Under real-world conditions, mRNA vaccine effectiveness after full immunization (at least 14 days after the second dose) was 90% against SARS-CoV-2 infections regardless of symptom status, while vaccine effectiveness for partial immunization (at least 14 days after the first dose but before second dose) was 80%, according to Centers for Disease Control and Prevention in its Morbidity and Mortality Weekly Report (MMWR).
The vaccines included in the study were Pfizer-BioNTech’s BNT162b2 and Moderna’s mRNA-1273.
To arrive at the results, CDC tested a cohort of healthcare personnel, first responders and other essential frontline workers in eight U.S. locations for SARS-CoV-2 infections every week from December 14, 2020 through March 13, 2021 regardless of symptom status. The agency also tested study participants at the onset of symptoms consistent with COVID-19–associated illness.
Among 3,950 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. Among unvaccinated participants, 1.38 SARS-CoV-2 infections were confirmed by reverse transcription-polymerase chain reaction (RT-PCR) per 1,000 person-days.
In contrast, among fully immunized (at least 14 days after second dose) people,l 0.04 infections per 1,000 person-days were reported, and among partially immunized (at least 14 days after first dose and before second dose) persons, 0.19 infections per 1,000 person-days were reported.
Participants included physicians and other clinical leads (primary healthcare personnel) (21.1%), nurses and other allied healthcare personnel (33.8%), first responders (21.6%), and other essential and frontline workers (23.5%). The majority of participants were female (62.1%), aged 18–49 years (71.9%), White (86.3%), and non-Hispanic (82.9%) and had no chronic medical conditions (68.9%).
Most (75.0%) of the participants received one or more doses of vaccine during the study period); 477 (12.1%) received their first dose and had not received their second dose by the end of the study period, and 2,479 (62.8%) received both recommended mRNA vaccine doses. Most (60.5%) were vaccinated with their first dose during December 14-31, 2020. Both mRNA vaccine products were administered to participants in all locations but differed in the timing of their availability; 62.7% of vaccinated participants received Pfizer-BioNTech vaccine and 29.6% received the Moderna vaccine.
