This past summer, the Centers for Disease Control (CDC) funded a study on the transmission potential of vaccinated and unvaccinated persons infected with SARS-CoV-2 at a federal prison in Texas. Specifically, the study focused on a virus outbreak in July 2021. Regular tests and smears were gathered from an established group of prison inmates in the following weeks. The study, titled “Transmission potential of vaccinated and unvaccinated persons infected with the SARS-CoV-2 Delta variant in a federal prison, July—August 2021,” found absolutely no difference between the vaccinated and unvaccinated inmates, concluding:

“As this field continues to develop, clinicians and public health practitioners should consider vaccinated persons who become infected with SARS-CoV-2 to be no less infectious than unvaccinated persons. These findings are critically important, especially in congregate settings where viral transmission can lead to large outbreaks.”

Undoubtedly, the analysis, which occurred during a SARS-CoV-2 Delta variant outbreak, detected no statistically significant difference in the duration of viral culture positivity between fully vaccinated participants (median: 5 days) and those who were not fully vaccinated. 

The study research immediately revealed that, while “highly effective in preventing severe illness and death” from SARS-CoV-2, COVID-19 vaccines are not 100% effective in preventing infection. Indeed, some infections among vaccinated persons are expected to occur. In further explaining the basis for the study, researchers note that the extent to which vaccinated persons who become infected contribute to the spread of the virus is not yet well understood. In fact, additional research reveals comparable COVID-19 virus transmission potential regardless of vaccination status. 

SARS-CoV-2 RT-PCR test positivity (A) and viral culture test positivity (B) stratified by vaccination status and prior infection status for enrolled participants, Federal prison, Texas, July 12— Aug. 9, 2021 Panels illustrate the proportions of specimens for which RT-PCR test results (panel A) or viral culture test results (panel B) were positive, stratified by both vaccination status and history of prior SARS-CoV-2 infection. Solid bars indicate results for participants with no known prior infections, and striped bars indicate results for participants with documented prior infections. Specimens were included as presumptive negative results if no culture was performed but were accompanied by negative RT-PCR results or positive RT-PCR results with Ct>35. Onset was determined to be either a) date of the first onset of self-reported symptom(s) meeting the case definition of COVID-19 or b) date of first positive diagnostic SARS-CoV-2 test, whichever occurred first. Results are depicted only for days 0, 3, 5, 7, and 9 since onset, representing days for which 100% of eligible specimens had viral culture performed. Bar labels indicate the number of specimens collected from participants in each group for each day. P-values are reported at the top of each daily grouping and correspond to Fisher’s exact test of proportions across the four groups.

The Prison Outbreak and Testing

The SARS-CoV-2 outbreak at the federal prison occurred on July 12, 2021, and affected inmate housing units. The CDC and the Federal Bureau of Prisons (BOP) arrived at the prison to investigate the outbreak. Inmates were eligible to enroll in the study if they had tested positive for SARS-CoV-2 between July 12 (the start of the outbreak) and Aug. 4, 2021.

Specimen collection took place from July 18 – Aug. 9, 2021. CDC and BOP staff collected one nasal specimen daily for ten consecutive days from participants who had tested positive, beginning on July 19. Likewise, for cases identified after July 19, data collection started on the date of participants’ first positive test.  

During the study, consenting inmates with confirmed SARS-CoV-2 infection provided mid-turbinate nasal specimens daily for ten consecutive days and reported symptoms through a questionnaire. They were also placed under quarantine precautions. Real-time reverse transcription-polymerase chain reaction (RT-PCR), viral whole-genome sequencing, and viral culture were performed on these nasal specimens. Duration of RT-PCR positivity and viral culture positivity was assessed using survival analysis—the analysis of time-to-event data.

To help in case-finding, the quarantined inmates were tested every other day beginning on July 19 or on their first full day of quarantine. Those who tested positive during quarantine were invited to participate in the ten consecutive days of specimen collection. All participants were asked to supply a specimen on Aug. 6 to provide additional data on viral shedding, “which corresponds to a late timepoint in infection for most participants.”

On the tenth day of specimen collection, participating inmates were asked to complete a written questionnaire to report COVID-19-like symptoms during their illness, including the date of symptom onset and symptom duration. For all participants, information on demographic characteristics, COVID-19 vaccination history, previous positive SARS-CoV-2 diagnostic tests, and underlying medical conditions was pulled from BOP electronic medical records.

RT-PCR Cycle Threshold distributions for enrolled participants with confirmed SARS-CoV-2 infection, Federal prison, Texas, July 12—Aug. 9, 2021 Panels illustrate daily medians and interquartile ranges (IQRs) for reverse transcription-polymerase chain reaction (RT-PCR) cycle threshold (Ct) values among specimens with positive RT-PCR results. Solid lines indicate median Ct values and shaded regions indicate IQRs. Percentages at the top of each panel indicate the proportion of specimens with negative RT-PCR results each day Onset was determined to be either a) date of the first onset of self-reported symptom(s) meeting the case definition of COVID-19 or b) date of first positive diagnostic SARS-CoV-2 test, whichever occurred first. Panel A depicts RT-PCR positivity by vaccination status (not fully vaccinated participants include 2 participants who received only the first dose of a two-dose COVID-19 vaccine series). Panel B depicts positivity by vaccine product among fully vaccinated participants. Panel C depicts positivity according to the time from completion of a COVID-19 vaccine/series to onset. Panel D depicts positivity according to the history of known prior SARS-CoV-2 infection.

Results Show Little Difference in Vaccinated & Unvaccinated

Study results reveal that during the high-transmission COVID-19 outbreak of the Delta variant, researchers “failed to find different durations of RT-PCR positivity, Ct values, or durations of viral culture positivity in fully vaccinated persons compared with persons who were not fully vaccinated.” Additionally, four inmates were hospitalized; three of them were unvaccinated. One unvaccinated person was in intensive care, which included mechanical ventilation. The study noted this patient ultimately died.

The researchers determined that since viral infections in vaccinated persons can result from either a “failure to mount a protective immune response following initial vaccination or a gradual waning of immunological protection following initially robust protection,” the infectiousness of vaccinated persons may be inconsistent. They added that it is conceivable some participants in the evaluation who became infected despite being vaccinated had “weak or waning vaccine-induced protection” and were thus equal to unvaccinated persons in the markers of transmission potential. Explained in further detail in the preprint, the study author’s observed:

“Collectively, our findings suggest that, as evidence continues to emerge in this developing field, vaccinated persons who become infected should be regarded as not significantly less infectious than unvaccinated persons for the purposes of public health action.

In this investigation, we found no statistically significant difference in transmission potential between vaccinated persons and persons who were not fully vaccinated. Our data add to a growing body of evidence characterizing transmission potential from vaccinated persons.