One dose of Pfizer or AstraZeneca vaccine cuts COVID-19 spread by up to 50% within a household, a study found

• One dose of a Pfizer of AstraZeneca vaccine cut household transmission by up to 50% in a study.
• Protection kicked in from 14 days after vaccination regardless of age, Public Health England said.
• It remains unclear how long protection from one dose lasts for.
• See more stories on Insider’s business page.

A single dose of the Pfizer or AstraZeneca COVID-19 vaccine cut the spread of coronavirus within a household by up to 50% in a new study.

People who had one dose of either vaccine were between 40% and 50% less likely to pass on the virus to people in their household, 21 days after the shot, compared to those who weren’t vaccinated at all, the study authors from Public Health England said in a preprint paper posted Wednesday. 

Protection kicked in at around 14 days after vaccination, with similar levels of protection regardless of age, PHE said in a press release.

Both vaccines — one made by Pfizer and German partner BioNTech, the other by AstraZeneca and the University of Oxford — are two-dose vaccines.

The virus is highly likely to spread from person to person within a household.  PHE said it expected similar results in other high-risk settings, such as shared accommodations and prisons.

“Not only do vaccines reduce the severity of illness and prevent hundreds of deaths every day, we now see they also have an additional impact on reducing the chance of passing COVID-19 on to others,” Mary Ramsay, head of PHE, said in a statement.

The study has not yet been peer-reviewed by experts.

So far, more than 33 million Brits have had one vaccine dose, according to government figures. In the US, more than 141 million people have received a single COVID-19 shot, according to the Centers for Disease Control and Prevention (CDC).

Pfizer’s vaccine is authorized in the US, but AstraZeneca’s is not.

Read more: COVID-19 Vaccine Tracker: AstraZeneca’s shot proves safe and effective, and is headed to the FDA

The researchers looked at 57,000 contacts of people who had a COVID-19 vaccine, and compared them to a group of nearly 1 million contacts for unvaccinated people.

Given the researchers looked at lab-confirmed cases only, it was not possible to conclude from the research how well vaccines stop people spreading asymptomatic COVID-19. 

Dr. Peter English, a retired consultant in communicable disease control, said in a statement that the authors may have actually underestimated the vaccines’ effectiveness in preventing transmission, because the vaccinated person’s contacts could have caught COVID-19 from someone else.

“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,” he said.

A UK-based study reported by Insider Thursday showed a single dose of either Pfizer or AstraZeneca’s vaccine reduced symptomatic and asymptomatic coronavirus infections overall by more than 65%. 

Deborah Dunn-Walters, British Society for Immunology COVID-19 Taskforce chair and professor of immunology at the University of Surrey, said in a statement that the study was welcomed, but there was still much to learn about how COVID-19 vaccines affect transmission. 

“It is still very important for us all to get two doses of the COVID vaccine to ensure we receive the optimal and longest lasting protection, both for ourselves and our communities,” she said. 

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