COVID-19 vaccine is associated with fewer asymptomatic SARS-CoV-2 infections

STATUE: First author Diego Hijano, MD, of the St. Jude Department of Infectious Diseases, studied how the COVID-19 vaccine reduced symptomatic and asymptomatic infections in workers. view more

Credit: St. Jude Children’s Research Hospital

Vaccination drastically reduced symptomatic and asymptomatic COVID-19 infections in St. Jude Children’s Research Hospital employees compared to their unvaccinated peers, according to a research letter published today in the Journal of the American Medical Association.

The study is one of the first to show a link between COVID-19 vaccination and less asymptomatic infections. When the Pfizer-BioNTech BNT162b2 vaccine was approved for use in the US, the vaccine was reported to be very effective in preventing laboratory-confirmed COVID-19. Data from clinical studies suggested that the two-dose regimen reduced symptomatic illness, including hospitalization and death. But an association with reduced asymptomatic infection was unclear.

“While further research is needed to prevent infections, including in people who have no symptoms, vaccination is likely to reduce transmission of SARS-CoV-2,” said Diego Hijano, MD, of the St. Jude Department of Health. Infectious Diseases. He and Li Tang, Ph.D., of St. Jude Biostatistics, are the first authors of the report. Tang is also the corresponding author.

The study involved 5,217 St. Jude employees who were eligible for vaccination under Tennessee State guidelines between December 17, 2020 and March 20, 2021. More than 58% of workers were vaccinated during that time. Most workers received both doses.

Overall, vaccination reduced the risk of asymptomatic and symptomatic SARS-CoV-2 infection by 79% in vaccinated workers compared to their unvaccinated colleagues. An analysis of asymptomatic infections only found that vaccination reduced the risk by 72%.

The protection was even greater for workers who had taken two doses. One week or more after receiving the second dose, vaccinated workers were 96% less likely than unvaccinated workers to be infected with SARS-CoV-2. When researchers only looked at asymptomatic infections, vaccination reduced the risk by 90%.

Find SARS-CoV-2 infections

The research stems from a program St. Jude leaders began in March 2020 to protect patients and employees from the pandemic virus.

The effort included targeted testing for workers with COVID-19 symptoms or known exposure to the pandemic virus. The plan also included routine lab testing of asymptomatic workers. Nasal swabs were collected from self-reported asymptomatic campus workers at least weekly to perform a polymerase chain reaction to detect asymptomatic SARS-CoV-2 infection.

“This study was possible because St. Jude invested in resources to determine how best to control the disease and to protect our patients and employees,” said Tang. “Few places then or now offer such broad asymptomatic tests.”

Hijano said, “Testing has been invaluable to the institutional COVID-19 mitigation plan. Ultimately, testing also serves as a unique tool that helps fill critical knowledge gaps.”

Results in figures

During the study, 236 of the 5,217 employees included in the analysis tested positive for SARS-CoV-2. They included 185 unvaccinated workers and 51 of the 3,052 workers who received at least one dose of the vaccine.

Nearly half of the positive cases, 108, reported no symptoms on testing. The asymptomatic cases involved 20 workers who received one dose of vaccine and three who tested positive within seven days of the second dose. “The results are a reminder of the many hidden cases in the population, making it a major challenge to contain the virus,” said Tang.

The study group included a cross-section of workers with regard to race and gender. More than 80% of the employees were under 65 years of age. The vaccinated group comprised a higher percentage of the health care workers, 47%, than the unvaccinated workers, 25.7%.


Authors and Funding

The senior authors are James Hoffman, Pharm.D., And Randall Hayden, MD, of St. Jude. The other authors are Aditya Gaur, Terrence Geiger and Ellis Neufeld, all from St. Jude.

The research was funded in part by ALSAC, the St. Jude fundraising and awareness organization.

St. Jude Children’s Research Hospital

St. Jude Children’s Research Hospital is leading the way the world understands, treats and cures childhood cancer and other life-threatening diseases. It is the only National Cancer Institute-designated Comprehensive Cancer Center dedicated solely to children. Treatments developed in St. Jude have helped the overall childhood cancer survival rate to rise from 20% to 80% since the hospital opened more than 50 years ago. St. Jude freely shares the breakthroughs it is making, and every child rescued in St. Jude means doctors and scientists around the world can use that knowledge to save thousands more children. Families never receive a bill from St. Jude for treatment, travel, housing, and food – because all a family has to worry about is helping their child survive. For more information, visit or follow St. Jude on social media at @stjuderesearch.

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