Virus spreads among families without any symptoms showing

The coronavirus spreads more easily among people living together without the infected person displaying symptoms according to a major new study of a Chinese city.

Coronavirus spreads more easily among people living together without the infected person displaying symptoms.

New modelling research, published in The Lancet Infectious Diseases journal, suggests the coronavirus (SARS-CoV-2) that causes Covid-19 may spread more in households than severe acute respiratory syndrome (SARS) or Middle East respiratory syndrome (MERS).

The estimates are the first of their kind to quantify symptomless transmission.

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, and that older people (aged 60 years or more) were most susceptible to household infection with SARS-CoV-2.

The study of people living together and family members (not living at the same address), and non-household contacts like friends and co-workers, 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.

Dr Yang Yang, from the University of Florida 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 transmission of Covid-19 is suspected to have contributed substantially to the rise in cases in China following the introduction of lockdown measures.

But little research has assessed the spread of disease at the household level.

Previous estimates of household infections are specific to the setting where the data were obtained, and represent the proportion of infections among all traced contacts, which does not fully account for the difference in individual exposure history, or the fact that infections may not necessarily be secondary, and could be tertiary—ie, the possibility of transmission among contacts themselves, or infection risks from objects such as clothes, utensils, and furniture.

In the study, researchers developed a transmission model that accounted for individual-level exposure, tertiary transmission, potential exposure to untraced infection sources, and asymptomatic infections.

Using data gathered by the Guangzhou Center for Disease Control and Prevention (CDC) on 215 primary Covid-19 cases (ie, with no known source of exposure, or assumed to have been infected outside Guangzhou), 134 secondary/tertiary cases, and 1,964 of their close contacts between January 7 and February 18, 2020, the study estimated the secondary attack rate (the probability that an infected person transmits the disease to a susceptible individual) among people living together and family members, and non-household contacts.

Close contacts—unprotected individuals who had been within a metre of a person with Covid-19 less than 2 days before their symptoms developed—were traced, quarantined, and tested for SARS-CoV-2 on days 1 and 14.

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 5 days and a maximum infectious period of 13 days (including up to 5 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.