Serial interval

Source: By Abhishek De: The Indian Express

Ever since the Covid-19 outbreak, there has been a pandemic of epidemiological terms that have now become commonplace. A recent research paper published in the journal Science, written by Benjamin Cowling and other researchers at the University of Hong Kong, said China, which has now gone over a month without any locally transmitted Covid-19 cases, was able to contain Covid-19 due to its ability to manage the serial interval. 

The serial interval is the duration between symptom onset of a primary case and symptom onset of secondary cases (contacts) generated by the primary case. In simple terms, the serial interval is the gap between the onset of Covid-19 symptoms in Person A and Person B, who is infected by Person A.

The term was first used by British physician William Pickles, who had initially referred to it as transmission interval with reference to a hepatitis epidemic in the United Kingdom during 1942-45. Later, another British physician RE Hope Simpson used the term serial interval, defining it as the interval between successive illness onsets.

The researchers, in their paper titled “Serial interval of SARS-CoV-2 was shortened over time by non-pharmaceutical interventions”, explained that the serial interval depends on other epidemiological parameters such as the incubation period, which is the time between a person’s exposure to the virus and symptom onset, and the reproduction rate or R naught, the number of people who will be infected by one infected person.

The serial interval helps to gauge the effectiveness of infection control interventions besides indicating rising population immunity and forecast future incidence. Thus, the more quickly persons who contracted Covid-19 are identified and isolated, the shorter the serial interval becomes and cuts down opportunities for transmission of the virus. To manage serial intervals, a robust system of contact tracing, quarantine, and isolation protocols should be in place.

In the paper, the researchers found that the serial interval in Wuhan came down from 7.8 days to 2.6 days between early January and early February. Quarantining contacts within 1 day from symptom onset helped reduce Covid-19 transmission by 60 per cent, the researchers said.

This, the researchers suggest, was made possible due to aggressive contact tracing, quarantine, and isolation, thereby ensuring that infected patients, because they were isolated, could not infect any more people later in the infection cycle. The report also mentioned that interventions such as suspension of intra- and intercity travel, and different forms of social distancing widely implemented in different Chinese cities kept the serial interval low.

Similarly, a separate study by researchers in Zurich and Seoul, published in the International Journal of Infectious Diseases, found that the serial interval in South Korea, another country whose Covid-19 response has been touted as a success story, was estimated to be 3.63 days, crediting the effectiveness of the country’s intensive contact tracing efforts.