NIH Clinical Trial Examines Mixed COVID-19 Vaccine Booster Doses

The NIH has initiated a phase 1/2 clinical trial to evaluate the safety and immunogenicity of mixed COVID-19 vaccine schedules with a booster dose.

The National Institutes of Health has started a phase 1/2 clinical trial (NCT04889209) to investigate the safety and immunogenicity of mixed boosted COVID-19 vaccination regimens, the agency recently announced.

The National Institute of Allergy and Infectious Diseases (NIAID) is leading and funding the clinical trial.

“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,” said NIAID Director Anthony S. Fauci, MD, in a June 1, 2021 press release. “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.”

The trial will include approximately 150 adults who have already been fully vaccinated with 1 of the 3 COVID-19 vaccines currently available in the US under FDA Emergency Use Authorization (EUA):

Each vaccine group will enroll about 25 individuals aged 18-55 years and approximately 25 individuals aged ≥56 years. Participants will receive a single booster dose of the Moderna COVID-19 vaccine 12 to 20 weeks after completing their initial vaccination regimen.

Participants will be followed for 1 year after receiving the booster vaccination, and they will be asked to complete telephone check-ins and various in-person follow-up visits. Trial investigators will evaluate participants for safety and any side effects post-vaccination.



Participants will also periodically provide blood samples so investigators can evaluate immune responses against current circulating strains of SARS-CoV-2 and emerging variants, according to the press release.

If participants develop laboratory-confirmed symptomatic COVID-19, researchers will perform genetic sequence analyses on the participant samples to determine if a variant caused the infection.

Individuals who have not received a COVID-19 vaccine are also eligible to enroll in a separate trial cohort. Initially, these volunteers will receive the 2-dose Moderna COVID-19 vaccine and will be assigned to receive a booster dose of a different vaccine after about 12-20 weeks.

The trial has an adaptive design and may add arms as vaccines are awarded EUA and/or variant lineage vaccines become available for evaluation, according to the press release.

Initial trial results are expected in late summer 2021.