In a recent publication in NEJM, researchers published data from the ongoing NIH-sponsored trial that resulted in the FDA authorizing the mix-and-match strategy for COVID-19 vaccine boosters last fall.
In this phase 1-2, open-label, nonrandomized, adaptive-design clinical trial (NCT04889209) conducted at 10 sites in the U.S., adults who had completed a COVID-19 vaccine regimen at least 12 weeks earlier and had no reported history of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection received a booster injection with one of three vaccines: mRNA-1273 (Moderna) at a dose of 100 mcg, Ad26.COV2.S (Johnson & Johnson–Janssen) at a dose of 5x1010 virus particles, or BNT162b2 (Pfizer–BioNTech) at a dose of 30 mcg. The primary end points were safety, reactogenicity, and humoral immunogenicity on trial Days 15 and 29.
The study enrolled 458 participants in the trial from May 29, 2021, to August 13, 2021, at 10 sites in the U.S. All participants had completed a COVID-19 vaccine regimen at least 12 weeks earlier and received one of the three authorized vaccines as a booster—153 received a booster dose of the Pfizer vaccine, 154 received a booster dose of the Moderna vaccine, 150 received a booster dose of the Johnson & Johnson vaccine, and one participant did not receive the assigned vaccine.
Reactogenicity was comparable to that reported for the primary series. Researchers indicated that more than half the recipients reported having injection-site pain, malaise, headache, or myalgia. For all combinations, antibody neutralizing titers against a SARS-CoV-2 D614G pseudovirus increased by a factor of 4 to 73, and binding titers increased by a factor of 5 to 55.
Homologous boosters increased neutralizing antibody titers by a factor of 4 to 20, whereas heterologous boosters increased titers by a factor of 6 to 73. Spike-specific T-cell responses increased in all but the homologous Ad26.COV2.S-boosted subgroup. CD8+ T-cell levels were more robust in the Ad26.COV2.S-primed recipients, and heterologous boosting with the Ad26.COV2.S vaccine substantially increased spike-specific CD8+ T cells in the mRNA vaccine recipients.
The authors wrote, "In this preliminary trial, we found that boosting with any of the three vaccines that are currently authorized for emergency use in the United States will stimulate an anamnestic response in persons who have previously received a primary series of any of these vaccines."
The researchers concluded that both homologous and heterologous booster vaccines had an acceptable safety profile and were immunogenic in adults who had completed a primary COVID-19 vaccine regimen at least 12 weeks earlier.
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