One-tenth of all patients infected with the coronavirus disease (Covid-19) become “super-spreaders” and account for 60% of new infections, while 70% of infected patients do not transmit the disease to anyone else , shows a study on transmission patterns of Sars-CoV-2 published in the journal Science: it is the first time that numbers have been put to axioms that most people accept.
The contact tracing study, perhaps the largest epidemiological analysis of Covid-19 conducted in the world, analyzed disease transmission patterns in at least 575,000 people who were exposed to nearly 85,000 confirmed cases of Covid-19 in two states of India: 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 Center for Disease Dynamics, Economics and Politics (CDDEP), also found that the case fatality rate (CFR) is higher in the 40 age group. to 69 years in India than in other seriously affected countries and that proximity travel is one of the highest high-risk behaviors for the transmission of Covid-19.
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The trend in the CFR, however, falls after the age of 80, where India remains in line with other countries, a phenomenon that experts say is due to selection bias, as Indians living beyond that age tend to be healthier and therefore able to fight. better out of sickness.
The study, ‘Epidemiology and dynamics of COVID-19 transmission in two states of India’, published in Science, also found that the average hospital stay before death from Covid-19 in the two states was around five days , compared to 13 days. in the U.S.
Transmission factors
One of the key findings of the report was that there was great variation in the number of people to whom infected people transmitted the virus; although many were not infecting anyone, a handful of “super-spreaders” were responsible for most of the new infections. . Of the 88,616 index patients studied in the report, 70.7% did not actually 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 new infections observed, which the researchers said was the largest recorded demonstration of “over-spread.”
“Super spread is often mistaken for being just an event. In our study we found that some people, around one-tenth of the population of infected individuals, are much more likely to transmit the virus than others. This probably plays an important role in all countries, not just India, ”said Laxminarayan, lead author of the study.
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In fact, most experts have come to accept the role of super diffusers, although, so far, no studies have quantified their impact.
The analysis classified all contacts into high- and low-risk incidents. If people shared a home and had close or direct physical contact with the index case, they were considered to be high-risk contacts. Low-risk contacts were those who shared the same space (eg, room) as the index case without meeting the high-risk exposure criteria. He also looked at the types of interaction and settings that caused the Covid-19 transmissions.
Proximity travel (sharing an enclosed space in a car or bus), the study found, carried the highest risk of people infecting others and had a “ secondary attack rate ” (SAR) – the proportion of all contacts tested, testing positive – 79.3%.
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It also found a high prevalence of infection among children who were case contacts close to their own age, with children in the five to 17-year age group being a more likely proportion of infecting others. For every 100 index patients among the age group 5 to 17 years, 15.4 of their contacts tested positive, the highest proportion among the age groups analyzed. Those between the ages of 18 and 39 were in second place, both infecting 12.3% of their contacts.
“We have underestimated the role of children as an important part of the chain of transmission. This has implications for the reopening of schools and situations where children are likely to become infected outside the home while transmitting the virus to those within the home, ”Laxminarayan said.
Experts said that the super-spreader concept gives ordinary people more reason to be careful about factors like mask hygiene.
“It is always a small fraction of infected people who transmit the disease the most and most people do not transmit the virus at all or transmit very little. This is the first study in India that has confirmed with data that the superpreader effect is prevalent here. The implication for us is that no one really knows that whoever they meet will be a super-spreader, so it is even more important for us to continue to wear masks rigorously and to be more careful, ”said Dr. Shahid Jameel, Virologist and School Director of Trivedi Biosciences at Ashoka University.
Mortality findings
The study also found a wide range in the breakdown of the CFR by age in the country, very much in line with what scientists have found internationally. The case fatality rates ranged from 0.05% for the five to 17-year-old age groups to 16.6% for those 85 and older.
Unlike the findings in high-income countries like the US and parts of Europe, in India, deaths were largely concentrated among people ages 50 to 64. In fact, infected people in the age group 40 to 69 were more likely to die from Covid-19 in India than in other countries.
“This probably reflects the higher prevalence of diabetes, hypertension and other comorbidities 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 comorbidity.
Experts echoed Laxminarayan’s finding, saying that comorbidities such as diabetes, in particular, are more common in India and can influence CFR for people over 40.
“The 40 to 70 year old age group in India tends to have quite a high number of comorbidities because Indians are genetically predisposed to diabetes anyway. This would greatly burden the CFR in that age group, ”said Dr. Jacob John. “Add to this that Indians tend to have to watch live in more polluted environments, so our lungs suffer years of damage when we reach our 40s. So that can also be a factor, ”he said.
“The interesting thing is that people over 65 have a lower CFR than abroad. This may be due to a selection bias because in India our life expectancy is, on average, about 10 years lower than in the Western world. So those of us who survive are generally people who are in good health, so they end up fighting disease more effectively, ”said Dr. Jameel.
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