Individuals fully vaccinated against COVID-19 who have a breakthrough infection can still pass the infection to household contacts, according to new research published in The Lancet Infectious Diseases.

The UK study aimed to investigate SARS-CoV-2 transmission and viral load in vaccinated and unvaccinated people with mild Delta variant infection. A total of 621 individuals were recruited for the study. Participants were tested daily to detect infection; swab samples were used to analyze viral load. Transmission risk was assessed by vaccination status for household contacts exposed to the index case (first detected case in a household).

Among study participants, 205 household contacts were exposed to a Delta variant index case; 62% (n=126) received 2 vaccine doses, 19% (n=39) received 1 vaccine dose, and 19% (n=40) were unvaccinated.

Continue Reading

Fifty-three household contacts tested positive for COVID-19. Among these individuals, 25% (31/126) were fully vaccinated, 38% (15/40) were unvaccinated, and 18% (7/39) were partially vaccinated (received 1 dose more than 7 days before enrollment).

While vaccinated individuals infected with the Delta variant experienced a faster decline in viral load compared with unvaccinated individuals, the study authors noted that peak viral load was found to be similar between those with breakthrough infections and unvaccinated cases.

“By carrying out repeated and frequent sampling from contacts of COVID-19 cases, we found that vaccinated people can contract and pass on infection within households, including to vaccinated household members,” said Dr Anika Singanayagam, co-lead author of the study. “Our findings provide important insights into the effect of vaccination in the face of new variants, and specifically, why the Delta variant is continuing to cause high COVID-19 case numbers around the world, even in countries with high vaccination rates.”


Singanayagam A, Hakki S, Dunning J, et al. Community transmission and viral load kinetics of the SARS-CoV-2 delta (B.1.617.2) variant in vaccinated and unvaccinated individuals in the UK: a prospective, longitudinal, cohort study. Published online October 28, 2021. The Lancet Infectious Diseases. doi:10.1016/S1473-3099(21)00648-4

This article originally appeared on MPR