May 14, 2021

Asymptomatic and symptomatic SARS-CoV-2 infections among hospital employees after BNT162b2 vaccination

Findings from a study published in JAMA show an association between vaccination with BNT162b2 (Pfizer-BioNTech) in hospital employees and a decreased risk of symptomatic and asymptomatic infections with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

“Although an association between vaccination and a reduction in symptomatic [coronavirus disease 2019 (COVID-19)] has been well described, an association with asymptomatic infection remains unclear,” wrote Li Tang, St Jude Children’s Research Hospital, Memphis, Tennessee, and colleagues.

On December 17, 2020, vaccination with BNT162b2 was initiated at St Jude Children’s Research Hospital. For the study, vaccinated employees receiving BNT162b2 were followed up from the date they received their first vaccine dose, while unvaccinated employees were followed up from December 17, 2020, or their first asymptomatic screen result, whichever was later. Individuals with prior COVID-19 exposure were excluded from the study.

Between December 17, 2020, and March 20, 2021, 3,052 hospital employees received at least 1 BNT162b2 dose, and 2,776 received 2 doses. Meanwhile, 2,165 employees were unvaccinated. In the vaccinated group, 66.0% were women and 88.7% were younger than 65 years. Meanwhile, in the unvaccinated group, 58.3% were women and 84.8% were younger than 65 years. Median follow-up was 81 days in the unvaccinated group and 72 days among vaccinated employees.

Among vaccinated employees, 51 tested positive for SARS-CoV-2 during follow-up, of whom 29 (56.9%) were diagnosed through asymptomatic screening. Of the vaccinated employees who were SARS-CoV-2 positive, 41 and 10 tested positive before and after the second dose, respectively. Among unvaccinated employees, 185 tested positive, of whom 79 (42.7%) were asymptomatic. 

Study data showed an incidence rate ratio (IRR) of 0.21 (95% confidence interval [CI], 0.15-0.28) for any SARS-CoV-2 infection, 0.28 (95% CI, 0.18-0.42) for asymptomatic screen results, and 0.16 (95% CI, 0.10-0.25) for symptomatic or known exposure cases. On the other hand, the IRR within the first 11 days after the first dose of vaccine was 0.58 to 0.60 for all 3 outcomes. 

Additionally, the IRR for positive results via asymptomatic screening from 12 days after the first vaccine dose until the second dose of vaccine (median interval between doses, 21 days [range, 11-49 days]) was 0.58 (95% CI, 0.30-1.13), while the IRRs within 7 days after the second dose and 7 days or more after the second vaccine dose were 0.35 (95% CI, 0.11-1.09) and 0.10 (95% CI, 0.04-0.22), respectively. Meanwhile, there were no positive symptomatic or known exposure cases more than 7 days after the second dose. 

The authors noted that unvaccinated employees had higher incidences of positive test results via asymptomatic screening, for symptoms, or for known exposure and that an association between vaccination with BNT162b2 in hospital employees and a decreased risk of asymptomatic infections with SARS-CoV-2 was observed.

“Further research is needed to determine whether a reduction in risk of asymptomatic infection leads to reduced transmission,” the authors added.

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