Recent COVID vaccination cuts household transmission by half

Getting vaccinated against COVID-19 could significantly reduce the risk of passing the virus on to people you live with, according to a household transmission study published in JAMA Network Open. Study participants who had received a COVID-19 vaccine within the past six months were nearly half as likely to infect other members of their household, according to findings reported by corresponding author Sarah E. Smith-Jeffcoat, M.P.H., of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention (CDC), and her colleagues.

As part of the CDC's Respiratory Infection: Gauge of Household Transmission (RIGHT) study, the researchers enrolled 362 primary case participants —meaning the first person in each household to test positive — along with 763 of their household contacts across New York, Tennessee and Washington between January 2024 and January 2025. Participants collected daily nasal swabs for 10 days and were tested for the COVID-19 virus regardless of whether they felt sick.

The study found that household contacts of the primary case participants vaccinated within the past six months were significantly less likely to become infected than the household contacts of the primary case participants who were unvaccinated. The adjusted relative risk was 0.57, translating to a vaccine effectiveness against transmission of 43%.

The researchers’ analysis of the data suggests that those vaccinated earlier— between seven and 12 months earlier and more than a year earlier —  were also less likely to transmit the COVID-19 virus, but the difference did not reach the standards used to gauge statistical significance and therefore can’t be said to represent a true difference. Still, Smith-Jeffcoat and her colleagues said in the JAMA Network Open article that the pattern hints that reduced risk of transmitting the COVID-19 virus wanes with time after vaccination, a phenomenon seen with vaccination and protection against severe illness.

In the study, 62.4% of all household contacts tested positive during follow-up, illustrating how efficiently the COVID-19 virus moves through shared living spaces.

Protection appeared to run in one direction only, as household contact vaccination status did not significantly reduce the likelihood of becoming infected. The authors note this may reflect limited statistical power rather than a true absence of effect, pointing to larger cohort studies that have shown meaningful vaccine effectiveness against infection.

Conducted nearly four years into the pandemic, at a time when population immunity from prior infection was high, the study found roughly 43% of participants had detectable COVID-19 anti-nucleocapsid antibodies at baseline. Although the study was not designed to investigate the mechanism by which vaccination reduces transmission, the authors noted that prior immunity boosted by recent vaccination may result in more efficient neutralization of COVID-19, decreasing infectiousness.

The findings may be relevant beyond the household setting, including at healthcare facilities and congregate work or living environments where exposure intensity is comparably high, the authors noted. They explained that one possible way to protect other household members in high-exposure settings is to get a COVID-19 vaccine and that vaccination benefits not only the person getting the shot but also others by decreasing transmission.

Smith-Jeffcoat and her co-authors noted several limitations of their study. Participants had to seek outpatient care and testing soon after symptom onset, which may not represent the broader population. The model did not account for multiple chains of transmission or ongoing infection risk from outside the household. Additionally, unmeasured confounders could influence the estimates. Prior confirmed SARS-CoV-2 infection was also not formally incorporated into the vaccine effectiveness model, which could bias estimates toward lower effectiveness, although the authors noted these estimates may be interpreted as reflecting the incremental benefit of vaccination given high background seroprevalence.

“This cohort study found that at a time when the US population had high levels of SARS-CoV-2 immunity, recent COVID-19 vaccination was associated with a decreased risk of SARS-CoV-2 transmission to others by nearly one-half,” Smith-Jeffcoat and her colleagues wrote. “While the individual benefit of COVID-19 vaccination may vary by age and chronic medical conditions, COVID-19 vaccination may have indirect benefits by decreasing transmission and thus reducing overall exposure to SARS-CoV-2.”