On March 29, 2021, this report was posted as an MMWR Early Release on the MMWR website (https://www.cdc.gov/mmwr)Messenger RNA (mRNA) BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna) COVID-19 vaccines have been shown to be effective in preventing symptomatic COVID-19 in randomized placebo-controlled Phase III trials (1,2); however, the benefits of these vaccines for preventing asymptomatic and symptomatic SARS-CoV-2 (the virus that causes COVID-19) infection, particularly when administered in real-world conditions, is less well understood. Using prospective cohorts of health care personnel, first responders, and other essential and frontline workers* in eight U.S. locations during December 14, 2020-March 13, 2021, CDC routinely tested for SARS-CoV-2 infections every week regardless of symptom status and 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 (≥14 days after second dose) persons, 0.04 infections per 1,000 person-days were reported, and among partially immunized (≥14 days after first dose and * Occupational categories: primary health care personnel (physicians, physician assistants, nurse practitioners, and dentists), other allied health care personnel (nurses, therapists, technicians, medical assistants, orderlies, and all other persons providing clinical support in inpatient or outpatient settings), first responders (firefighters, law enforcement, corrections, and emergency medical technicians), other essential and frontline workers (workers in hospitality, delivery, and retail; teachers; and all other occupations that require contact within 3 feet of the public, customers, or coworkers as a routine part of their job). † An additional five participants received the Janssen COVID-19 vaccine (Johnson & Johnson), resulting in 2,961 vaccinated participants. § Person-days is an estimate of the time-at-risk (to SARS-CoV-2 infection) that each participant contributed to the study.

No abstract

BACKGROUND: Information is limited on messenger RNA (mRNA) BNT162b2 (Pfizer–BioNTech) and mRNA–1273 (Moderna) COVID–19 vaccine effectiveness (VE) in preventing SARS–CoV–2 infection or attenuating disease when administered in real–world conditions. METHODS: Prospective cohorts of 3,975 healthcare personnel, first responders, and other essential and frontline workers completed weekly SARS–CoV–2 testing during December 14 2020—April 10 2021. Self–collected mid–turbinate nasal swabs were tested by qualitative and quantitative reverse–transcription—polymerase–chain–reaction (RT–PCR). VE was calculated as 100%× (1−hazard ratio); adjusted VE was calculated using vaccination propensity weights and adjustments for site, occupation, and local virus circulation . RESULTS: SARS–CoV–2 was detected in 204 (5.1%) participants; 16 were partially (≥14 days post–dose–1 to 13 days after dose–2) or fully (≥14 days post–dose–2) vaccinated, and 156 were unvaccinated; 32 with indeterminate status (<14 days after dose–1) were excluded. Adjusted mRNA VE of full vaccination was 91% (95% confidence interval [CI]=76%—97%) against symptomatic or asymptomatic SARS–CoV–2 infection; VE of partial vaccination was 81% (95% CI=64%–90%). Among partially or fully vaccinated participants with SARS–CoV–2 infection, mean viral RNA load (Log10 copies/mL) was 40% lower (95% CI=16%–57%), the risk of self–reported febrile COVID–19 was 58% lower (Risk Ratio=0.42, 95% CI=0.18–0.98), and 2.3 fewer days (95% CI=0.8–3.7) were spent sick in bed compared to unvaccinated infected participants. CONCLUSIONS: Authorized mRNA vaccines were highly effective among working–age adults in preventing SARS–CoV–2 infections when administered in real–world conditions and attenuated viral RNA load, febrile symptoms, and illness duration among those with breakthrough infection despite vaccination.

C oronaviruses are a family of RNA viruses whose large genomes, propensity for mutation, and frequent recombination events have resulted in a diversity of strains and species that are capable of rapid adaptation to new hosts and ecologic environments (1). This viral plasticity has garnered widespread concern because of zoonotic potential and the consequences of new emergence events in both human and animal populations. The emergence of a new strain of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes coronavirus disease (COVID-19) has once again demonstrated the role of the family Coronaviridae in causing human disease outbreaks. SARS-CoV-2, a novel betacoronavirus, was identifi ed in human patients from Wuhan, China, during December 2019 and has resulted in a global pandemic, an unprecedented public health emergency, and untold economic and societal repercussions worldwide. Similar to the 2002-2003 severe acute respiratory syndrome (SARS) epidemic, a live animal market where hundreds of animal species were sold is suspected to be associated with the emergence or early spread of COVID-19 in humans (2). Although COVID-19 is novel in the breadth of the human outbreak, several pathogenic alphacoronaviruses and betacoronaviruses have shown similar patterns of emergence. As early as the 1930s, coronaviruses pathogenic to livestock, companion animals, and laboratory animals were identifi ed (3). During the 1960s, 2 human coronaviruses, HCoV-229E and HCoV-OC43, were detected in patients who had common colds (4,5). Although it is speculated that HCoV-OC43 might also have emerged through a global pandemic in the late 1800s (6), the 2002-2003 SARS outbreak is the fi rst known global epidemic caused by a coronavirus. The SARS epidemic triggered research within this viral family (3). This research led to detection of 2 new human coronaviruses, HCoV-NL63 and HCoV-HKU1 (7,8). HCoV-229E, HCoV-OC43, HCoV-NL63, and HCoV-HKU1 are now accepted as globally endemic common cold species that are typically associated with mild-to-moderate respiratory illness. In 2012, the most deadly human coronavirus to date was detected in the Arabian Peninsula: Middle East respiratory syndrome coronavirus (MERS-CoV) (9). A cumulative body of research on these and other coronaviruses has shown that most alphacoronaviruses and betacoronaviruses infecting humans have come from animal hosts and that both historic patterns and coronavirus biology establish an urgent ongoing threat to human and animal health (10).

In a cohort of essential workers in the United States previously infected with SARS-CoV-2, risk factors for reinfection included being unvaccinated, infrequent mask use, time since first infection, and being non-Hispanic Black. Protecting workers from reinfection requires a multipronged approach including up-to-date vaccination, mask use as recommended, and reduction in underlying health disparities.