Single COVID-19 vaccine dose can reduce household transmission by up to half: researchers

A single dose of the COVID-19 vaccine can limit household transmission by up to half, according to a study by Public Health England released on Wednesday, signaling the effectiveness of the vaccines at preventing spread. 

The research, which has yet to be peer-reviewed, found that people who got COVID-19 three weeks after receiving one dose of either the Pfizer-BioNTech or AstraZeneca vaccines were between 38 percent and 49 percent less likely to give the virus to those in their household, compared to those who didn’t receive any vaccine dose.

Public Health England analyzed more than 57,000 contacts from 24,000 households in which a person who had received a vaccine dose tested positive for the virus and compared it with almost 1 million contacts of cases among unvaccinated people. 

Cases among household contacts were considered secondary cases if they received a positive COVID-19 test two to 14 days after the initial case. 

The researchers found protection was present about 14 days after vaccination and remained steady despite the different ages of cases or contact. The measured protection is in addition to the 60 percent to 65 percent decreased risk of the vaccinated person developing symptomatic COVID-19 four weeks after one dose. 

Researchers said the results would likely be similar in other high-risk transmission locations besides households, such as shared accommodations and prisons.

British Health Secretary Matt Hancock called the study “terrific news,” saying, “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.”

“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,” he added in a statement.

A vast majority of the participants were under 60 years old, although Public Health England noted other research has shown the two vaccines reduce infections and deaths among the older population. Previous analysis concluded that 10,400 deaths among older adults were prevented by the end of March.

The researchers plan to next examine how two doses of the COVID-19 vaccine can contribute to the reduced risk of transmission in households. Both the Pfizer-BioNTech and AstraZeneca vaccines require two doses.

But Mary Ramsay, head of immunization at Public Health England, still encouraged caution, saying “even if you have been vaccinated, it is really important that you continue to act like you have the virus, practise good hand hygiene and follow social distancing guidance.”

Vaccine recipients are still able to test positive for COVID-19, even after being fully vaccinated, because vaccines are not 100 percent effective. But the vaccines have been found to decrease the risk of severe illness and death from COVID-19.