In a recent study posted to medRxiv*, researchers evaluated infectiousness of vaccine breakthrough infections and reinfections during severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron wave.
Background
SARS-CoV-2 transmission dynamics have changed throughout the coronavirus disease 2019 (COVID-19) pandemic due to vaccination coverage and the appearance of novel mutants. In the initial phase of the pandemic, infections were recorded in the susceptible population; however, reinfections and vaccine breakthrough infections are now increasingly prevalent. Data on the infectiousness of reinfections/breakthrough infections with the SARS-CoV-2 Omicron variant are limited.
It is essential to assess the dynamics of the Omicron variant and the impact of prior infection or vaccination, especially in high-risk and vulnerable populations, including incarcerated individuals. The incarcerated population has been disproportionately affected by the COVID-19 pandemic, as SARS-CoV-2 transmission remains high in prisons, partly due to overcrowding, poor ventilation, and introductions from other sources.
The study and findings
In the present study, researchers assessed the infectiousness of SARS-CoV-2 infections in vaccinated or previously-infected individuals (convalescents) relative to non-vaccinated or infection-naïve subjects incarcerated in the United States (US) state prisons. The authors collected records of housing data and SARS-CoV-2 infections from 35 adult institutions in the state prison system of California. Data were analyzed for five months (December 2021 – May 2022).
An index case was a resident with a positive SARS-CoV-2 test. A breakthrough case was defined as a SARS-CoV-2 infection occurring at least 14 days after the first vaccination, besides not testing SARS-CoV-2-positive in the past 90 days. Reinfection was defined as a positive SARS-CoV-2 test in those with a prior infection, provided that the time between two positive tests was more than 90 days.
Close contacts were those who shared the cell with the index case for a minimum of one night when the case was still infectious. Secondary cases were close contacts who tested positive within three days of first exposure or 14 days of last exposure to the index case.
In this period, on average, SARS-CoV-2 testing was performed 8.1 times per resident, with 11.7 mean days between tests. There were 1261 index cases included in the study based on the following criteria: positive diagnostic test and continuous incarceration from April 2020 or earlier, with a valid contact in a closed-door, shared cell.
Close contacts were tested within two days of first exposure and 14 days of last exposure. The mean exposure duration of close contacts with index cases was 2.3 days. Non-vaccinated and vaccinated index cases were matched by institutions and time. On average, 3.6 vaccinated index cases were matched to each non-vaccinated index case.
The unadjusted mean transmission risk to all close contacts was 29%, based on 2.3 mean days of exposure. Vaccinated cases had a 27% risk of transmission to close contacts, while non-vaccinated cases had a 36% risk. Cases with a previous infection had a lower risk (22%) of transmission to close contacts than naïve individuals (30%).
After adjusting vaccination and infection history of close contacts, the authors found that index cases vaccinated with a minimum of one vaccine dose had a 24% lower transmission risk to close contacts than non-vaccinated cases. Further, each additional vaccination (in index cases) was associated with a 12% decrease in the risk of transmission to close contacts.
Similarly, a prior infection had a 22% decrease in transmission risk from index cases. Notably, vaccination and previous infection in an index case were both associated with a 41% reduced risk of transmission. Further, they noted that the time since the last vaccination was inversely related to infectiousness.
However, the relationship between transmission risk and time since vaccination was statistically insignificant. The authors found that primary, breakthrough infections, reinfections, and breakthrough infections in convalescents contributed to 20%, 50%, 7%, and 22% of transmission to secondary cases, respectively.
Conclusions
In summary, prior infection and vaccination decreased the infectiousness of SARS-CoV-2 infections in prisons during the Omicron wave. Prior infection and vaccination were individually associated with a comparable decrease in infectiousness, and additional vaccine doses (boosters) or more recent vaccination caused a more significant reduction in infectiousness.
Notably, both vaccination and infection had an additive effect. Nevertheless, despite prior infection or vaccination, SARS-CoV-2 reinfections and breakthrough infections accounted for 80% of transmission. Vaccination or infection-induced immunity alone might not prevent COVID-19 in high-risk settings like prisons.
*Important notice
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
