HKUMed researchers have discovered that the serial interval of SARS-CoV-2 can be shortened substantially over time by non-pharmaceutical interventions. Conducted by scholars at the WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, LKS Faculty of Medicine at The University of Hong Kong (HKUMed) and collaborators from the University of Cambridge, the United Kingdom; Institut Pasteur, Paris, France; Dalian Minzu University, China; University of Texas at Austin, The United States; and Beijing Normal University, China, the research team evaluated the serial intervals (i.e. the time between illness onset in successive cases in a transmission chain) of COVID-19 by compiling a line-list database of transmission pairs in mainland China. They concluded that the serial interval distribution of COVID-19 was substantially shortened in response to interventions, in particular isolation of cases. Identifying these changes over time can offer a better measure for evaluating the effectiveness of control measures. The findings are now published in the journal Science.

The virus which causes COVID-19 spreads from one person to another, and the “serial interval distribution” is defined as the length of time between the symptom onset of consecutive cases in these chains of transmission. For example, if person A infects person B, the serial interval is the time between symptom onset of person A and symptom onset of person B. The distribution of serial intervals is important for estimating many other key epidemiological parameters (e.g. the reproduction number Rt), which are in turn used to predict disease trends and healthcare demands. Some previous studies have examined the serial interval distribution of COVID-19 in different locations, with estimates of the mean serial interval varying from 3.1 days to 7.5 days. These early studies each assumed the serial interval distribution was constant over time.

In the current study, the research team examined the serial interval distribution of COVID-19 and found that variation in the serial interval can occur. The mean serial intervals of COVID-19 shortened substantially from 7.8 days to 2.6 days within a month (January 9 to February 13, 2020).The serial intervals shortened because of the improvements in identification and isolation of cases, which reduced the chance of transmission later in illness. This work was based on a database of 1,407 COVID-19 transmission pairs, in which symptom onset dates and social relationships were available for both the infector and infectee of 677 transmission pairs.

The real-time assessment of changes in the serial interval distribution allows more accurate estimates of reproduction numbers than the traditional analysis assuming a constant serial interval distribution. These findings would improve assessment of transmission dynamics, forecasting future incidence, and estimating the impact of control measures.

About the WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, HKUMed

The School of Public Health, LKS Faculty of Medicine of The University of Hong Kong (HKUSPH) has a long and distinguished history in public health education and high impact research. With world leading research in infectious diseases as well as on non-communicable diseases of both local and global importance, the School has made significant contributions through its research and advocacy to improve the health of populations and individuals, both locally and globally. The School is a leading research and teaching hub in public health on influenza and other emerging viruses, control of non-communicable and infectious diseases, tobacco control, air pollution, psycho-oncology, behavioural sciences, exercise science, life-course epidemiology, and health economics, health services planning and management. Work done by HKUSPH researchers has informed international (e.g. the World Health Organization and the Food and Agriculture Organization), national and local public health policies.

The School of Public Health hosts the WHO Collaborating Centre (WHO CC) for Infectious Disease Epidemiology and Control. With a view to protecting the public's health in Hong Kong and across the region, the WHO CC aims to coordinating research on the control and prevention of infectious diseases and providing local and regional education and training in infectious disease epidemiology and control. Members of the WHO CC are involved in the response to COVID-19 and conducted a range of scientific research projects. The team has created a website to share the latest scientific findings and the implications for evidence-based public health policies on the ongoing COVID-19 outbreak.

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