April 1, 2021

CDC Study Shows Effectiveness of mRNA Vaccines Against SARS-CoV-2

A CDC study of prospective cohorts of essential and frontline workers shows that messenger RNA (mRNA) vaccines are effective for preventing SARS-CoV-2 infections in real-world conditions. The agency’s findings are published in the March 29 issue of CDC’s Morbidity and Mortality Weekly Report (MMWR). The study found BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna) COVID-19 vaccines to be 90 percent effective against symptomatic and asymptomatic SARS-CoV-2 infection at least 14 days after the workers’ second dose. Partially immunized workers also received protection: mRNA vaccines were found to be 80 percent effective against SARS-CoV-2 infection at least 14 days after the workers’ first dose but before they received the second.

During Dec. 14, 2020–March 13, 2021, CDC routinely tested 3,950 workers in eight U.S. locations who represented a range of occupational categories, including healthcare personnel; first responders; workers in hospitality, delivery, and retail; teachers; and other occupations that routinely require contact within three feet of coworkers or members of the public. The workers were enrolled in the study in July 2020. Participants were surveilled for COVID-19 symptoms through weekly text messages, emails, and direct participant or medical record reports. They also self-collected weekly nasal swabs regardless of symptom status, plus additional nasal swabs at the onset of any illness associated with COVID-19. These specimens were tested by reverse transcription-polymerase chain reaction (RT-PCR) assay for SARS-CoV-2 infection. Participants’ receipt of COVID-19 vaccines was also documented; by the end of the study period, 75 percent had received at least one dose of the Pfizer-BioNTech or Moderna mRNA vaccines and 62.8 percent had received both doses.

One hundred sixty-one PCR-confirmed SARS-CoV-2 infections were identified while participants were unvaccinated. When participants’ immune status was considered indeterminate—within 13 days of their first or second vaccine doses—researchers identified 33 PCR-confirmed infections, which were excluded from the results. Eight infections occurred in participants who were considered “partially immunized”: five infections were confirmed among participants who were at least 14 days beyond their first dose but did not receive their second dose within the study period, and an additional three infections were confirmed among participants at least 14 days after receipt of their first dose and through their receipt of the second. Finally, three infections were confirmed among participants with full immunization who had received their second vaccine doses at least 14 days prior to their positive test.

While the Pfizer-BioNTech and Moderna mRNA vaccines have been shown through randomized trials to be effective in preventing symptomatic COVID-19, it was not previously understood how well these vaccines worked under real-world conditions. According to CDC, the report demonstrates that mRNA vaccines can reduce the risk of SARS-CoV-2 infections regardless of symptom status. The agency stresses that this finding is especially important for healthcare personnel, first responders, and other essential workers, who have potential to transmit the virus due to frequent close contacts with patients, coworkers, and the public.

CDC’s report encourages caution when interpreting estimates of vaccine effectiveness due to “moderately wide [confidence intervals] attributable in part to the limited number of postimmunization PCR-confirmed infections observed.” The report is also subject to limitations related to the self-collection of specimens and shipment delays. A full explanation of the study’s limitations is available in the “discussion” section of the report.

CDC researchers recommend further research to determine the number of days required for a vaccinated person to develop immunity. To learn more, see the full MMWR report.