, such as those against measles, mumps, and rubella—better known as the MMR vaccine. The CDC recommends MMR vaccination in babies and boosters in children 4–6 years old, as well as for students attending college and adults who will be in settings or countries that pose high risk.

Current childhood vaccines contain live, replicating virus; often resulting in lifetime immunity. However, the current mRNA-based COVID-19 vaccines (Pfizer and Moderna) do not, which means their efficacy is likely to wane more quickly than other vaccines. Ongoing studies of vaccinated individuals are working to determine exactly if and when immunity falls to levels that no longer offer protection against SARS-CoV-2.

As part of the current NIH trial, adults who have been fully vaccinated with any of the three COVID-19 vaccines currently available in the United States—made by Pfizer, Moderna, and Johnson & Johnson—will receive boosters made by Moderna. A total of 150 people will be enrolled and separated into six groups. Each vaccine group will enroll about 25 people ages 18–55 years and approximately 25 people ages 56 years and older. Participants will receive a single booster dose of the Moderna vaccine as part of the trial 12 to 20 weeks following their initial vaccination regimen.

An additional cohort will be composed of adults who have not received an FDA authorized vaccine. These individuals will first receive two doses of the Moderna vaccine, followed by a booster 12 to 20 weeks later.

Participants will be followed for one year, meeting with investigators by phone and in-person. Periodic blood samples will be used to test immune responses against current circulating strains of the virus, as well as emerging variants. If a participant develops symptomatic COVID-19, genetic analysis will be performed to determine whether their infection was caused by a novel variant of the virus. Initial results are expected late this summer.