The US National Institutes of Health (NIH) has started a Phase 1/2 clinical trial in which adult volunteers who have been fully vaccinated against COVID-19 will receive booster doses of different COVID-19 vaccines to determine the safety and immunogenicity of mixed boosted regimens.

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"Although the vaccines currently authorised by the US 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", National Institute of Allergy and Infectious Diseases Director Anthony S. Fauci said. "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", Fauci, who is also a top adviser to President Joe Biden on the coronavirus pandemic.

The trial, to be led by Baylor College of Medicine, Houston, and the University of Maryland, College Park, will include approximately 150 individuals who already have received one of the three FDA authorised COVID-19 vaccines of -- Johnson & Johnson, Moderna or the Pfizer-BioNTech.

Each vaccine group will enrol about 25 people ages 18 to 55 years and approximately 25 people age 56 years and older. Initially, these volunteers will receive the two-dose Moderna COVID-19 vaccine regimen and will be assigned to receive a booster dose of a vaccine about 12 to 20 weeks later. People who have not yet received a vaccine are also eligible to enrol in the trial in a separate cohort.

All trial participants will be followed for one year after receiving their last vaccination as part of the study. Participants also will be asked to provide blood samples periodically so that trial investigators can evaluate immune responses against current circulating strains of SARS-CoV-2, as well as emerging variants. If trial participants develop laboratory-confirmed symptomatic COVID-19, investigators will perform genetic sequence analyses on the participant samples to see if a variant strain of SARS-CoV-2 caused the infection. Initial trial results are expected in late summer 2021.