In a publication in the BMJ, Chung et al conducted a test-negative design study to estimate the effectiveness of messenger (mRNA) COVID-19 vaccines (BNT162b2 [Pfizer-BioNTech] or mRNA-1273 Moderna) vaccine against symptomatic infection and severe outcomes (hospitalizations or death). The study involved 324,033 individuals aged16 years and older with symptoms of COVID-19 and who were tested for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Primary outcome measures included laboratory-confirmed SARS-CoV-2 via reverse transcription polymerase chain reaction and hospital admissions and deaths associated with SARS-CoV-2 infection.

Multivariable logistic regression was adjusted for personal and clinical characteristics correlated with SARS-CoV-2 and vaccine receipt to approximate vaccine effectiveness against symptomatic infection and severe outcomes. The researchers revealed that of the 324,033 individuals with symptoms, 53,270 (16.4%) were positive for SARS-CoV-2 and 21,272 (6.6%) received at least one dose of vaccine. Among participants who tested positive, 2,479 (4.7%) were admitted to hospital or died. Vaccine effectiveness against symptomatic infection observed >14 days after one dose was 60% (95% confidence interval 57%-64%), expanding from 48% (41%-54%) at 14 to 20 days after one dose to 71% (63%-78%) at 35 to 41 days. After two doses, vaccine effectiveness observed >7 days was 91% (89%-93%).

Vaccine effectiveness against hospital admission or death observed >14 days after one dose was 70% (60%-77%), expanding from 62% (44%-75%) at 14 to 20 days to 91% (73%-97%) at 35 days, while vaccine effectiveness observed >7 days after two doses was 98% (88%-100%). For adults aged >70 years, vaccine effectiveness estimates were observed to be lower for intervals shortly after one dose but were comparable to those for younger individuals for all intervals after 28 days. After two doses, high vaccine effectiveness was noticed against variants with the E484K mutation.

The authors indicated that protection against symptomatic infection and severe outcomes rises steadily after one dose of an mRNA COVID-19 vaccine. They also indicated that their findings for fully vaccinated individuals are analogous with efficacy estimates from clinical trials and other real-world effectiveness estimates reported in a range of settings.

The authors noted that vaccine effectiveness of one dose was observed to be lower, especially for older adults shortly after the first dose. The authors wrote, "Since vaccine effectiveness against symptomatic infection after one dose is only moderate, and among older adults appears to be modest even at 14-20 days, individuals need to be informed that besides the absence of benefit during the first two weeks (and likely longer for older adults) after one dose of a mRNA COVID-19 vaccine, they should continue to adhere to recommended public health measures, such as wearing a mask, physical distancing, and avoidance of social gatherings."

Chung et al concluded that two doses of mRNA COVID-19 vaccines were extremely effective against symptomatic infection and severe outcomes.

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