mRNA Vaccines Limit Severity of Rare Breakthrough Infections
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Information is limited regarding the effectiveness of the two-dose messenger RNA (mRNA) vaccines BNT162b2 (Pfizer–BioNTech) and mRNA-1273 (Moderna) in preventing infection with SARS-CoV-2 and in attenuating Covid-19 when administered in real-world conditions.

The team conducted a study involving 3975 health care personnel, first responders, and other essential and frontline workers. From December 14, 2020, to April 10, 2021, the participants completed weekly SARS-CoV-2 testing by providing mid-turbinate nasal swabs for qualitative and quantitative reverse-transcriptase–polymerase-chain-reaction (RT-PCR) analysis.

SARS-CoV-2 was detected in 204 participants (5%), of whom 5 were fully vaccinated, 11 partially vaccinated and 156 unvaccinated; the 32 participants with indeterminate vaccination status were excluded. Adjusted vaccine effectiveness was 91% with full vaccination and 81% with partial vaccination.

Among participants with SARS-CoV-2 infection, the mean viral RNA load was 40% lower in partially or fully vaccinated participants than in unvaccinated participants. In addition, the risk of febrile symptoms was 58% lower and the duration of illness was shorter, with 2.3 fewer days spent sick in bed.


Authorized mRNA vaccines were highly effective among working-age adults in preventing SARS-CoV-2 infection when administered in real-world conditions, and the vaccines attenuated the viral RNA load, risk of febrile symptoms, and duration of illness among those who had breakthrough infection despite vaccination.