Who to Boost When: The Effect of Age and Dosing Interval on Delta and Omicron COVID-19 Incidence in the Open-label Phase of the COVE Trial.

Background To help inform COVID-19 vaccination recommendations, we evaluated the impact of age and dosing interval on clinical benefit of a third dose of mRNA-1273. Methods Approximately 17 000 participants from the phase 3 Coronavirus Efficacy trial who previously received 2 doses of 100 µg mRNA-1273 were evaluated for COVID-19 between September 2021 and April 2022 during uptake of a third booster dose of 50 µg of mRNA-1273. Cox models assessed booster relative efficacy of a third dose. Results Initial booster relative efficacy against Delta COVID-19 was 83% (95% confidence interval, 60–93) 14 days postdose and 83% (67–91) 60 days later. Initial booster efficacy against Omicron COVID-19 was 56% (44–65) at 14 days postdose and 4% (−27 to 28) 120 days later. For those aged ≥65 years, initial booster efficacy against Omicron COVID-19 was 86% (69–93) compared with 50% (36–61) for those <65 years. Placebo crossover to 2 doses of mRNA-1273 induced a median 5-month difference from the second to third dose between the original randomized arms. Postboost, the mRNA-1273 arm had a 24% (16%, 32%) lower risk of Omicron COVID-19 compared to the placebo-mRNA-1273 arm. Modeling predicted a 41% postboost reduction in Omicron COVID-19 for a 15- versus 7-month interval between the second and third doses. Conclusions Boosting reduced Delta COVID-19 risk by 83% through 2 months and reduced Omicron COVID-19 risk by 56% but declined by 4 months. A 15- versus 7-month dosing interval predicted a 41% postboost reduction in Omicron COVID-19 but increased preboost risk. Primary Funding Source The National Institutes of Health/National Institute of Allergy and Infectious Diseases. Registration for the COVE Trial.  ClinicalTrials.gov ID# NCT04470427 [ABSTRACT FROM AUTHOR]