Fully vaccinated people with breakthrough SARS-CoV-2 infection are just as likely as unvaccinated individuals to transmit the virus to their household contacts, new research shows.

Despite it reducing the overall risk of infection, the findings suggest that COVID-19 vaccination alone is insufficient to prevent all transmission, the authors say.

Although the current vaccines had been shown to cut the risk household transmission of the Alpha variant by up to 50%, their efficacy in reducing onward infection for Delta variant cases was previously unclear.

The UK team analysed data from 233 contacts exposed to 163 PCR-positive COVID-19 cases — including 71 individuals with the Delta variant — to assess the impact of vaccination status on the secondary attack rate (SAR).