St Jude Releases Early Data on Asymptomatic Cases Among Vaccinated Healthcare Workers

By Allison Proffitt

May 12, 2021 | In a research letter published last week in JAMA, St Jude Children’s Research Hospital researchers reported the findings of an observational study done within St. Jude to track asymptomatic COVID-19 among vaccinated healthcare workers. St Jude authors report a decrease in both symptomatic and asymptomatic infection among vaccinated staff.

At the outbreak of the COVID-19 pandemic in 2020, St Jude launched a mitigation plan to keep both staff and the vulnerable St Jude patients safe on campus. By mid-March 2020, all St Jude employees were regularly tested for COVID-19 explained Diego Hijano, M.D., of the St. Jude Department of Infectious Diseases, and a co-first author on the JAMA paper (DOI: 10.1001/jama.2021.6564). St Jude’s asymptomatic screening program tests everyone who works at St Jude and comes on campus every 4-7 days with a PCR test for COVID-19. An off-site location offers testing for any employee who has symptoms or has been exposed to the virus.

While the approved COVID-19 vaccines have demonstrated a reduction in severe, symptomatic disease, the vaccine’s impact on asymptomatic infection has not been as closely tracked. Hijano and Li Tang, Ph.D., of St. Jude Biostatistics, decided to use the asymptomatic screening data as the foundation for an observational study looking at asymptomatic infection.

Once the Pfizer-BioNTech vaccine was made available to eligible St Jude staff in December 2020, researchers could compare screening results for both vaccinated and unvaccinated employees. “Our data actually provided a unique research tool to address some of the questions that were mapped out in the literature,” Tang said.

An observational study can be challenging and subject to biases, Tang said. “The two groups are not fully comparable; they’re not randomized. It’s hard to control all the underlying confounders. What we were trying to do was to mimic a clinical trial… in order to minimize the bias,” she said.

To do that, the researchers only considered screening results for employees who had not previously tested positive for COVID-19. Vaccinated individuals were included from the date of their first Pfizer-BioNTech injection; staff who received another vaccine were excluded. Unvaccinated individuals were included from the launch of the vaccination program in December 2020. Screening data were not included for any individual prior to their vaccination. In the context of this JAMA letter, the study period ended on March 20, 2021.

Within that timeframe, 3,052 St Jude workers received at least 1 dose of the Pfizer-BioNTech vaccine; 2,165 unvaccinated staff were included in the dataset. The employees were diverse, including both men and women, 40% were white, 24.6% were Black. A majority of the employees—84.8%—were younger than 65 years old.

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

Importantly, there were no positive symptomatic or known exposure cases more than 7 days after the second dose. Unvaccinated employees had higher cumulative incidence of a positive test result than vaccinated employees, and higher incidences of positive test results via asymptomatic screening, for symptoms, or for known exposure.

While the size and observational nature of the study do present some limitations, Hijano says that the findings reveal important additional information about how vaccines work.

“When we started with the vaccination program, the first thing we knew was that vaccines prevent you from going into the hospital or dying from COVID-19,” he said. “Then we learned a bit that vaccines will decrease the likelihood of you getting a symptomatic infection, or symptomatic COVID-19. But we didn’t know that it would actually reduce asymptomatic infection. And that’s very important, because asymptomatic infections have been one of the main drivers of the pandemic. Because you don’t have any symptoms, you don’t recognize that you’re infected, you’re going to spread the virus.”

Hijano and Tang are working on a full-length research article, they said, with a much larger cohort of participants now that multiple versions of a vaccine are available to almost everyone and covering a much longer follow-up period. “Those people who got the vaccine earlier, we have followed them for longer now. We hope to summarize that and get that released very soon. We are working together diligently on that,” Tang said.

So far, she said, the newest findings align with the current results and seem to hold true for both Pfizer and Moderna vaccines. “We are planning to show results for both [vaccines] in the full-length article,” Tang said. “It seems that the pattern is similar for the data we have observed so far.” One caveat: “We have very limited number of employees that have received the Johnson & Johnson, so we probably will not be able to address that one,” she added.