Covid spreads easily within families, symptomless spread common: Study

nNEW DELHI: The coronavirus disease (Covid-19) spreads easily among people living together even when an infected person is presymptomatic or asymptomatic, according to the first global estimates quantifying symptomless transmission of the infectious disease within homes.

People with Covid-19 are as infectious before they develop symptoms as later in their illness, with people who are 60 years older being most susceptible to infection, showed the analysis published in The Lancet Infectious Diseases journal.

The study said breaking the chain of transmission within households through timely tracing and quarantine of close contacts could reduce new infection during the incubation period.

The analysis was based on contact-tracing data from 349 people with Covid-19 and 1,964 of their close contacts in Guangzhou, China, with a population of 13 million. It included those living together, family not living at the same address, and non-household contacts like co-workers, friends and fellow commuters.

“The study confirms what we already know, which is that household and family clusters are the primary source of infection. The more virus you are exposed to, the more you are likely to get infected, and in highdensity housing, where large families live together, infected respiratory droplets can stay suspended in air and infect others,” said Dr V Ramana Dhara, additional professor, Indian Institute of Public Health, Hyderabad.

“What is significant is the establishment that incubation period is infectious, which means everybody needs to wear a mask all the time. Most people take off their masks inside homes because they feel safe and protected, but given the density of population in India, indoor containment will be a challenge,” said professor Dhara.

To study the household secondary attack rate (the probability of an infected person transmitting the disease to another) of Covid-19, researchers developed a transmission model for the study that accounted for individual level exposure, tertiary transmission, potential exposure to untraced infection sources, and asymptomatic infections.

Data gathered by the Guangzhou Center for Disease Control and Prevention on 215 primary Covid-19 cases, 134 secondary/tertiary cases, and 1,964 of their close contacts between January 7 and February 18 was used to estimate the secondary attack rate among people living together, family members not living together, and non-household contacts.

Among the 349 laboratory confirmed primary and secondary Covid-19 cases, 5% reported no symptoms during the follow-up period. The analyses estimated that the likelihood of secondary transmission (spread from to non-household contacts) was 2.4%, compared to the attack rate of 17.1% (around one in six) among people living at the same address, and 12.4% (about one in eight) among family members not living at the same address.

There is substantial infectivity during the incubation period, comparable to, and potentially higher than, during the illness period.

After one day of exposure (daily infection probability), family members were 39% less likely to become infected after symptoms emerged than during the incubation period, while those living together had 41% lower odds, although the difference was not statistically significant.

The high infectivity of the virus during the incubation period suggests that quarantining asymptomatic contacts may help preventing transmission. “The cornerstone of the response in any transmission scenario continues to be to find, isolate, test and care, and to trace and quarantine contact. That is the best defence against Covid-19,” said Dr Poonam Khetrapal Singh, regional director, World Health Organization, South East Asia Regional Office.

The risk of household infection was the highest among adults aged 60 years or more, who had an attack rate of 28% (about one in four) in those living together, 18.4% for family members not living together, according to the analysis. The chance of getting infected was the lowest in those aged 20 years or younger, where the attack rate was 6.4% for those living together and 5.2% for family not living in the same home.

The authors caution that the model is based on a series of assumptions, such as current knowledge about the transmission dynamics of Covid-19, about the length of incubation and how long symptomatic cases are infectious, changes in which may affect accuracy of the estimates.

In a comment in The Lancet, Dr Virginia Pitzer, associate professor of epidemiology, Yale School of Public Health, the US, who is not linked with the research, said: “The probability of transmission is substantially higher during the presymptomatic incubation period for Sarscov-2, , whereas little to no transmission occurred prior to onset of symptoms for Sars-cov (virus that causes severe acute respiratory syndrome, or Sars). Notably, authors estimate that prompt case isolation was only able to prevent 20-50% of secondary cases of Covid-19 in Guangzhou.”

AN ANALYSIS SAYS BREAKING THE CHAIN OF TRANSMISSION WITHIN HOUSEHOLDS THROUGH TIMELY TRACING AND QUARANTINE OF CLOSE CONTACTS COULD REDUCE NEW CASES