Study population and setting
Beginning on July 12, 2021, 18 people incarcerated in federal prison in Texas reported COVID-19 symptoms and received positive rapid antigen tests for SARS-CoV-2 infection. Following this, between July 12 and August 14, 2021, antigen testing was conducted on 233 people in the two interconnected units. Testing was conducted on July 12 to 13, July 14, July 19, July 22, August 2, and August 10 with a combination of rapid antigen and reverse transcriptase-polymerase chain reaction (RT-PCR) testing. Testing was also offered to staff members on a voluntary basis. A subset of 70 people who were incarcerated in the units consented to a secondary investigation with daily symptom reports and nasal swabs for up to 20 days after symptom onset. Viral culture was performed for RT-PCR-positive specimens among a subset of participants, and genomic sequencing was conducted for one specimen from each participant. Staff members were vaccinated only voluntarily and vaccination received outside of the correctional clinic was not recorded by the Federal Bureau of Prisons (BOP). BOP also collected vaccination status, demographic characteristics, and medical condition history from the electronic medical records (EMRs) of the 233 incarcerated people. Descriptive statistics were calculated, including the attack rate.
Summary of Main Findings
Of the 233 people living in the unit, 185 (79%) were fully vaccinated, 42 were unvaccinated (18%) and 6 (3%) were partially vaccinated at the time of data collection. Overall, there were 172 cases, with an attack rate of 74%. By vaccination status, of the 185 vaccinated people, the attack rate was 70% (129 cases), and for the 42 unvaccinated people, the attack rate was 93% (39 cases), which was a statistically significant difference in the attack rates. Four (2%) of these cases were hospitalized, with 3 of them unvaccinated and 1 fully vaccinated. Of those fully vaccinated with Moderna (N=50), the attack rate was 40% (20 cases), compared to 122 people vaccinated with Pfizer, who had an attack rate of 81% (99 cases), and 13 people with the Janssen vaccine who had an attack rate of 77% (10 cases), though it was not clear if these related to specific differences in vaccination or timing since vaccination, where Pfizer was offered first to participantsl. There was a statistically significant difference between these attack rates as well. Also, comparing the times since their full vaccination to the outbreak, among the fully vaccinated, the attack rate for those vaccinated between 2 weeks and 2 months ago was 61% (19 cases among 31 people), while for those vaccinated 4 to 6 months ago, the attack rate was 89% (83 cases among 93 individuals). Of the 275 staff members, 9 reported a positive SARS-CoV-2 test (3%). Genomic sequencing identified the Delta variant in 58 specimens from 58 people sequenced.
The study had electronic medical records for all incarcerated people, which allowed for ascertainment of not only vaccination status, but the date and product used, as well as underlying conditions. This granularity adds strength to the study in assessing the different factors potentially related to the attack rates. Their testing strategy across multiple timepoints also likely allowed them to identify many newer infections soon after those individuals became infected, which aided in correctly classifying new cases and determining time since vaccination and symptoms.
The study used a mix of rapid antigen tests and RT-PCR tests, which have varying sensitivities and specificities, as well as turnaround times. Therefore, ascertainment of a case may be delayed for an RT-PCR test or subject to a false negative on the rapid antigen test especially for asymptomatic cases, and it is not clear how this potential misclassification may have impacted the attack rate. One limitation was the lack of consistent testing for the staff. Staff members had to self-report tests outside of the correctional setting to the BOP, which likely reduced the number of tests actually reported. Additionally, it is not clear that the differences between vaccine product reflect actual differences in their effectiveness—particularly because individuals vaccinated greater than 4 months ago had a higher attack rate, and most of those vaccinated at that point were done so with the Pfizer vaccine because it was the first available in the correctional setting. Therefore, there may be confounding in the estimated difference in attack rate between these products.
This is one of the first studies examining the risk of infection among vaccinated people in prison since the proliferation of the Delta variant.
This review was posted on: 9 November 2021