How easily does coronavirus spread at home?

Covid-19 spreads easily among people who live together and other family members, even before an infected person shows any symptoms, new research shows. The study - published on 17 June in The Lancet Infectious Diseases journal - also said that the new coronavirus spreads among household members more easily than severe acute respiratory syndrome (SARS) or Middle East respiratory syndrome (MERS).
Those conclusions emerged from an analysis of contract-tracing data from 349 Covid-19 patients and 1 964 of their close contacts in Guangzhou, China.
The analysis found that people with Covid-19 were as infectious before they developed symptoms as during their actual illness.
Close contacts included people in the same household and family members who don't live together, as well as friends and coworkers.
Another key finding: People age 60 and older were most susceptible to Covid-19 infection.
"Our analyses suggest that the infectiousness of individuals with Covid-19 before they have symptoms is high and could substantially increase the difficulty of curbing the ongoing pandemic," said study co-leader Yang Yang, an associate professor in the College of Public Health and Health Professions at the University of Florida.
Researchers said their estimates are the first to quantify symptomless transmission of the new coronavirus.
The findings suggest that halting the chain of transmission within households and families could significantly reduce the number of Covid-19 cases. Doing so will require identifying and isolating infected people and then tracing and quarantining their close contacts, researchers said.
"Active case finding and isolation in conjunction with comprehensive contact tracing and quarantine will be key to preventing infected contacts from spreading the virus during their incubation periods, which will be crucial when easing lockdown restrictions on movement and mixing," Yang said in a journal news release.
Household transmission was believed to be a major factor in Covid-19 infections in China after lockdowns were imposed, but research into spread of the disease in households has been limited.
Understanding risk factors
This study estimated that the secondary attack rate (the probability that an infected person will transmit the disease to someone else) was 2.4% among contacts not in the same household. However, the researchers estimated the rate as 1 in 6 (17%) for people in the same household, and 1 in 8 (12.4%) among family members not living together.
"Family members such as parents and older children may not be living at the same address, which might explain why they appear at less risk of secondary infections than those living in the same household as the Covid-19 case," said study co-author Natalie Dean, an assistant professor of biostatistics at UF.
"While the likelihood of transmitting Covid-19 in households may seem quite low, it is around twice what has been estimated for SARS [4.6%-8%] and three times higher than for MERS [4%-5%], although these data are only based on a small number of studies," Dean said in the release.
The study also found that the risk of household infection is highest among adults 60 and older - an attack rate of 28% (more than 1 in 4) of those living together, and 18.4% (1 in 5) of other family members.
The risk is lowest in those 20 and younger, according to the study. Of those living together, the attack rate is 6.4% (1 in 15), and it's 5.2% (1 in 20) for other family members.
Virginia Pitzer, an associate professor of epidemiology at Yale School of Public Health, wrote an editorial that accompanied the findings.
"This study demonstrates the value of carefully collected contact tracing data to understand risk factors for transmission and susceptibility," she wrote.
"The findings confirm the relative importance of pre-symptomatic transmission and the relationship between older age and susceptibility, key insights which should inform design of intervention strategies."