COVID-19 Vaccines Reduce Asymptomatic Cases


Routine screening of employees at St. Jude Children’s Research Hospital show lower rates of asymptomatic SARS-CoV-2 infection among vaccinated workers.

The Pfizer-BioNTech mRNA vaccine prevents asymptomatic SARS-CoV-2 infections as well as severe COVID-19 from the virus, a recent study confirmed.

The research letter, published in JAMA, included 5217 employees at St. Jude Children’s Research Hospital, who were routinely screened for SARS-CoV-2 infection before and after vaccination.

“Asymptomatic infections have been the main driver of rapid dissemination of SARS-CoV-2 infection,” Diego Hijano, MD, MSc, Infectious Diseases Department, St. Jude Children’s Research Hospital, told Contagion. “While we knew that vaccines protected against severe COVID-19 (hospitalization, death) and had some evidence that protected against symptomatic infection, we did not know the effects the vaccine had on asymptomatic infections. The biggest takeaway of our study is that by decreasing the risks of asymptomatic infections, COVID-19 vaccines have great potential to significantly reduce viral spread and make a dent in mitigating this pandemic.”

Since March 2020, the hospital has conducted weekly COVID-19 tests of employees. The study tracked vaccinated and unvaccinated employees from the initiation of vaccination with the BNT162b2 vaccine by Pfizer-BioNTech on Dec. 17 until March 20. It included 3052 (58.5%) employees who received at least 1 vaccine dose, 2776 (53.2%) who received 2 doses and 2165 (41.5%) who were unvaccinated.

“To our knowledge, St. Jude Children’s Research Hospital is one of the very few institutions who invested in a regular on-campus asymptomatic employee screening program from the beginning of the pandemic, in order to protect our patients, their families and our employees,” Li Tang, PhD, Biostatistics Department, St. Jude Graduate School of Biomedical Sciences, St. Jude Children’s Research Hospital, told Contagion.

The hospital’s asymptomatic employee screening program, along with measures such as social distancing, wearing masks and contact tracing, helped minimize transmission of the virus and provided a tool for better understanding the spread of the disease, Tang said.

Among employees who had received at least one dose of the vaccine, 51 tested positive for SARS-CoV-2, including 29 (56.9%) identified through asymptomatic screening during the follow-up period. No symptomatic or known exposure cases were found more than 7 days after the second dose. Among unvaccinated employees, 185 tested positive, including 79 (42.7%) who were asymptomatic.

“Our findings filled in a knowledge gap that was not filled with existing literature about the vaccine’s effect on reducing asymptomatic infections,” Tang said. “We were confident about the vaccine, but we were happily surprised to see the vaccine was very comparably effective in reducing both asymptomatic and symptomatic infections.”

The incidence rate ratios of confirmed COVID-19 cases per person-days of follow-up was 0.21 (95% 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. Seven days or more after the second dose, the IRR for asymptomatic screen results was 0.10 (95% CI, 0.04-0.22).

“Our findings, along with finding from other studies in literature, reinforces the significance of getting more people vaccinated,” Tang said.

“We would like to emphasize the significance of the multi-layered mitigation program implemented at St. Jude during the pandemic, including asymptomatic screening and vaccination,” she added. “All measures together have helped to keep our patients safe during the pandemic. With the evidence added by our study, we would also like to emphasize the importance of encouraging coworkers and eligible patients to get vaccinated. By largely reducing asymptomatic infections in addition to symptomatic infections, vaccination has been shown as a powerful component in mitigating the pandemic.”

The investigators will continue to review the data and complete a more comprehensive analysis with a longer follow-up period. The data also will help the hospital adapt its COVID-19 prevention and mitigation practices.

The study is among a growing body of evidence supporting COVID-19 vaccines. A recent study out of Israel found that administration of two doses of the BNT162b2 vaccine was associated with greater than 90% prevention of COVID-19, including 91.5% against asymptomatic infection.

The U.S. Food and Drug Administration recently granted Emergency Use Authorization for Roche Diagnostics Solutions cobas SARS-CoV-2 Test to screen people without symptoms.

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