How the coronavirus spreads

The extensive use of epidemiological surveillance, isolation of infected patients and quarantine of exposed individuals in the Chinese city of Shenzhen, in the early months of the COVID-19 outbreak, allowed scientists to estimate important characteristics of this now-pandemic infectious disease, according to a new study.

Researchers from Johns Hopkins Bloomberg School of Public Health analysed data gathered by the Shenzhen Centre for Disease Control and Prevention on 391 people with COVID-19 – what are referred to as ‘index cases’, or the first identified cases in an outbreak – and 1286 of their close contacts, during the 30-day period between 14 January and 12 February.

Shenzhen is a densely populated city of about 13 million people, situated just north of Hong Kong.

The analysis, thought to be the first of its kind in such a well-documented and well-tested set of cases, revealed that the COVID-19 index cases had symptoms for about five days on average before being identified, but that contact tracing and extensive testing reduced this to about three days for infected contacts.

Among the contacts, household contacts were at particularly high risk of infection.

Children, while less likely to develop severe symptoms, were infected at about the same rate as adults.

“Having data on an early set of infected individuals as well as their contacts allowed us to tackle questions about COVID-19 transmission dynamics that had been hard to answer before,” says study co-author Associate Professor Justin Lessler.

The way an emergent pathogen is transmitted from one individual to the next can be difficult for epidemiologists to assess, once community spread has begun, since the potential paths of transmission become too complex to trace reliably.

Early in an outbreak, before community spread has started, epidemiologists can much more easily assess transmission dynamics by finding symptomatic people, and then tracing their recent contacts to see who got infected and who didn’t.

Prompted by reports from Hubei Province, where the COVID-19 outbreak appears to have started in the city of Wuhan in late 2019, the Shenzhen CDC in early January began gathering and testing suspected cases in the city; people who had recently travelled from Hubei, people with fevers in local hospitals, and others identified through community fever screening.

The public health agency also traced all recent close contacts of suspected cases.

Suspected or confirmed cases with symptoms were isolated in local hospitals, and asymptomatic people with test-confirmed exposures were quarantined.

The researchers found that among the 391 confirmed cases of COVID-19 during this period, men (187 cases) and women (204 cases) were about equally represented, though men were about 2.5 times more likely to show severe symptoms.

Children also were about as likely as adults to be infected, though they were less likely to have severe symptoms.

Only about nine per cent of the 391 infected people showed severe symptoms at the time they were first evaluated by doctors.

Among the secondary cases discovered by contact tracing, 20 per cent reported no symptoms at the time they were first evaluated – suggesting that a significant proportion of coronavirus carriers are ‘silent carriers’, at least during the early stage of infection.

The average age of the 391 infected people was about 45.

The sample of 391 people included subsets for whom the timing of key events was known with high confidence, enabling the researchers to estimate key time intervals for COVID-19.

For example, the ‘incubation period’, from exposure to symptom onset, had an estimated median value of 4.8 days.

The median recovery time – the interval from symptom onset to having no symptoms and also testing negative for viral RNA – was 23 days for 60-69 year olds, 22 days for 50–59 year olds, and 19 days for 20–29 year olds.

The ‘attack rate’, or the proportion of close contacts of a confirmed case that are infected, was 6.6 per cent when the researchers assumed that all close contacts were tested and all positive results were recorded.

The attack rate was higher – 11.2 per cent – for household contacts.

The ‘serial interval’, which approximates the interval between one person’s becoming infected and infecting another, had an average value of 6.3 days.

Assoc. Prof. Lessler notes, however, that the research is based only on the known infected contacts.

“We don’t have the full picture,” he explained. “There wasn’t data on all the unknown contacts of each case, such as the people who rode on buses with them or passed them on the street.”

Moreover, the researchers found that some infected individuals accounted for many further infections, implying that such super-spreaders could relatively easily reignite outbreaks.

“Our analysis suggested that about 80 per cent of infections among contacts were caused by just 8.9 per cent of our index cases,” says study co-author Qifang Bi.

These estimates for key COVID-19 intervals and rates will help epidemiologists, pharmaceutical scientists, and public health officials around the world shape their goals and policies to meet the challenge of the pandemic, the researchers say.

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Written by Ben

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