Follow-up of vaccinated and unvaccinated individuals at St. Jude Children’s Research Hospital, USA, revealed asymptomatic infection reduced by 90% and symptomatic infections reduced by up to 96% in vaccinated people compared to those not vaccinated.
The Pfizer/BioNTech mRNA was approved for use in the USA in December 2020. Clinical trials reported an efficacy of 94.8%, and it has been shown to reduce symptomatic infection of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, it is not yet known how it affects asymptomatic infection.
Since March 2020, St. Jude Children’s Research Hospital has conducted routine screening of asymptomatic workers and testing of symptomatic people and people with known exposure to the virus. Nasal swabs were collected about once a week from asymptomatic individuals to test for SARS-CoV-2.
After the vaccination of eligible employees began, vaccinated individuals were followed up from their first dose, and unvaccinated employees were followed up until December, or their test was positive. This comparison allowed the researchers to find associations between vaccination and asymptomatic infection. They reported their results recently in JAMA.
Screening vaccinated and unvaccinated employees
Of the 5,217 workers eligible for vaccination, by March 20, 2021, about 58% received at least one dose, and about 53% had received both doses.
The team found 51 tested positive for the virus during the follow-up, 41 before the second dose, and 10 after the second dose, among those who were vaccinated. Among the unvaccinated workers, 185 were positive, and about 43% were asymptomatic.
Overall, among the people who tested positive, about half did not have any symptoms. “The results are a reminder of the many hidden cases in the population, which makes containing the virus a big challenge,” said Li Tang, lead author of the study.
The results showed that vaccination reduced the risk of SARS-CoV-2 infection (symptomatic and asymptomatic) by about 79% compared to unvaccinated individuals. When analyzing only asymptomatic infections, the team found a decrease of about 72% in the risk of vaccination.
Receiving two doses of the vaccine protected people even more. After a week or more of the second dose, vaccinated individuals were 96% less likely than those who were not vaccinated to get infected. Vaccination reduced asymptomatic infection risk by 90%.
This said, there are some limitations to the study, such as a small sample size, short follow-up time, and the higher likelihood of risk-prone behavior by the people who chose not to get vaccinated.
While further research is needed, by preventing infections, including in people who have no symptoms, there is a high possibility that vaccination will decrease transmission of SARS-CoV-2,” Diego Hijano, another author of the study said.