(HealthDay News) — For individuals who receive two doses of mRNA-1273 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine, vaccine efficacy (VE) against symptomatic COVID-19 appears to peak at 120 days after the first dose, according to a research letter published online June 8 in JAMA Network Open.

Dan-Yu Lin, Ph.D., from the University of North Carolina at Chapel Hill, and colleagues considered the mRNA-1273 P301 cohort study, which is an ongoing phase 3, randomized, placebo-controlled trial of 30,415 U.S. adults evaluating the efficacy and safety of the mRNA-1273 SARS-CoV-2 vaccine. The VE against symptomatic COVID-19 was estimated at 94.1 and 93.2% in an interim analysis and at completion of the blinded phase, respectively. The per-protocol population (28,451 participants who tested negative for SARS-CoV-2 at baseline and received two vaccine doses by the end of the blinded phase) was included in this cohort study.

The researchers found that the placebo group included 14,164 patients with 769 cases of COVID-19, while the vaccine group included 14,287 patients with 56 cases of COVID-19. At 40 days after dose 1, VE reached 92.6% and increased to a peak of 94.1% at 120 days. At approximately 120 days, the VE started to decline and reached 89.6% at 200 days. Even at 200 days after dose 1, the level of protection was high, although there was uncertainty estimating VE near the end of blinded follow-up.

“Few cases of COVID-19 occurred after six months in our study, making it difficult to precisely estimate the degree of waning at the end of the blinded follow-up,” the authors write.

One author disclosed financial ties to Pfizer and was involved in COVID-19 vaccine research; a second author disclosed ties to Moderna.

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