Pfizer Covid-19 vaccine linked with fewer asymptomatic infections: Study

People who received the Pfizer COVID-19 vaccine at a US hospital had dramatically lower symptomatic and asymptomatic infections compared with their unvaccinated peers, according to a study.

The research, published in the Journal of the American Medical Association, is among the first to show an association between COVID-19 Pfizer-BioNTech vaccination and fewer asymptomatic infections.

"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," said Diego Hijano from the St. Jude Children's Research Hospital in the US.

The study involved 5,217 St. Jude employees who were eligible under Tennessee state guidelines for vaccination between December 17, 2020, and March 20, 2021.

More than 58 per cent of employees were vaccinated during that period. Most workers received both doses.

Overall, vaccination reduced the risk of asymptomatic and symptomatic SARS-CoV-2 infection by 79 per cent in vaccinated employees compared with their unvaccinated colleagues, the researchers said.

An analysis of asymptomatic infections alone found vaccination reduced the risk by 72 per cent, they said.

Researchers found that protection was even greater for employees who completed two doses.

A week or more after receiving the second dose, vaccinated employees were 96 per cent less likely than unvaccinated workers to become infected with SARS-CoV-2, they said.

When researchers looked just at asymptomatic infections, vaccination reduced the risk by 90 per cent.

During the study, 236 of the 5,217 employees included in the analysis tested positive for SARS-CoV-2.

They included 185 unvaccinated employees and 51 of the 3,052 workers who had received at least one dose of the vaccine.

Almost half of the positive cases, 108, reported no symptoms upon testing, the researchers said.

The asymptomatic cases included 20 employees who had received one vaccine dose and three who tested positive within seven days of the second dose, they said.