The NIH Begins Trial on Mixing COVID Vaccine Boosters

June 2, 2021

A trial funded by the NIH has entered Phase 1/2 testing to study COVID-19 vaccine boosters from different manufacturers on fully vaccinated adults

While it has not been determined whether boosters will be necessary, Dr. Fauci has stated that “we need to prepare for the possibility… to counter waning immunity and to keep pace with an evolving virus” (1).
The trial includes 150 participants who have been fully immunized with either the Johnson & Johnson, Moderna, or Pfizer vaccine. “Twelve to 20 weeks following their initial vaccination regimen, participants will receive a single booster dose of the Moderna COVID-19 vaccine as part of the trial.” Those who have not been fully vaccinated are eligible to enroll as part of a separate cohort, in which volunteers will receive two doses of the Moderna vaccine and will receive a third dose of a vaccine 12-20 weeks later. 

“All trial participants will be followed for one year after receiving their last vaccination as part of the study,” and will provide blood samples to evaluate the antibody response to current and variant strains of SARS-CoV-2. The initial results are expected by the end of Summer 2021. 

This is the first US trial to test the effects of mixing vaccines, while the Com-COV study began earlier this spring. Com-COV is a “UK multi-centre, participant-masked, randomized heterologous prime-boost COVID-19 vaccination study,” monitoring the effects of mixing vaccine doses between AstraZeneca, Pfizer, Moderna, and Novavax (2). 

While participants are experiencing higher rates of adverse events, the vaccine is still being deemed safe as the side effects are not severe. Reported side effects include fever, chills, headache, fatigue, joint pain, malaise, and aching muscles. Symptoms have reportedly been “short-lived, and there [are] no concerns from the limited hematology and biochemistry data available.”

Preliminary data shows that those vaccinated with both the AstraZeneca and Pfizer vaccines are producing robust antibodies to SARS-CoV-2. Studies incorporating the Moderna and Novavax vaccines are also ongoing. Data around the primary immunological outcome is planned to be released in June 2021. 


Update June 3*

Canada has also instituted a new policy in which citizens may receive a Pfizer booster after one dose of the AstraZeneca vaccine. This is due to vaccine shortages across Canada; currently, less than 6% of Canada’s population has been fully vaccinated.

The National Advisory Committee on Immunization also stated that Moderna and Pfizer vaccines may be used interchangeably between doses, although it’s recommended that people complete their vaccination schedule using the same manufacturer.