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.
Conclusions:
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.
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