It is clear that vaccinations do not completely prevent the transmission of Sars-CoV-2. It is unclear whether and how the immunizations affect the transmission of the virus. An investigation with large amounts of data from prisoners in the USA now comes to meaningful results.
A US research team has investigated the extent to which vaccinations can reduce the transmission of Sars-CoV-2. For this purpose, researchers led by Sophia Tan and Nathan Lo from the University of California in San Francisco evaluated the anonymous data from the “California Department of Corrections and Rehabilitation”.
These included information on Covid-19 test results, vaccination status and cell arrangements for a total of 111,687 inmates from 35 different prisons. The data analyzed is from December 15, 2021 to May 20, 2023, when the omicron variant first became dominant. 97 percent of the inmates included in the study are male.
So far, it has been difficult to make scientifically sound statements about the extent to which Covid-19 pathogens can be transmitted after vaccination. Too many factors in everyday life, such as wearing a mask or keeping your distance, would influence the infectivity results. The researchers therefore resorted to data from inmates in prisons. These were better suited for the study because in prisons a number of the influencing factors that lead to transmission are compulsorily regulated and comparable.
The evaluation of the data showed that Covid-19 vaccinations reduce the risk of infection by eleven percent compared to the infectiousness of unvaccinated people. Overall, the absolute risk of infection remained high, even for vaccinated prisoners, the team writes about the study results in the journal “Nature Medicine”. Because despite the high vaccination rate of 81 percent among prisoners, according to the study, 22,334 prisoners were infected with the Covid 19 pathogen within the investigation period.
In addition, the experts were able to see in their data that infected people who had previously been vaccinated within a period of two months passed the pathogen on to the prisoner in the same cell in 28 percent of all cases than unvaccinated infected people (36 percent). However, this effect decreased the longer the vaccination was given. It decreased by six percent every five weeks.
The least infectious were the prison inmates who had already had a Covid 19 infection and were vaccinated. This group of people was 40 percent less likely to infect others compared to the unvaccinated and previously uninfected groups. The research group attributes around half of this effect to the vaccinations.
“People are least contagious within the first two months after vaccination,” Lo said. This suggests that booster shots and large-scale vaccination campaigns can play a role in reducing transmission during waves of infection.
