The science behind super-spreader events

Updated on Apr 17, 2021 06:44 AM IST

The pandemic is raging in India due to multiple reasons, including changes in the behaviour of people (whether vaccinated or unvaccinated), change in the behaviour of virus (in terms of new variants), as well as avoidable mass gatherings

Representational image. (AP)
Representational image. (AP)
ByRajinder K Dhamija

Chris Moore, an expert in virus-spread modelling at the Santa Fe Institute, New Mexico, recently wrote that a virus’s ability to infect is not entirely a property of that virus or its virulence alone, but of “how the virus and human society interact”.

The pandemic is raging in India due to multiple reasons, including changes in the behaviour of people (whether vaccinated or unvaccinated), change in the behaviour of virus (in terms of new variants), as well as avoidable mass gatherings — where few infected persons can spur a large outbreak among participants, and which increase the risk of contagion.

A super-spreading event, popularly known as SSEV in science, is a physical phenomenon in which communicable diseases spread faster than usual. These situations provide ideal ground for the virus to propagate among crowds in close contact settings, further compounded by actions such as singing, shouting and heavy breathing. Many who get infected then go back to their cities, towns or villages, carrying the virus with them and putting larger populations at risk.

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As clinical and research scientists, we have learned a lot more about Covid-19 in past 15 months and the data is unambiguous — super-spreading incidents represent an important engine of transmission. They not only propel the growth of new infections but can also facilitate genetic drifts in virus genome that might be responsible for some new strains and mutations.

Size matters. As the numbers increases, so does the risk of transmitting the virus to a wider cluster. A large assembly size increases the possibility that someone present in the group will be asymptomatic but infectious. Many studies suggest that 10-20% of infected people are responsible for 80% of the spread of Sars-Cov-2. Infected individuals can remain asymptomatic in the first five to six days, and around 40% of transmission occurs during this period. The longer they come into contact with other people (especially if that contact is longer than 15 minutes, which is the case in most of these events), the more ideal it is for the virus to spread quickly and rampantly.

Persons shedding high volume of virus can result in new infections that are of high viral replication, generating a ripple effect, causing severe disease with higher morbidity and mortality. A study from South India published in Science last year revealed that in case super-spreading events predominate, then as low as 8% of infected people may be responsible for 60-80% of the secondary cases of Covid-19 in the community. This is crucial as it increases the effective R0 of virus. Cases could, thus, explode in a very short period of time if we continue to have series of these events, thereby causing what is called Domino’s effect.

Amid the surge of Covid-19 infections, such events are still taking place. The following steps must be taken immediately.

One, science tells us that if we clamp down on these large mass gatherings of susceptible people, and the virus is denied favourable circumstances for a spread, the rate of infections will slow.

Second, individuals who head back to their homes after such an event, and may be carrying the virus, should be mandated to be under self-isolation for a week after their return to slow the further spread.

Third, extensive testing should be carried out in as many of these people on their return to their homes. Research shows that if we encourage people to get tested, they become aware of their infection status and the consequent dangers, and are more likely to adhere to non pharmacological measures and change their behaviour.

Fourth, the concept of backward contact tracing needs to be aggressively pursued, which can help find other infectious people from the same event. This can be quite useful in detecting more chains of transmission and slowing the virus.

The virus is rampant and here to stay. We need to be vigilant, more now than ever.

Rajinder K Dhamija is the head of neurology department, Lady Hardinge Medical College and SSK Hospital, New Delhi. He was, formerly, a WHO Fellow at National Institute of Epidemiology

The views expressed are personal

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