New data released Monday from a US Centers for Disease Control and Prevention (CDC) study of vaccinated healthcare workers show that mRNA-based coronavirus vaccines by Pfizer and Moderna were 80% effective at preventing infections after one dose, with efficacy climbing to 90% two weeks after the second dose. The findings indicate that "authorised mRNA COVID-19 vaccines are highly effective…for preventing SARS-CoV-2 infection, regardless of symptom status, among working-age adults in real-world conditions," the agency said in its latest Morbidity and Mortality Weekly Report.
Pfizer's vaccine BNT162b2, which it co-developed with BioNTech, became the first COVID-19 vaccine to be greenlighted in the US after the FDA authorised it for emergency use on December 11. An emergency authorisation for Moderna's mRNA-1273 followed a week later.
The CDC study looked at prospective cohorts of 3950 healthcare personnel, first responders and other essential and frontline workers who completed weekly SARS-CoV-2 testing for 13 consecutive weeks during the period from December 14, 2020, and March 13, 2021. Among the 2961 vaccinated individuals in the study, 62.8% had received both recommended mRNA doses and 12.1% received only one dose. The remaining 989 people were unvaccinated. Participants had no previous laboratory documentation of COVID-19 infection.
The CDC said that among unvaccinated participants, 1.38 SARS-CoV-2 infections were confirmed by RT-PCR testing per 1000 person-days. By contrast, infection rates were 0.19 among partially immunised workers at least 14 days post-vaccination, but before the second dose, and 0.04 among fully vaccinated people at least 14 days after the second dose. "Estimated mRNA vaccine effectiveness for prevention of infection, adjusted for study site, was 90% for full immunisation and 80% for partial immunisation," according to the CDC report.
CDC director Rochelle Walensky called the study "tremendously encouraging," adding that it "underscored the importance of getting both of the recommended doses of the vaccine in order to get the greatest level of protection against COVID-19, especially as our concerns about variants escalate." The findings are the first real-world data from the US, and are in line with similar research elsewhere, including a recent Israeli study that found BNT162b2 to be 97% effective in preventing symptomatic COVID-19, including severe disease and death.
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