Single dose of vaccine halves transmission of virus

A single dose of a COVID-19 vaccine can cut household transmission of the novel coronavirus by up to half, a new study in the United Kingdom has found.

Researchers from Public Health England, or PHE, found that people who had received one shot of the Pfizer or AstraZeneca vaccine and who had become infected were between 38 percent and 49 percent less likely to pass on the virus to housemates than infected individuals who had not received a vaccine.

While vaccines have been proven to reduce infection and severe illness, the PHE data provides some of the first evidence that shots can also reduce transmission of the virus.

"This is an extremely encouraging set of findings," said Peter English, a former chairman of the British Medical Association's Public Health Medicine Committee, who was not involved in the study.

English said the "really important" new data suggests that herd immunity may be achievable thanks to COVID-19 vaccines, because they decrease the chance of passing the virus to others.

"The evidence was already mounting that vaccination will prevent people from becoming infected," he added. "This study shows that even if people who are vaccinated do become infected, they are considerably less likely to be infectious, and to pass the infection on to others."

PHE reached its conclusions by comparing how easily unvaccinated individuals spread the virus within their households against similar spread in homes where a vaccinated person had caught the virus. The study included more than 57,000 cases of infection in vaccinated people, and nearly 1 million unvaccinated cases.

Households are particularly high-risk settings for transmission and provide early evidence on the impact of vaccines in preventing onward transmission, the study authors said. The PHE study supports previous evidence that vaccinated people tend to have lower viral loads if they catch the novel coronavirus.

"This is terrific news－we already know vaccines save lives and this study is the most comprehensive real-world data showing they also cut transmission of this deadly virus," said the UK's health secretary, Matt Hancock. "It further reinforces that vaccines are the best way out of this pandemic as they protect you and they may prevent you from unknowingly infecting someone in your household. I urge everybody to get their vaccines as soon as they are eligible."

Mary Ramsay, the head of immunization at PHE, said while the findings are "very encouraging", people who have been vaccinated should still behave responsibly.

"Even if you have been vaccinated, it is really important that you continue to act like you have the virus, practice good hand hygiene and follow social distancing guidance," Ramsay said.