Vaccinated Individuals Can Still Pass COVID-19 in Household Settings


Among those who were infected with the delta variant (126), 25% were fully immunized and 38% were unvaccinated, study results show.

Individuals who are vaccinated with the delta variant of COVID-19 can overcome the infection more quickly than those who are unvaccinated, but the peak viral load is similar to those who are unvaccinated in households, new study results show.

“Our findings show that vaccination alone is not enough to prevent people from being infected with the delta variant and spreading it in household settings,” Ajit Lalvani, a professor at the Imperial College in London, said in a statement. “The ongoing transmission we are seeing between vaccinated people makes it essential for unvaccinated people to get vaccinated to protect themselves from acquiring infection and severe COVID-19, especially as more people will be spending time inside in close proximity during the winter months.”

The risk of transmission based on vaccination status was analyzed for household contacts exposed to the delta variant via a polymerase chain reaction (PCR) test on swab sampled provided daily by each individual for 14 to 20 days. The changes of the amount of virus in an individual’s nose and throat and viral load were estimated by using a modelling PCR data to compare the cases of delta infection between fully vaccinated and unvaccinated individuals.

A total of 205 household contacts of the delta variant were identified, and 53 individuals tested positive for COVID-19. Of those, 62% had received 2 vaccination doses, 19% had received 1 dose, and 19% were unvaccinated.

Among those who were infected with the delta variant (126), 25% were fully vaccinated, and 38% were unvaccinated.

The viral load declined more rapidly for those who were fully vaccinated than those who were unvaccinated. However, investigators noted that vaccinated individuals did not have a lower peak viral load than those who were vaccinated.

Among those who were vaccinated and infected, the median length of time since vaccination was 101 compared with 64 days since vaccination for those who remained uninfected.

The study included 621 individuals who had mild or asymptomatic COVID-19, identified by the United Kingdom contact tracing system, between September 2020 and September 2021.

Demographic and vaccination status was collected on enrollment and the individuals had daily PCR tests to detect infection. Individuals were considered unvaccinated if they had not received a single COVID-19 dose at least 7 days before enrollment; they were partially vaccinated if they had received 1 dose more than 7 days before enrollment; and they were fully vaccinated if they had 2 doses more than 7 days beforehand.

The results were published in The Lancet Infectious Diseases.


The Lancet Infectious Diseases: COVID-19 vaccines lower risk of infection with delta variant, but infection can still be passed on in household settings. EurekAlert. News release. October 28, 2021. Accessed November 1, 2021.

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