This work, carried out in Californian prisons
and which examined transmission between people living in the same cell, indeed demonstrate that COVID-19 vaccines as well as one or more past infections might have had a direct impact on the transmission of Omicron. The study thus describes these advantages of vaccination and booster, even in settings where many people are still infectedto reduce transmission.
The cumulative effects of vaccinations, boosters and previous infections
The study analyzed anonymized data from the California Department of Corrections and Rehabilitation including COVID-19 test results, vaccination status and whereregardings of 111,687 residents, 97% of whom were male, between December 15, 2021 and May 20 2022. Analysis reveals that -here in prisons-:
- breakthrough infections (infection following vaccination) are common, despite a relatively high vaccination rate (2-dose regimen) of 81%;
- the rate of severe forms of COVID remains low: over a follow-up period of 5 months, 22,334 infections by SARS-CoV-2 Omicron were identified, resulting in 31 hospitalizations and no deaths by COVID-19;
- vaccinated residents who developed breakthrough infections are significantly less likely to transmit the virus: 28% vs 36% for unvaccinated residents;
- the risk of transmission increases in a dose-dependent manner with the time since the last vaccination: specifically, this risk of transmission increases by 6% for every 5 weeks since the last injection;
- the natural immunity conferred by a past infection also provides a protective effect, and the risk of transmission of the virus is estimated at 23% for a person who has developed a reinfection vs 33% for a person who has never developed an infection: this also suggests an inverse dose-dependent correlation between the number of past infections and the risk of transmission;
- hybrid immunity, linked to both infection(s) and vaccination, is associated with a 40% reduction in the risk of transmission; natural immunity and vaccine immunity appear to have a similar impact in this reduction in the transmission rate (50%/50%).
Vaccination, an essential protection once morest transmission: the study also confirms, like previous research, the additional protection that vaccination confers on previously infected people. It will thus be noted that:
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the risk of transmission is reduced by 11% for each additional dose of vaccine.
“These results are particularly relevant for improving the health of the prison population, but not only”, comments lead author Dr. Nathan Lo, MD, PhD, a research scientist in the Division of HIV and Infectious Diseases at UCSF. Although the researchers point out that despite this transmission-reducing effect of vaccination, there is still – at least in this study – strong transmission of the virus.
“People are the least contagious in the two months following vaccination, which calls for not weakening on the importance and frequency of reminders”.
