The University of Pittsburgh is enrolling participants for a trial of booster shots that mix vaccines against covid-19.

The goal is to see, for example, if a dose of Pfizer after a round of Moderna gives people stronger and broader protection.

Pitt’s Pittsburgh Vaccine Trials Unit (PVTU) is one of 12 sites nationwide participating in a National Institute of Allergy and Infectious Diseases clinical trial to test booster doses of a covid-19 vaccine.

The trial aims to study the safety and immune response of a mixed-booster regimen, in which people receive a booster dose of a covid-19 vaccine that is different from the one they initially received. The researchers seek to know what combination of boosters would be safe and effective if boosters are recommended, said Dr. Judy Martin, a professor of pediatrics at Pitt’s School of Medicine and a member of Pitt’s Center for Vaccine Research. They are not charged with determining whether booster doses are necessary; that policy decision depends on factors like covid-19 variants.

“If eventually booster doses are recommended in order to enhance the immune response, we need to figure out what is going to be the best combination of vaccines,” Martin said.

A booster dose would mean a third shot for those initially vaccinated with the two-dose vaccines from Pfizer or Moderna, or a second vaccine for those who originally received the one-dose option from Johnson & Johnson.

The study is divided into two cohorts.

One group will include fully vaccinated adults who received any of the three covid-19 vaccines available in the U.S. Twelve to 20 weeks after their initial vaccination regimen, participants will receive a single booster dose of a different covid-19 vaccine.

Mixing vaccine types could provide better immune responses for certain individuals, such as organ transplant recipients, whose initial immunity was found to be weak.

“All sorts of combinations will be looked at,” she said.

The second group will be comprised of approximately 250 unvaccinated adults. They will receive the two-dose Moderna vaccine, followed by a booster dose of a vaccine about 12 to 20 weeks after the initial two-dose regimen.

Throughout the year, volunteers will provide blood samples, which will help researchers gauge their immune response against new variants of covid-19. If a participant contracts covid-19 during the study, investigators will determine whether a covid-19 variant caused the infection.

Collecting blood samples from those who were not previously vaccinated will be a key element of the study, Martin said, explaining that they’ll collect blood specimens from those participants before their first dose of the vaccine, between vaccine doses, after their second dose and after their booster.

“We can really see how the immune response changes over time,” she said.

Study participants will be followed for a year — but initial results are expected in late summer 2021.

Other combinations of vaccines may be studied in the future as part of this ongoing research.

Potential participants are able to consult with experts about risks, benefits and the details of the study before committing. Martin said that while medical researchers are always grateful to volunteers in clinical trials, they especially appreciate participation in a study like this one.

“It’s an important study with new questions to be asked,” she said.

For additional information or to enroll in the trial, visit PVTU.org.