Moderna's COVID Vax Produces Antibody Responses 6 Months Later
— But neutralizing activity declined over time, and not equally among age groups
"Antibodies persisted 6 months after the second dose of Moderna's COVID-19 vaccine in younger and middle-age adults, but were diminished in older adults when using more sensitive assays, according to data from an ongoing phase I trial.
Antibody activity was high across groups at day 209, but assessment with a "live virus" neutralization test found geometric mean titers declined more over time in participants ages 56 and older compared with adults ages 18-55, reported Mehul Suthar, PhD, of Emory University School of Medicine in Decatur, Georgia, and colleagues in a correspondence piece in the New England Journal of Medicine.hey noted the durability of protection is currently unknown, and examined binding and neutralizing antibodies elicited by the Moderna COVID-19 vaccine in 33 healthy adult participants in an ongoing phase I trial, who were stratified by age. They measured responses at 180 days after the second dose of vaccine, or day 209, using three distinct serologic assays.
Suthar's group found nearly all participants "had detectable activity in a pseudovirus neutralization assay," and all participants had detectable activity on the "more sensitive live-virus focus-reduction neutralization mNeon-Green test," but it was this assay for which geometric mean titers were lower in older adults compared with adults ages 18-55.
In that assay, neutralizing activity in the older groups declined approximately 10-fold at 6 months from the peak 14 days after the second vaccine dose; in those 18-55, activity was down by roughly half. But activity levels in the older groups remained more than 10-fold above pre-vaccination baseline at the 6-month mark.
The researchers estimated a half-life of binding antibodies of 52 days after day 43 for all participants using a model assuming a steady decay rate over time, and the neutralizing antibody half-life was 202 days for live-virus neutralization using a model that assumed decay rates decrease over time. The model with the steady decay rate was the best fit for binding, and the one with the decreasing decay rate was best for neutralization, which was "consistent with published observations of convalescent patients with COVID-19 through 8 months after symptom onset," Suthar and colleagues wrote.
Suthar and colleagues noted that "the antibody titers and assays that best correlate with vaccine efficacy are not currently known," and ongoing studies continue to monitor immune response beyond 6 months, as well as examine the impact of a potential booster dose against COVID-19 variants."
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