Coronavirus may spread more easily among people living together and family members with the likelihood of household infection highest among older adults aged 60 or more, according to new research published in The Lancet Infectious Diseases journal.

The analysis, based on contact tracing data from 349 people with Covid-19 and 1,964 of their close contacts in Guangzhou (the most populated city in southern China), found people with Covid-19 were at least as infectious before they developed symptoms as during their actual illness.

The study of people living together and family members (not living at the same address), and non-household contacts (eg, friends, co-workers, passengers) suggests that breaking the chain of transmission within households through timely tracing and quarantine of close contacts, in addition to case finding and isolation, could have a huge impact on reducing the number of Covid-19 cases.

While the model has been updated to reflect the current knowledge about the transmission dynamics of Covid-19, the authors caution that it is based on a series of assumptions, for example about the length of incubation and how long symptomatic cases are infectious, that are yet to be confirmed, and might affect the accuracy of the estimates.

Dr Yang Yang from the University of Florida in the USA who co-led the research, said: “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.

“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.”

Household infection is highest among older adults aged 60 or more

The study also modelled the effects of age and sex on the infectivity of Covid-19 cases and susceptibility of their close contacts. For the primary results, researchers assumed an average incubation period of five days and a maximum infectious period of 13 days (including up to five days before illness onset). Among the 349 laboratory-confirmed primary and secondary Covid-19 cases, 19 (5%) reported no symptoms during the follow-up period.

The analyses estimated that the likelihood of secondary transmission—spread from an infected person to non-household contacts—was 2.4%. The likelihood of passing on the virus was higher among people living together and family members, with an attack rate of 17.1% (or around 1 in 6) among people living at the same address, and 12.4% (about 1 in 8) among family members.

The model also suggests that the likelihood of household infection is highest among older adults aged 60 or more (attack rate of 28% or around 1 in 4 of those living together, 18.4% or about 1 in 5 family members), and lowest in those aged 20 years or younger (attack rate 6.4% or around 1 in 15 of those living together, 5.2% or about 1 in 20 family members).

Writing in a linked Comment, Dr Virginia Pitzer (who was not involved in the study) from Yale School of Public Health in the USA, said: “This study demonstrates the value of carefully collected contact tracing data to understand risk factors for transmission and susceptibility. 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.”