Recent COVID-19 vaccination was associated with a lower risk of SARS-CoV-2 transmission in a prospective household transmission study conducted from January 2024 to January 2025, published in JAMA Network Open.
Researchers enrolled 362 patients with laboratory-confirmed SARS-CoV-2 infection identified as the first symptomatic case in their household, along with 763 household contacts, through outpatient settings in New York, Tennessee, and Washington as part of the Respiratory Infection: Gauge of Household Transmission study.
Household contacts collected daily nasal swabs for 10 days regardless of symptoms. SARS-CoV-2 infection was detected in 476 contacts, yielding a secondary infection risk of 62%.
Contacts of primary case participants vaccinated within 6 months of symptom onset had a 43% lower adjusted risk of infection compared with contacts of unvaccinated primary case participants. The adjusted relative risk in that group was 0.57.
No statistically significant association was observed when the primary case participant’s most recent vaccination was 7 to 12 months or more than 12 months prior to symptom onset.
Researchers did not identify statistically significant protection against infection based on the household contact’s own vaccination status in this high-exposure setting. Point estimates for vaccine effectiveness against infection were also null or slightly negative across all vaccination categories, with no consistent trend toward protection among vaccinated household contacts.
Vaccination status was categorized by time from the most recent verified COVID-19 vaccine dose to the primary case participant’s symptom onset. The multivariable model accounted for vaccination status of both the primary case participant and household contact, age of both participants, enrollment state, household size, and enrollment period.
Peak enrollment occurred in August 2024, and circulating SARS-CoV-2 lineages varied during the study period.
The observational study design precluded conclusions about causality. Enrollment required health care-seeking and SARS-CoV-2 testing within 6 days of illness onset, which may limit generalizability. The analysis also did not account for multiple household transmission chains or outside exposures.
Confirmed prior SARS-CoV-2 infection was not incorporated into the vaccine effectiveness model, which the researchers noted could bias effectiveness estimates downward.
Anti-nucleocapsid antibodies were detected in 43% of both primary case participants and household contacts at baseline, suggesting substantial prior infection in the cohort. Researchers noted that the findings may therefore reflect the incremental benefit of recent vaccination in a population with high baseline SARS-CoV-2 immunity.
“COVID-19 vaccination may have an indirect benefit of decreasing transmission and thus reducing overall exposure to SARS-CoV-2,” the researchers wrote.
Disclosures: The study was supported by the Centers for Disease Control and Prevention. Several researchers reported grants, consulting fees, or other relationships with the CDC, National Institutes of Health, pharmaceutical companies, or other organizations. The researchers stated that the findings and conclusions do not necessarily represent the official position of the CDC.
Source: JAMA Network Open
