Nature Medicine: During the first wave of Omicron, vaccination helped limit the spread of COVID-19

Studies in California prisons have found that recent vaccinations and boosters have reduced contagiousness, but the risk of infection remains high

According to researchers at the University of California, San Francisco who analyzed transmission between people living in the same cell, vaccination helped limit the spread
of COVID-19 in California prisons during the first wave of Omicron.

This study demonstrates the benefits of vaccination and booster vaccination in reducing transmission, even in settings where many people are still infected
.
It shows the cumulative effect and additional protection
of vaccination on previously infected people.
With each additional dose, the likelihood of transmission decreases by 11%.

Nathan Lo, M.
D.
, a faculty researcher in the Department of HIV, Infectious Diseases and Global Medicine at UCSF and senior author of the study, said, "Many of the benefits of vaccines in reducing contagiousness come from people who have received boosters as well as people
who have recently been vaccinated.
" The study was published on January 2, 2022 in the journal Nature Medicine
.
"Our findings are particularly relevant
to improving the health of incarcerated populations.
"

The researchers analyzed unidentified data
collected by the California Department of Corrections and Rehabilitation (CDCR).
This includes COVID-19 test results, vaccine status, and housing location for 111687 residents between December 15, 2021 and May 20, 2022, 97% of whom were men
.

Despite a relatively high primary vaccination rate of 81% among residents, breakthrough infections are still common
.
However, the incidence of serious disease is low
.
In just over five months, there were 22,334 confirmed SARS-CoV-2 Omicron infections, 31 hospitalizations, and no COVID-19 deaths
.

Residents who received the breakthrough infection vaccine were significantly less likely to transmit the infection: 28% vs 36% of unvaccinated people
.
But since someone's last vaccination, the likelihood of transmission increases by 6%
every five weeks.

Natural immunity from previous infections also has a protective effect, with a 23% risk of spreading the virus in a reinfected person compared to a 33%
risk of transmitting the virus in a person who has never been infected.

Those who develop mixed immunity through infection and vaccination are 40%
less likely to spread the virus.
Half of the protection comes from immunity gained when fighting infection, and the other half comes from vaccination.

The researchers said they were pleased to see that vaccination provides additional protection even for those already infected, but they were surprised by the extent to which the infection continues to spread, despite the relatively high
vaccination rates among residents.

"Regardless of the benefits you see in vaccination and prior infections, there is still a significant amount of transmission in this study, and we hope these findings will support ongoing efforts
to protect this vulnerable group.
"

This includes efforts to keep residents up to date with vaccinations and increasing vaccination rates for prison staff, with only 73% receiving the main series at the time of the study
.

The general lift rate can also be significantly improved
.
During the study period, only 59 percent of residents and 41 percent of staff received all doses
recommended by the Centers for Disease Control and Prevention (CDC) based on age and health status.

"People were least contagious in the two months following vaccination, suggesting that boosters and large-scale time-to-time vaccination campaigns may play a role
in reducing surging transmission.
" "As vulnerable groups remain at high risk of infection, new incentives are needed
.
"