Covid, previous infection and vaccine make it less contagious

A new study from the USA confirms the protective qualities of the so-called ‘hybrid immunity’ and the added value guaranteed by the Covid vaccination to natural immunity: people who have both a previous Sars-CoV-2 infection and a recent vaccine behind them have 40% less likely to transmit the virus.

Previous infection and recent vaccination make it less contagious

It is one of the main results that emerged from a study conducted in California prisons and published in ‘Nature Medicine’. The authors found that recent vaccinations and boosters reduce infectivity, even though they admit the risk of infection remains high. In particular what was observed during the first Omicron wave is that, when the vaccination and booster were done recently, they helped to limit the spread of Covid within the examined prisons.

The analysis was conducted by researchers at the University of California San Francisco, who examined the contagion between people living in the same cell.

The study demonstrates the benefits of vaccination and booster shots in reducing transmission, even in settings where many people are still getting infected. And it shows the cumulative effects of the booster and the additional protection the vaccination gives to those who are also previously infected. The probability of transmission is decreased by 11% for each additional dose.

“A lot of the benefits of vaccines in reducing infectivity were related to people who had booster shots or were recently vaccinated,” he explained. Nathan LoUCSF researcher and senior author of the study.

The study of inmates in California prisons

The scientists analyzed deidentified data collected by the California Department of Corrections and Rehabilitation (Cdcr): results of the Covid tests, vaccination status, location of accommodation for 111,687 residents in prisons, 97% of whom were male, between 15 December 2021 and 20 May 2022. Breakthrough infections, i.e. in vaccinated subjects, they were common, despite the relatively high vaccination rate of residents, 81% with primary courses. But the serious illness rate was low.

Infected vaccinated people transmit the virus less

In just over five months, there were 22,334 confirmed Omicron infections, 31 hospitalizations and no Covid deaths. Infected vaccinated people had a significantly lower probability of transmitting the virus: 28% against 36% of the unvaccinated. But the probability of transmission has increased by 6% for every 5 weeks since the last vaccination.

Natural immunity obtained from a previous infection also had a protective effect and the risk of transmitting the virus was 23% for those who had a reinfection compared to 33% for those who had never been infected. Prisoners with ‘hybrid’ immunity, from previous infection and vaccination, were 40% less likely to transmit the virus. Half of this protection came from immunity, which is acquired from fighting infection and the other half from vaccination, the authors estimate.

The researchers said they were gratified to see that the vaccination also confers additional protection on those already infected with Covid, but were surprised by how much the infection has continued to spread, despite the relatively high vaccination rates of prisoners.

High circulation of the virus despite vaccines

“Regardless of the benefits that are seen in vaccination and prior infection, there is still a high amount of transmission in this study,” he said Sophia Tan, a researcher in Lo’s lab and first author of the study. “We hope these findings will support ongoing efforts to protect this vulnerable population.”

This includes making efforts to keep the prison population up to date with boosters and increasing the vaccination rate of prison staff, only 73 percent of whom had received the primary series at the time of the study. The overall booster rate could also be improved, the authors note, given that at the time of the research, only 59% of inmates and 41% of staff had received all the doses recommended by the US Centers for Disease Control and Prevention. diseases (Cdc), based on age and state of health.

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“Within two months after vaccination, people are the least contagious, indicating that booster shots and large Covid vaccination campaigns may play a role in reducing peak transmission,” Lo concluded. “New ideas are needed as the risk of infection in this vulnerable population remains very high.”