COVID-19 vaccines administered in the United States were approved for use because they dramatically reduced the risk of developing the disease when tested in clinical trials. However, these trials did not test the vaccines’ ability to prevent a coronavirus infection – the first step on the road to COVID-19.
Researchers suspect that the vaccines to some extent counteract infections. Two new studies strengthen their case.
Both studies compare coronavirus infection rates among vaccinated and non-vaccinated individuals working at a single medical center. And in both cases, vaccination was actually associated with a significantly lower risk of testing positive for an infection.
The hospital staff are good students because they were among the very first to have access to COVID-19 vaccines. That means they have a longer track record to extract when it comes to assessing the performance of the shots.
Another upside: Many hospitals routinely screen their staff for coronavirus infections. It makes it possible to identify people who look perfectly healthy but who have SARS-CoV-2 virus in their system – and have the potential to spread it to others.
Unlike in a clinical trial, hospital workers in these two studies decided for themselves whether or not to receive the COVID-19 vaccine. There was nothing random about it.
This means that if the coronavirus infection rates are different for vaccinated and non-vaccinated workers, it may be due to factors other than the vaccine itself. Perhaps the people who made a point of getting shot were also more likely to wear face masks, wash their hands thoroughly, or take other actions to avoid getting sick.
Until better data are available, studies like these can still provide insight into whether COVID-19 vaccines prevent infections as well as diseases.
The first report comes from St. Jude Children’s Research Hospital in Memphis, where workers began receiving the Pfizer BioNTech vaccine on December 17th. By March 20, 3,052 workers had received at least one dose, including 2,776 who received both doses. A further 2,165 workers eligible for the vaccine refused to take it.
During the three months, 51 people tested positive for a coronavirus infection after receiving their first dose of vaccine, and 29 of them never developed any COVID-19 symptoms. Meanwhile, 185 of the unvaccinated people became infected and 79 of them were asymptomatic.
After crushing all the numbers, biostatistician Li Tang and her St. Jude colleagues found that people who had had at least one dose of vaccine were 79% less likely to become infected with coronavirus than their unvaccinated colleagues. They were also 72% less likely to develop an infection that was asymptomatic.
This apparent protection was more potent in humans who received both doses of the vaccine and had time for the second dose to kick in. Hospital staff who were at least seven days out of their second shot were 90% less likely than their unvaccinated colleagues to become infected, and when they did, their infections did not produce symptoms.
The second study took place at Tel Aviv Sourasky Medical Center in Israel, where workers began receiving the Pfizer BioNTech vaccine on December 20th. By February 25, 5,953 of them had received at least one dose, and all but 436 had received both doses. Another 757 workers remained unvaccinated.
In all, these workers were tested for coronavirus infections 16,224 times, and 243 of these tests came back positive. In 149 of these cases, the infection led to COVID-19 symptoms – 64 of them in people who had been vaccinated and 85 in the much smaller group of those who had not.
Researchers led by Dr. Yoel Angel calculated that people who were fully vaccinated were 97% less likely to develop an infection with symptoms than their unvaccinated peers. Even among those who were only partially vaccinated, the risk of a symptomatic infection was 89% lower.
Another 63 people who had had at least one dose of vaccine were found to have asymptomatic coronavirus infections, as were 31 workers who skipped the vaccine.
The researchers calculated that people who were fully vaccinated were 86% less likely to develop an asymptomatic infection than their non-vaccinated counterparts. Even those who had only one dose of vaccine saw their risk decrease by 36%.
The results of asymptomatic infections are especially important because people who have SARS-CoV-2 and do not know it are thought to make up somewhere between 40% and 45% of coronavirus cases – and they could spread the virus to others without even realized it, wrote Angel and his colleagues.
In other words, the study results suggest that COVID-19 vaccines have the potential to greatly reduce the threat from silent spreaders.
Both studies were published Thursday in the Journal of the American Medical Assn.
This story originally appeared in the Los Angeles Times.