Image of Ajit Lalvani, DM; Image credit: Imperial College London
Ajit Lalvani, DM; Image credit: Imperial College London

People who are fully vaccinated against COVID-19 are just as infectious as their unvaccinated counterparts, albeit for a shorter period of time, finds an unusual new household study from the United Kingdom. 

The study enrolled 621 participants with mild or asymptomatic COVID-19 caused by the highly contagious delta variant. They were identified using the UK’s contact tracing system between September 2020 and September 2021. All were tested daily for detectable infection using PCR tests, whether symptomatic or not.

Two doses of a COVID-19 vaccine have been shown to lessen the risk of contracting COVID-19, and fully vaccinated people are known to clear the virus more quickly. But the current study found that, at the peak of infection, vaccinated participants carry a viral load that is similar to that of unvaccinated individuals.

This may explain why vaccinated participants easily transmitted the delta variant of the SARS-CoV-2 virus in their households, including to other fully vaccinated contacts, according to study lead Ajit Lalvani, DM, of Imperial College London. The findings may also explain why the delta variant is causing high case numbers worldwide even in population with high vaccination rates, he and his colleagues explained.

Protecting vulnerable groups

Although vaccines remain highly effective at preventing severe disease and deaths from COVID-19, the results suggest that vaccination is “not sufficient to prevent transmission of the delta variant in household settings with prolonged exposures,” the researchers wrote. It is advisable for people at risk of disease progression and severe illness to take additional precautions, such as getting a booster shot, they added.

“Our analysis suggests that direct protection of individuals at risk of severe outcomes, via vaccination and non-pharmacological interventions, will remain central to containing the burden of disease caused by the delta variant,” they concluded.

The study was published in the Lancet.