Important insights into COVID-19 for worldwide strategic planning

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Researchers in China and the United States have analyzed cases of early severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and their close contacts to provide further insights into the disease’s transmission and severity and the effectiveness of control measures.

The study suggests that contact tracing should be supported and that it may be important to focus on children in terms of intervention, even if children do not become ill.

The team says their results provide an evidence-base for estimating the impact of the virus, assessing control measures, and guiding the worldwide response.

“This study is, to our knowledge, the first analysis of SARS-CoV-2 transmission and COVID-19 natural history based on a large primary dataset of cases and close contacts, for which the mode of surveillance (ie, symptom-based versus contact-based) was sufficiently documented and RT-PCR testing was nearly universal,” say Tiejian Feng (Shenzhen Center for Disease Control and Prevention) and colleagues.

About the pandemic

Since the coronavirus disease 2019 (COVID-19) epidemic emerged in Wuhan city, China, in 2019, it has quickly become a pandemic that has since caused more than 216,000 deaths worldwide.

Part of the scientific community’s rapid response has been to investigate and report on the many aspects of SARS-CoV-2 transmission and its natural history, but important questions remain unanswered.

The fast spread of the virus in Wuhan prompted careful surveillance in the city of Shenzhen, which has provided data that can be used to measure transmission characteristics, disease course, and the effectiveness of control measures and screening.

What has the current study involved?

Between January 14th and February 12th this year, the Shenzhen Center for Disease Control and Prevention identified 391 people with SARS-CoV-2 cases and 1,286 people in close contact with them.

As reported in The Lancet, Feng and colleagues have compared cases that were identified using symptomatic surveillance and contact tracing and estimated the time since symptom onset to key events such as confirmation, isolation, admission to hospital, and recovery. They also estimated transmission metrics and factors that affect the risk of transmission.

“We present one of the first estimates of the serial interval, secondary household attack rate, and dispersion (i.e., a tendency towards super spreading) for SARS-CoV-2 based on active surveillance data,” writes the team.

Study findings

The average age of cases was 45 years, and the number of infections was balanced between men and women.

On first assessment, 256 of 391 (91%) cases had mild or moderate disease, and 3% had severe disease. By February 22nd, three people had died, and 225 had recovered, with recovery taking an average of 21 days.

“This work further supports the understanding of COVID-19 as a disease with a fairly short incubation period (mean 4–6 days) but a long clinical course, with patients taking many weeks to die or recover,” said Feng and team.

Following symptom onset, cases were isolated for a mean of 4.6 days, with this reducing by 1.9 days once contact tracing was introduced.

Household contacts were at six times the risk of becoming infected than other close contacts, and people traveling with a case were at seven times the risk.

The rate of secondary household attacks was 11·2%, meaning fewer than one in six contacts sharing the same household as an infected person became infected. Children were at no less risk of infection than adults, although their symptoms were less likely to be severe. The observed reproductive number, which is the average number of secondary cases arising from one case, was 0·4, and the mean serial interval (time between symptom onset in one case and onset in close contact) was 6·3 days.

Low rates of transmission

“We were able to estimate important transmission parameters directly, and show that, at least among observed contacts, transmission rates are low,” write the researchers. “Estimates of the distribution of time between symptom onset and case isolation by surveillance type reveal that heightened surveillance combined with case isolation could plausibly account for these low rates of transmission.”

However, they add that the overall effect of isolation and contact tracing is not guaranteed and much depends on the number of asymptomatic people.

Moreover, the finding that children are at a similar risk of infection to the general population means they should be included in studies of transmission and control, says the team.

“These findings are important for understanding the burden of COVID-19 and for strategic planning across the world,” concludes Feng and colleagues. “The resulting estimates provide important inputs for interpreting surveillance data, evaluating interventions, and setting public health policy.”

Journal reference:

Feng T, et al. Epidemiology and transmission of COVID-19 in 391 cases and 1286 of their close contacts in Shenzhen, China: a retrospective cohort study. The Lancet 2020. DOI:,

Sally Robertson

Written by

Sally Robertson

Sally first developed an interest in medical communications when she took on the role of Journal Development Editor for BioMed Central (BMC), after having graduated with a degree in biomedical science from Greenwich University.


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