Analysis of coronavirus tracing data provides key insights into its spread

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A study published in The Lancet Infectious Diseases describes the analysis of epidemiological data collected in Shenzhen (China) in the early months of the COVID-19 outbreak. Researchers from Johns Hopkins Bloomberg School of Public Health (MD, USA) analyzed data gathered by the Shenzhen Center for Disease Control and Prevention (CDC) on a group of individuals infected with the virus and their close contacts over a 30-day period. Using this information, the team were able to estimate how the coronavirus spreads.  

It can be difficult to assess how an emergent pathogen is transmitted from one individual to the next once community spread has begun. However, early in an outbreak, epidemiologists can much more easily assess transmission dynamics of a virus by identifying individual cases and performing contact tracing to monitor its spread.

Following reports from Hubei Province, the suspected origin of the outbreak, the Shenzhen CDC began gathering and testing suspected cases in the city, along with recent close contacts. Suspected or confirmed cases with symptoms were isolated, while asymptomatic individuals with test-confirmed exposures were quarantined.

The data was collected from 14 January – 12 February, based on 391 people with COVID-19, the first identified cases, known as ‘index cases,’ and 1286 of their close contacts.

“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,” commented study co-senior author Justin Lessler (Bloomberg School of Public Health).

The analysis revealed that men (187 cases) and women (204 cases) were about equally represented, though men were approximately 2.5 times more likely to display severe symptoms. Children, while less likely to develop severe symptoms, had a similar rate of infection to adults.

Approximately 9% of the infected cohort displayed severe symptoms at the time of medical evaluation, while 20% of the secondary cases discovered through contact tracing reported no symptoms when they were first evaluated, suggesting that a significant proportion of coronavirus carriers are ‘silent carriers.’

The median time for the incubation period – from exposure to symptom onset – was 4.8 days, while the median recovery time – from symptom onset to having to symptoms and testing negative for viral RNA – was 23 days for 60-69 year olds, 22 days for 50-59 years and 19 days for 20-29 years.

Household contacts were at particularly high risk of infection, with an ‘attack rate’ – the proportion of close contacts that are infected – of 11.2%, compared with 6.6% for contacts outside the household. Researchers also observed an average of 6.3 days between one person’s becoming infected and infecting another.

They calculated an observed reproduction number of 0.4, however this was attributed to Shenzhen CDC’s efforts to contact trace and isolate, along with the potential for unknown contacts of each case.

Further, the researchers discovered that some infected individuals accounted for many further infections. “Our analysis suggested that about 80 percent of infections among contacts were caused by just 8.9 percent of our index cases,” reported co-first author Qifang Bi (Bloomberg School of Public Health). This implies that such superspreaders could spark further outbreaks.

The researchers anticipate that their analysis will help epidemiologists, pharmaceutical scientists and public health officials as they face the challenge of the pandemic.

Sources: Bi Q, Wu Y, Mei S et al. Epidemiology and transmission of COVID-19 in 391 cases and 1286 of their close contacts in Shenzhen, China: a retrospective cohort study. Lancet Infect. Dis. doi:10.1016/S1473-3099(20)30287-5 (2020); www.jhsph.edu/news/news-releases/2020/study-from-chinese-city-of-shenzhen-outside-hong-kong-in-southern-china-provides-key-insights-on-how-coronavirus-spreads.html

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