Now, new research is providing some answers. CDC researchers report that, under real-world conditions, mRNA vaccine effectiveness of full immunization 2 weeks or more after the second dose was 90% against SARS-CoV-2 infections regardless of symptom status. Furthermore, according to the article in the Morbidity & Mortality Weekly Report, vaccine effectiveness of partial immunization, essentially 14 or more days after the first dose but before the second dose, was 80%.
“The findings complement and expand upon these preceding reports by demonstrating that the vaccines can also reduce the risk for infection regardless of COVID-19–associated illness symptom status. Reducing the risk for transmissible infection, which can occur among persons with asymptomatic infection or among persons several days before symptoms onset, is especially important among healthcare personnel, first responders, and other essential and frontline workers given their potential to transmit the virus through frequent close contact with patients and the public.”
Researchers reached those conclusions based on prospective cohorts of 3,950 healthcare personnel, first responders, and other essential and frontline workers who completed weekly SARS-CoV-2 testing for 13 consecutive weeks.
“Authorized mRNA COVID-19 vaccines are effective for preventing SARS-CoV-2 infection in real-world conditions,” according to the CDC, which recommends COVID-19 vaccination for all eligible Americans.
Study participants came from eight United States locations during December 14, 2020–March 13, 2021; they were tested for SARS-CoV-2 infections every week regardless of symptom status and at the onset of symptoms was consistent with COVID-19–associated illness. Locations participating were associated with HEROES-RECOVER, a network of longitudinal cohorts in eight locations—Phoenix, Tucson, and other areas in Arizona; Miami; Duluth, MN; Portland, OR; Temple, TX, and Salt Lake City, UT.
Among the nearly 4,000 participants with no previous laboratory documentation of SARS-CoV-2 infection, 2,479 (62.8%) received both recommended mRNA doses and 477 (12.1%) received only one dose of mRNA vaccine. With COVID-19 infections confirmed by reverse transcription–polymerase chain reaction (RT-PCR) per 1,000 person-days, researchers determined the following:
• Among unvaccinated participants, 1.38 SARS-CoV-2 infections were confirmed.
• Among fully immunized (≥14 days after second dose) persons, 0.04 infections per 1,000 person-days were reported,
• Among those partially immunized (≥14 days after first dose and before second dose) persons, 0.19 infections per 1,000 person-days were reported.
“Estimated mRNA vaccine effectiveness for prevention of infection, adjusted for study site, was 90% for full immunization and 80% for partial immunization,” according to the authors. “These findings indicate that authorized mRNA COVID-19 vaccines are effective for preventing SARS-CoV-2 infection, regardless of symptom status, among working-age adults in real-world conditions.”
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