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As the world continues to grapple with the aftermath of the COVID-19 pandemic, the role of superspreaders is being continually scrutinised to find out why this pandemic has spread so quickly to all parts of the world.

As of 25 May, nearly 5.5 million people have been affected by the new coronavirus infection globally, with over 345,000 people having lost their lives to this disease which was first reported in late 2019. The outbreak of COVID-19 has since prompted countries all over the world to enforce restrictions on the movement of people with large-scale lockdowns.

Many superspreading events around the world have been identified as the reason behind the sharp rise in COVID-19 cases in those cities and regions, including the recent instance of a superspreader at a South Korean nightclub that led to 50 new cases. Prior to that, stringent lockdown policies and countrywide testing had ensured South Korea had managed to control the spread of the disease after the initial outbreak earlier this year.

Closer home, a religious congregation in the heart of the Indian capital also gave rise to a number of new COVID-19 cases around the country, although authorities were not able to identify superspreaders among them.

A new study on the pre-print server medRxiv questions the viability of lockdowns and whether they can be imposed in selective cases, keeping superspreading events in mind, as the transmission of the disease has not been established as a uniform phenomenon.

Read more: COVID-19 India timeline

  1. Who are superspreaders?
  2. How not to become a superspreader

Superspreading events (SSE) are not limited to infectious diseases that have emerged recently. According to the United States' Center for Disease Control and Prevention, an asymptomatic carrier of typhoid went on to infect 50 people in the early 20th century. 

Researchers thereafter arrived at a 20/80 rule, suggesting that about one in five people are responsible for 80% of all new infections. This empirical rule, according to research, explains the spread of diseases—those who end up spreading the infection at an abnormally higher rate are known as superspreaders.

There are several theories about what makes a person a superspreader, but there isn't any conclusive evidence to explain it yet. It is thought to have some link with the immune system of the person, as superspreaders tend to be highly infectious despite being asymptomatic—they don't have any symptoms, so they often do not realise that they could be transmitting the infection to others.

Some theories also suggest it could have something to do with the viral load on a person who has been infected, but the general overview is that it is virtually impossible to identify a superspreader before such an event has taken place.

While it is next to impossible to identify a superspreader—even for an infectious disease like COVID-19—unless a superspreading event occurs, researchers at Copenhagen University and Roskilde University have now created a model to analyse superspreading events to come up with counter-measures in various settings, including homes, workplaces as well as other public spaces.

The model, however, requires the use of intensive care units (ICU) of hospitals to be able to measure the impact of the outbreak, as each infected person is allowed to infect another person over a period of time to be able to study the growth rate of the virus, which is calculated with an R0 figure of 2.9.

This study is aimed at identifying clusters and any patterns in the spread of the infection among a certain population. It argues that based on this model, superspreaders can be identified as coming in contact with others at a pre-decided time interval. Reducing the number of people a superspreader comes into contact with by 75% can halt the epidemic. Of course, this requires identifying superspreaders and restricting their movements.

The findings also delve deeper, as separate lockdown events in different social settings can restrict the spread of the disease. For instance, if social contact is restricted or avoided at home, the peak of the epidemic drops by a third, while at a workplace, such restrictions can reduce the peak by half.

According to the findings of the study, random social contacts in public transport, public gatherings or frequent trips to the market, bars or restaurants "is what drives the epidemic". 

According to the researchers, "Superspreaders matter greatly in a mitigation scenario. Epidemics driven by superspreaders are less sensitive to reductions in close contacts at home or work/school, but highly affected by changes in their random contacts."

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References

  1. Sneppen K and Simonsen L. Impact of Superspreaders on dissemination and mitigation of COVID-19 medRxiv. 2020 May. [Internet]
  2. Stein RA. Super-spreaders in infectious diseases International Journal of Infectious Diseases. 2011 Aug; 15(8): 510-513.
  3. Hamner L et al. High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice — Skagit County, Washington, March 2020 Morbidity and Mortality Weekly Report, CDC. 2020 May; 69(19);606–610.
  4. Frieden TR and Lee CT. Identifying and Interrupting Superspreading Events—Implications for Control of Severe Acute Respiratory Syndrome Coronavirus 2 Emerging Infectious Diseases. 2020 Jun; 26(6):1059-1066.
  5. Woolhouse ME et al. Heterogeneities in the Transmission of Infectious Agents: Implications for the Design of Control Programs Proc Natl Acad Sci USA. 1997 Jan; 94(1):338-42.
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