THURSDAY, July 1, 2021 (HealthDay News) — Authorized mRNA vaccines are effective for preventing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, and attenuate viral load, risk of febrile symptoms, and duration of illness in cases of breakthrough infection, according to a study published online June 30 in the New England Journal of Medicine.

Mark G. Thompson, Ph.D., from the U.S. Centers for Disease Control and Prevention in Atlanta, and colleagues conducted a prospective cohort study involving 3,975 health care personnel, first responders, and other essential and frontline workers to examine the effectiveness of two-dose mRNA vaccines in preventing SARS-CoV-2 infection in real-world conditions. Participants completed weekly SARS-CoV-2 testing from Dec. 14, 2020, to April 10, 2021.

The researchers found that SARS-CoV-2 was detected in 204 participants: five, 11, and 156 of whom were fully vaccinated (≥14 days after dose two), partially vaccinated (≥14 days after dose one and <14 days after dose two), and unvaccinated, respectively; 32 participants had indeterminate vaccine status (<14 days after dose one) and were excluded. The adjusted vaccine effectiveness was 91 and 81 percent, respectively, with full and partial vaccination. Among those with SARS-CoV-2 infection, compared with unvaccinated participants, partially or fully vaccinated participants had a 40 percent lower mean viral RNA load. In addition, they had a lower risk of febrile symptoms (relative risk, 0.42) and shorter duration of illness (2.3 fewer days spent sick in bed).

“Authorized mRNA vaccines were highly effective among working-age adults in preventing SARS-CoV-2 infection when administered in real-world conditions,” the authors write.

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