Atlanta, GA—While interim results from a phase III trial of the Moderna mRNA-1273 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine indicated 94% efficacy in preventing coronavirus disease 2019 (Covid-19), the durability of protection has remained unknown.

A research letter published by the New England Journal of Medicine suggests the vaccine remains effective for at least 6 months after the first dose.

The letter describes mRNA1273-elicited binding and neutralizing antibodies in 33 healthy adult participants in an ongoing phase I trial. Emory University Medical School–led researchers focused on participants, who were stratified by age, at 180 days after the second dose of 100 mcg (day 209).

The report notes that antibody activity remained high in all age groups at that point, explaining, “Binding antibodies, measured by means of an enzyme-linked immunosorbent assay against SARS-CoV-2 spike receptor–binding domain, had geometric mean end-point titers (GMTs) of 92,451 (95% confidence interval [CI], 57,148 to 149,562) in participants 18 to 55 years of age, 62,424 (95% CI, 36,765 to 105,990) in those 56 to 70 years of age, and 49,373 (95% CI, 25,171 to 96,849) in those 71 years of age or older.”

The researchers add that nearly all participants had detectable activity in a pseudovirus neutralization assay, with 50% inhibitory dilution (ID50) GMTs of 80 (95% CI, 40-135), 57 (95% CI, 30-106), and 59 (95% CI, 29-121), respectively. The study team also administered the more sensitive live-virus focus-reduction neutralization mNeonGreen test. While all the participants had detectable activity, with ID50 GMTs of 406 (95% CI, 286-578), 171 (95% CI, 95-307), and 131 (95% CI, 69-251), respectively, those GMTs were lower in participants aged 56 to 70 years (P = .02) and in those aged 71 years or older (P = .004) than in those aged 18 to 55 years.

The authors write that the estimated half-life of binding antibodies after day 43 for all the participants was 52 days (95% CI, 46-58), which was calculated using an exponential decay model that assumes a steady decay rate over time, and 109 days (95% CI, 92-136), which was calculated with the use of a power-law model (at day 119), which assumes that decay rates decrease over time.

In addition, the neutralizing antibody half-life estimates in the two models were 69 days (95% CI, 61-76) and 173 days (95% CI, 144-225) for pseudovirus neutralization and 68 days (95% CI, 61-75) and 202 days (95% CI, 159-272) for live-virus neutralization.

The authors point out that their results are consistent with published observations of convalescent patients with COVID-19 through 8 months after symptom onset.

“Although the antibody titers and assays that best correlate with vaccine efficacy are not currently known, antibodies that were elicited by mRNA-1273 persisted through 6 months after the second dose, as detected by three distinct serologic assays,” the researchers write. “Ongoing studies are monitoring immune responses beyond 6 months as well as determining the effect of a booster dose to extend the duration and breadth of activity against emerging viral variants. Our data show antibody persistence and thus support the use of this vaccine in addressing the Covid-19 pandemic.”

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