Beijing, June 18 : Novel coronavirus that causes Covid-19 may spread more easily among people living together and family members than severe acute respiratory syndrome (SARS) or Middle East respiratory syndrome (MERS), says a new study 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 in China, found people with Covid-19 were at least as infectious before they developed symptoms as during their actual illness, and that older person (aged 60 years or more) were most susceptible to household infection with SARS-CoV-2 (coronavirus). "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 researcher Dr Yang Yang from the University of Florida in the US. 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, 134 secondary/tertiary cases, and 1,964 of their close contacts between January 7 and February 18, the study estimated the secondary attack rate 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 one 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 . Among the 349 laboratory-confirmed primary and secondary Covid-19 cases, 19 (five per cent) 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 per cent. The likelihood of passing on the virus was higher among people living together and family members, with an attack rate of 17.1 per cent among people living at the same address, and 12 per cent among family members. The model also suggests that the likelihood of household infection is highest among older adults aged 60 or more and lowest in those aged 20 years or younger. "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 co-author Dr Natalie Dean from the University of Florida in the US. 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. /IANS