The coronavirus SARS-CoV-2 that causes covid-19 can more easily spread among people living in the same household and members of the same family who do not live in the same direction that the coronavirus that causes severe acute respiratory syndrome (SARS) or respiratory Middle East syndrome (MERS). The study, published in “The Lancet Infectious Diseases”, is the first to quantify the transmission of SARS-CoV-2 in people without symptoms.
The analysis was based on data from the tracking of 349 people with covid-19 and 1.964 of their closest contacts (friends, coworkers, passengers on a train or bus) in the city of Guangzhou (the largest city in southern of China) and has been seen that people with covid-19 were equally infectious before symptoms develop as during their illness. In addition, it confirms that people over the age of 60 years were more susceptible to the infection home with SARS-CoV-2.
The authors of the study assert that their data suggest the importance of breaking the chain of transmission within households through tracking and timely quarantine of close contacts, in addition to case finding and isolation. All of this, they write could have a large impact on the reduction of the number of cases of covid-19.
And although they note their model has been updated to reflect the current knowledge on the transmission dynamics of covid-19, the authors warn that it is based on a series of assumptions, for example, over the period of incubation and the time in which a person is symptomatic, has the ability to infect.
The data suggest the importance of breaking the chain of transmission within households through tracking and timely quarantine of close contacts, in addition to case finding and isolation
“Our analyses suggest that the ability of infecting of asymptomatic persons with covid-19 is high and could increase the difficulty of curbing the pandemic,” says Yang Yang, of the University of Florida (USA). That’s why, he adds, “the active search of cases and isolation along with the tracking integral of contacts, and the quarantine will be key to avoid infected contacts to propagate the virus during its incubation period, which will be crucial to ease the restrictions on movement and measures of confinement”.
It is suspected that the transmission home of covid-19 has contributed to the increase in cases in China after the introduction of containment measures. However, there has been little research that has evaluated the spread of the disease to the family level.
In this study, the researchers have developed a transmission model that accounted for exposure at the individual level, the transmission to third parties, the potential exposure to sources of infection not traced, and the asymptomatic infections. Using data collected by the Centre of Guangzhou for the Control and Prevention of Diseases (CDC) of 215 cases primary covid-19 -that is to say, without a known source of exposure, or assumed to be infected outside of Guangzhou-, 134 cases to secondary / tertiary and 1.964 of your close contacts between the 7 January and 18 February 2020, the work estimated the rate of secondary infection (the probability that an infected person transmits the disease to an individual susceptible) between people who live together and the members of the family, and contacts who do not live at the same address.
The study also assessed the effects of sex and age on the infectivity of cases of covid-19 and the susceptibility of their close contacts
Close contacts (individuals without the protection that had been within one meter of a person with covid-19 less than 2 days before the occurrence of its symptoms) also were tracked, quarantined and tested for SARS-CoV-2.
The study also assessed the effects of sex and age on the infectivity of cases of covid-19 and the susceptibility of their contacts nearby. The researchers took a period of incubation media of 5 days and a period of infectious maximum of 13 days (even up to 5 days before the onset of the disease). Among the 349 cases of covid-19 primary and secondary laboratory-confirmed, 19 (5%) did not report symptoms during the follow-up period.
The analysis estimated that the probability of secondary transmission (spread from an infected person to contact non-household) was 2.4%. The likelihood of transmitting the virus was greatest among people who live together and the members of the family, with a rate of spread of 17.1 per cent (or about 1 of every 6) between the people who live at the same address, and 12.4% (approximately 1 out of every 8) between the family members.
“The members of a family, parents and older children may not be living at the same address, which might explain why they seem to have less risk of secondary infections than those who live in the same household as the case covid-19,” says Natalie Dean, co-author of the study. “Although the likelihood of transmitting covid-19 in households, it may seem quite low, is about twice what has been estimated for the SARS (4.6-8%) and three times higher than for the MERS (4-5%)” .
The model also suggests that the probability of infection in the home is highest among those older than 60 years (28% or about 1 of every 4 of those living together, 18.4 per cent, or 1 of every 5 members of the family), and lowest in those under 20 years of age (by 6.4% or about 1 of every 15 of those living together, 5.2 percent, or approximately 1 out of every 20 members of the family).
The estimates also suggest an ability to infect important during the period of incubation, which is comparable to and potentially higher than during the period of the disease.
If the reproductive number local continues to be less than one, the infection cannot spread effectively
The high capacity to infect the virus during the incubation period suggests that the quarantine of contacts asymptomatic could have been avoided with more streams later
The researchers reproductive number local -the average number of infections that a case covid-19 can generate during the entire period infectious through people who are living together and family members and contacts are not family – was 0.5. However, if the isolation of cases or quarantine of their contacts are not hubietambién calculated that elran implemented, the reproductive number local estimated would have been a 20-50% higher, increasing to 0.6-0,76. If the reproductive number local continues to be less than one, the infection cannot spread effectively, explained.
“Although the effect of the isolation of cases appears to be moderate, the high ability to infect the virus during the incubation period suggests that the quarantine of contacts asymptomatic could have been avoided with more transmissions later,” says Qin-Long Jing from the Guangzhou CDC
The authors acknowledge some important limitations: the fact that they were not able to quantify reliably the ability to infect the asymptomatic people or that the model assumed that the asymptomatic infections have the same infectivity that symptomatic cases during their incubation period, which may not be accurate.
Also note that some cases primary imported could have been infected locally, and that some infections or asymptomatic cases may have been missed by the tracing of contacts, or by tests, false-negative, which could underestimate the rate of secondary attack. Finally, the rapid isolation of cases and quarantine of their close contacts would have limited the number of transmissions when the cases were sick and affected the precision of the estimates.
In a comment, Virginia Pitzerm, who was not involved in the study, of the School of Public Health, Yale (U.S.), points out that the “key difference between SARS-CoV-2 and SARS-CoV is the probability of transmission is substantially greater during the incubation period presymptomatic SARS-CoV-2, whereas there was little or no transmission before the onset of symptoms of SARS-CoV. This made the SARS-CoV were a lot easier to control through the isolation of cases and quarantine of contacts.” According to this report, the isolation immediately of cases could only prevent 20-50% of secondary cases of COVID-19 in Guangzhou.
And he concludes: “The findings confirm the relative importance of the transmission pre-symptomatic and the relationship between age and susceptibility, key ideas that should serve to design strategies of intervention”. .