1 in 5 people with COVID-19 pass it to others in household, study finds

Aug. 27 (UPI) -- Nearly 1 in 5 people with COVID-19 pass the virus to other household members, an analysis published Friday by JAMA Network Open found.

About 19% of households in which a person infected with the coronavirus see second person test positive for COVID-19, the data showed.

This rate of transmission, referred to as a secondary attack rate, is higher than that of the flu, which may be as low as 1%, though the most contagious strains can be as high as 15%, research suggests.

The rate of COVID-19 spread within households, however, could be reduced by as much as 50% when all of the residents have been vaccinated, according to the researchers.

"[Our] analysis suggests that the household remains an important site of [COVID-19] transmission," the researchers, from the University of Florida and the University of Washington, wrote. "More transmissible variants may be associated with further changes."

The analysis included data from 87 studies published since the start of the pandemic that collectively included virus transmission information on nearly 1.25 million household contacts of confirmed COVID-19 cases in 37 countries.

It updated a similar report published by the same team of researchers in December, which reported a similar rate of household transmission, with 37 additional studies.

A separate study published earlier this year found that those infected with the coronavirus pass it to about 10% of their close contacts.

Taken together, the 87 studies included in the new analysis reported an average secondary attack rate of 19%, meaning that number of households with one virus cases developed a second one.

However, the secondary attack rate for the virus more than doubled from 13% from January and February 2020, when the pandemic began in China and the virus began to spread globally, to 31% between July 2020 and March of this year.

This increase in household transmission may be due to increased testing identifying new cases and more contagious virus variants, such as the Delta variant, among other factors, according to the researchers.

For example, studies conducted in China and Singapore reported lower household secondary attack rates, perhaps influenced by stricter quarantine requirements, the researchers said.

Across all of the studies in the analysis, household transmission was higher among spouses -- up to 39% -- and in those with chronic health conditions, at up to 50%.

Many of the included studies were conducted before COVID-19 vaccines were available, which may reduce household spread of the virus by up to 50% when all residents have been vaccinated, research suggests.

"Emerging data suggest that vaccination may not only be associated with the prevention of [COVID-19] infections among vaccinated individuals, but may also be associated with reductions in transmission to unvaccinated household contacts," the researchers wrote.