20% pass Covid to other 80%: Study

NEW DELHI: A little under 20% of infected people were responsible for passing on Covid-19 to the remaining 80%, according to a study of the first 1,038 cases in Hong Kong that suggests not everyone who gets the virus may pass it on, while a few who do may infect several more.

This unpredictability is one of the epidemiological unknowns that has persisted, close to six months after the disease was declared a pandemic.

The latest study now makes a case for avoiding large social gathering, which appears to have particular potential for superspreading events that can ignite a major outbreak.

“Our findings indicate that there is substantial potential for SARS-CoV-2 superspreading. SARS-CoV-2 exhibits a high degree of individual transmission heterogeneity,” said the study published on Thursday in Nature by authors from University of Hong Kong’s School of Public Health.

The researchers analysed contact tracing data from 1,038 confirmed between January 23 and April 28, where they identified all local clusters of infection. “We identified 4–7 SSEs (superspreading events) across 51 clusters (309 cases) and estimated that 19% (95% confidence interval, 15–24%) of cases seeded 80% of all local transmission,” they said.

The largest cluster comprising 106 cases was linked to a collection of four bars and members of a band that played at one of the bars.

The infections began at one bar where two customers and two bar staff fell ill, while a band member who played at this location later fell ill.

This person was also at one of the other bars, as where several other musicians who had overlapping gigs, the authors said.

“Gatherings in social settings such as bars, restaurants, weddings and religious sites appear to be at increased risk of superspreading events. Transmission in social settings was significantly associated with an increased number of secondary cases compared to transmission observed in family households,” the paper said.

Similar findings have been made by researchers in the United States.

In a study of cases in Georgia led by researchers from Emory University, 2% of all infections were directly responsible for 20% of total cases.The study was uploaded on preprint server medRxiv on August 20.