Asymptomatic, symptomatic COVID-19 after Pfizer vaccination in routinely screened HCWs

Editor's Note

This study from St Jude Children’s Research Hospital, Memphis,  finds an association between vaccination with the Pfizer-BioNTech vaccine in hospital employees and a decreased risk of symptomatic and asymptomatic COVID-19 infections.

Between December 17, 2020, and March 20, 2021, 5,217 healthcare workers (HCWs) met vaccination criteria—3,052 (58.5%) received at least one dose, and 2,776 (53.2%) received 2 doses of the vaccine; 2165 (41.5%) were unvaccinated.

Of vaccinated HCWs, 51 tested positive for COVID-19 during follow-up (41 HCWs before and 10 after the second dose); 29 (56.9%) were diagnosed through asymptomatic screening.

Among unvaccinated HCWs, 185 tested positive for COVID-19, and 79 (42.7%) were asymptomatic.

The estimated incidence rate ratio (IRR) was 0.21 for any COVID-19 infection, 0.28 for asymptomatic infections, and 0.16 for symptomatic infections. The IRR within the first 11 days after the first dose was 0.58 to 0.60 for all three outcomes.

The IRR for positive test results via screening of asymptomatic HCWs was 0.58 from 12 days after the first dose and before the second dose, 0.35 within 6 days after the second dose, and 0.10 at 7 days or more after the second dose. There were no positive symptomatic cases more than 7 days after the second dose.

Unvaccinated HCWs had more positive test results than vaccinated HCWs and more positive test results via asymptomatic screening for symptoms or known exposure.

Further research is needed to determine whether a reduced risk of asymptomatic infection leads to reduced transmission, the researchers say.