COVID booster shots cut Omicron transmission in closed environments

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In a recent paper published in the journal Nature Medicine, researchers in the United States investigated the infectiousness of incarcerated individuals with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron infections in prisons, high-risk settings where transmission is intense.

Study: Infectiousness of SARS-CoV-2 breakthrough infections and reinfections during the Omicron wave. Image Credit: p.ill.i / ShutterstockStudy: Infectiousness of SARS-CoV-2 breakthrough infections and reinfections during the Omicron wave. Image Credit: p.ill.i / Shutterstock


Despite high coronavirus disease 2019 (COVID-19) vaccination rates among residents, SARS-CoV-2 transmits at a higher rate in prisons, partly due to overcrowding, poor ventilation, and viral entrance from community sources. Consequently, there is an urgent need to understand how widespread vaccination, including booster doses and even prior infection, shifted the course of SARS-CoV-2 transmission dynamics and led to the advent of novel and highly transmissible SARS-CoV-2 variants, such as Omicron.

Also, the data on the infectiousness of breakthrough infections and reinfections by SARS-CoV-2 Omicron in vaccinated and previously infected individuals remains limited, particularly in prisons. In other words, studies still need to capture the extensive effects of the transmission environment while accounting for interaction with prior SARS-CoV-2 infection.

About the study

In the present study, researchers analyzed SARS-CoV-2 surveillance data from 35 prisons in California, United States of America (USA), between December 2021 and May 2022, a period that corresponded to the first five months of the COVID-19 wave when Omicron subvariants BA.1/BA.2 were predominant.

They assessed the infectiousness of Omicron-infected confirmed index cases, which covered vaccinated individuals and those with prior SARS-CoV-2 infection who shared a closed cell partitioned by walls. The researchers compared these results with other unvaccinated and uninfected individuals incarcerated in any participating California state prison in the USA. In addition, they stratified the results by each prisoner's vaccination status and COVID-19 history.

The team matched 273 unvaccinated index cases and 953 vaccinated index cases by the institution within 30 days and by a propensity score for receipt of vaccination. Further, they matched a mean of 3.5 vaccinated index cases to every unvaccinated index case. Overall, the balance was good across matched index cases.

Study findings

During the five-month study period, in a study population of 111,687 (97% males), there were 22,334 confirmed COVID-19 cases and 31 COVID-19-related hospitalizations. All participating California state prisons tested their residents for COVID-19 on average 8.1 times over the five-month study duration, with the average time between tests being 11.7 days.

All those serving continuous incarceration since 1 April 2020, having close contact in a shared, closed-door cell, and not testing COVID-19-positive in the preceding 90 days when tested COVID-19-positive constituted an index case. The researchers identified 1,226 index cases over the study period.

The mean duration of exposure of close contacts to index cases for vaccinated and unvaccinated index cases was 2.2 days and 2.4 days, respectively. However, the mean duration from a close contact's first exposure to testing was 6.2 days, irrespective of whether exposure occurred from a vaccinated or an unvaccinated index case. Likewise, the mean duration of the follow-up COVID-19 testing in close contacts was 10 and 10.6 days after first exposure for unvaccinated and vaccinated index cases, respectively. Vaccination of index cases did not change the dispersal of secondary cases from the exposure time. Thus, it was comparable between the two cohorts, 6.7 versus 5.7 days.

In adjustment analyses, the Poisson regression model estimated that index cases vaccinated with ≥1 COVID-19 vaccine dose had, on average, a 22% lower risk of transmitting infection than unvaccinated index cases. Each additional dose further reduced the risk of transmission to close contact by an average of 11%.

A previous infection by SARS-CoV-2 lowered the risk of transmission from the index case by 23%, and having vaccination and SARS-CoV-2 infection together reduced this risk by 40%, based on a linear combination of regression coefficients. A more recent vaccination also led to more significant reductions in infectiousness. However, the researchers could not identify any interaction between vaccination and prior natural infection associated with transmission risk; thus, perhaps both exerted their effects independently.


Although reductions in transmission risk associated with vaccination and prior infection were additive and vaccination reduced infectiousness even in individuals with immunity acquired by prior infection, both could not eliminate the risk of SARS-CoV-2 transmission, especially in higher-risk settings, such as prisons.

Nevertheless, the study demonstrated how each dose of the vaccine, especially boosters provided an additional relative reduction in infectiousness. In addition, the indirect effects of COVID-19 vaccination slowed down SARS-CoV-2 transmission, which, combined with its direct benefits, highlights the importance of COVID-19 vaccination.

Together, these findings could inform public health policy particularly relevant to populations in high-density congregate human habitations, such as prisons. Given the importance of the timeliness of vaccination, the study also raised the possibility of timed mass vaccination in prisons during COVID-19 surges to slow down viral transmission. Other measures, such as depopulating and improving ventilation in prisons, could also protect incarcerated populations.

Journal reference:
Neha Mathur

Written by

Neha Mathur

Neha is a digital marketing professional based in Gurugram, India. She has a Master’s degree from the University of Rajasthan with a specialization in Biotechnology in 2008. She has experience in pre-clinical research as part of her research project in The Department of Toxicology at the prestigious Central Drug Research Institute (CDRI), Lucknow, India. She also holds a certification in C++ programming.


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