A tenth of all patients infected with the coronavirus disease (Covid-19) become “superspreaders” and account for 60% of new infections, while 70% of infected patients do not pass the disease to anyone else, shows a study on transmission patterns of Sars-CoV-2 published in the journal Science — the first time numbers are being put to axioms that most people accept.

The contact tracing study, perhaps the largest Covid-19 epidemiological analyses conducted in the world, looked at disease transmission patterns in at least 575,000 people who were exposed to nearly 85,000 confirmed cases of Covid-19 in two Indian states — Andhra Pradesh and Tamil Nadu.

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The study also established that children are “active transmitters” of Covid-19.

A group of researchers, led by Ramanan Laxminarayan, director of the Washington-based Centre for Disease Dynamics, Economics and Policy (CDDEP), also found that the case fatality rate (CFR) is higher in the age group of 40 to 69 years in India than in other severely hit countries and that close proximity travel is one of the biggest high-risk behaviours for Covid-19 transmission.

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The trend in the CFR, however, drops off after the age of 80, where India stands in line with other countries — a phenomenon experts said was because of selections bias, as Indians who live past that age tend to be healthier and can thus fight off the disease better.

The study, ‘Epidemiology and transmission dynamics of COVID-19 in two Indian states’, published in Science, also found that the median hospital stay before death due to the Covid-19 in the two states was around five days, compared to 13 days in the United States.

Transmission factors

One of the key findings of the report was that there was a wide variance in the number of people to whom infected people passed on the virus — while many were not infecting anyone, a handful of “superspreaders” were responsible for a majority of new infections. Of the 88,616 index patients studied in the report, 70.7% did not go on to infect anyone else, the researchers found.

At the other end of the spectrum, follow-up testing of exposed contacts revealed that 8% of index patients accounted for 60% of observed new infections, what researchers said was the largest recorded demonstration of “superspreading”.

“Superspreading is often mistaken to be just an event. In our study we find that some people, about a tenth of the population of infected individuals are far more likely to transmit the virus than others. This probably plays a big role in all countries, not just in India,” said Laxminarayan, the study’s lead author.

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Indeed, most experts have come to accept the role of superspreaders, though, until now, no study has quantified their impact.

The analysis classified all contact into high- and low-risk incidents. If people shared a house and had close-range or direct physical contact with the index case, they were deemed as high risk contact. Low risk contacts were those who shared the same space (e.g. room) as the index case without meeting the high-risk exposure criteria. It also looked at interaction types and setting that caused Covid-19 transmissions.

Close proximity travel (sharing a closed space in a car or a bus), the study showed, carries the highest risk of people infecting others, and had a ‘secondary attack rate’ (SAR) – the proportion of all tested contacts, who test positive — of 79.3%.

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It also found high prevalence of infection among children who were contacts of cases around their own age, with children in the age group of five to 17 years as a proportion most likely to infect others. For every 100 index patients among the five to 17 year age group, 15.4 of their contacts tested positive, the highest proportion among the age groups analysed. Those between the ages of 18 and 39 years, were at the second spot, both infecting 12.3% of their contacts.

“We have underestimated the role of children as an important part of the transmission chain. This has implications for school re-openings and situations where children are likely to get infected outside the household while passing on the virus to those within the household,” Laxminarayan said.

Experts said the concept of superspreader gives the common people more reasons to be careful about factors like mask hygiene.

“It always a small fraction of infected people who spread the disease more and most people don’t spread the virus at all or spread very little. This is the first study in India that has confirmed with data that the superspreader effect is prevalent here. The implication for us is that no one really knows that who they meet would be a superspreader so it becomes all the more important for us to continue to wear masks rigorously and be all the more careful,” said Dr Shahid Jameel, virologist and director of Trivedi School of Biosciences at Ashoka Univeristy.

Mortality findings

The study also found wide range in age-wise breakup of CFR in the country — much along the lines of what scientists have found internationally. Case-fatality ratios spanned 0.05% for age groups of five to 17 years to 16.6% for people who were 85 years or above.

Unlike the findings in high-income countries such as the US and parts of Europe, in India, the fatalities were largely concentrated among people between the ages of 50 to 64 years. In fact, infected people in the 40 to 69 years age group were more likely to die from Covid-19 in India than they were to other countries.

“This likely reflects the higher prevalence of diabetes, hypertension and other co-morbidities in the Indian population,” Laxminarayan said.

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The report also found that men were 62% more likely to die from Covid-19 than women and that 3. 63% of those who died had at least one co-morbidity.

Experts echoed Laxminarayan’s finding and said co-morbidities such as diabetes, in particular, is more common in India and may influence the CFR for people above 40 years of age.

“The age group of 40 to 70 years in India would tend to have quite high number of co-morbidities because Indians anyway are genetically predisposed to diabetes. This would heavily cause the weighing down of the CFR in that age group,” said Dr Jacob John. “Add to this that Indians tend to have to see live in a more polluted environments, so our lungs take years of damage by the time we are in our 40s. So that can be a factor as well,” he said.

“The interesting thing is that people who are above 65 have a lower CFR than abroad. This may be caused by a selection bias because in India our life expectancy is on an average 10 or so years lower than the western world. So those of us who survive are generally people who are in good health, so they end up fighting off the disease more effectively,” Dr Jameel said.

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