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The largest COVID-19 study highlights the role of super-spreaders

In the bleak ranking of the worst COVID-19 outbreaks, the United States, with 7.2 million infections, is likely to be eclipsed only by India, which has 1 million fewer cases but is recovering rapidly.

Yet parts of India have led the world in one aspect of the pandemic response: contact tracing – the laborious, time-sensitive and painstaking work to identify people who have been exposed to a known infected person.

Extensive contact tracing in two southern Indian states offers the strongest evidence that some super-spreading individuals are responsible for a disproportionate share of new coronavirus infections, according to a study published Wednesday in the journal Science. It also suggests that children are more efficient transmitters of the virus than is believed.

A team of Indian and US researchers looked at data from 575,071

individuals who were tested after coming into contact with 84,965 people with confirmed cases of COVID-19. This is an average of seven contacts per case and a cohort more than 10 times larger than in a previous study from South Korea that mapped the mode of transmission of the virus.

“It is by far the largest epidemiological study on COVID,” said lead author Ramanan Laxminarayan of the Center for Disease Dynamics, Economics and Policy, in New Delhi.

Laxminarayan and his colleagues found that only 8 percent of people with COVID-19 accounted for 60 percent of the new infections seen among contacts. Meanwhile, 7 out of 10 COVID-19 patients were not linked to new cases.

The finding underscores the essential role of super-spreaders in the COVID-19 pandemic: one individual or event, such as in a poorly ventilated indoor space, can trigger a large number of new infections, while others may not transmit the virus at all.

In the new study, the researchers tracked down 78 people who had shared a bus or train with one of eight known infected people and sat within three rows of that person for more than six hours. Healthcare professionals visited these contacts at their home to conduct follow-up screenings and determined that nearly 80% of them had contracted the coronavirus.

In contrast, people who were known to be exposed to infected individuals in low-risk environments, such as being in the same room but more than three feet away, were only infected 1.6 percent of the time.

“Super-broadcast events are the rule rather than the exception,” Laxminarayan said. “It has many implications for COVID modeling, for how to keep places safe.”

The study suggests that super spread events are influenced by behavior: proximity to an infected person, duration of contact and environmental conditions determine the level of risk. It does not examine whether some infected people spread the virus more efficiently due to biological factors, a question scientists are still trying to answer.

The findings could help guide security measures in places such as gyms, churches and choral practice spaces that have been the site of previous super broadcast events.

People wait for COVID-19 test results at a New Delhi government hospital

People wait for the results of their COVID-19 tests at a government hospital in New Delhi.

(Manish Swarup / Associated Press)

The study also found that although children under the age of 17 were the least likely to die from COVID-19, they passed the virus at similar rates to the rest of the population, underscoring the idea that the disease does not spare young people. One figure in particular has implications for the reopening of schools: children aged 5 to 17 have transmitted the virus to 18% of close contacts of their age.

Antonio Salas, a Spanish researcher who has studied the role of super-diffusers in the pandemic, said the results of the study on children were important in light of “previous reports suggesting a minor role for children in the pandemic.”

“National policies on how to proceed with children in schools and other social activities could change dramatically if the scientific evidence supports the idea that children can infect as efficiently as adults and, even more so, they could even behave like super- speakers, ”said Salas, who was not involved in the India study.

As India’s coronavirus workload has doubled in the past month, from 3 million to over 6 million, the study authors said their work showed one strength of the country’s response: the ability to mobilize a large number of health professionals and public officials to conduct contact tracing, identify high-risk individuals and closely follow their cases.

The two Indian states in the study, Andhra Pradesh and Tamil Nadu, have a combined population of 128 million and boast one of the largest health workforce and the highest levels of public health spending in the country. Both have turned to disease surveillance networks set up years ago during the AIDS epidemic to carry out house-to-house checks at levels never seen in other parts of the world.

In Tamil Nadu, which includes the coastal metropolis of Chennai (formerly known as Madras), thousands of public and private health workers have been trained as contact detectors and health surveillance agents. The state has had experience in identifying populations at risk from its battle against AIDS, which has hit Tamil Nadu harder than anywhere else in the country.

Once a person tested positive for coronavirus, a public health worker interviewed them by phone about where they were and who they had met in the previous two weeks. Immediate family members were immediately examined and isolated; others were identified through phone calls and text messages.

Almost all of the contacts traced were people known to the infected person, so many strangers were lost. However, in Chennai, health workers have identified and tested an average of 17 contacts per infected person up to August 1.

While contact tracing is key to identifying, isolating and testing people vulnerable to infections and helping countries like South Korea avoid disastrous epidemics, an effective system has eluded most of the United States due to a shortage of skilled and of financing.

The Los Angeles County contact tracking program failed to stop major outbreaks and was haunted by language barriers, slow turnaround times for test results, and inaccurate information from patients. The county has hired approximately 2,600 contact tracers to cover a population of 10 million. Madurai, a semi-urban district in Tamil Nadu, had an equal number of health workers covering a population one-third the size, said Chandra Mohan, a state official who helped oversee the response and was a co-author of the study.

Although poorer nations now account for the majority of COVID-19 cases in the world, much of what is known about how coronavirus is transmitted comes from relatively small-scale studies in China, the United States, and wealthy nations of Europe.

India imposed one of the toughest lockdowns anywhere in late March, measures the new study found have significantly slowed the spread of the virus. While Prime Minister Narendra Modi’s government eased restrictions to revive the economy, the disease has returned, not surprisingly. But Mohan said the contact-tracking program saved lives.

Tamil Nadu has recorded nearly 600,000 infections and 9,400 deaths, which translates to 13 deaths per 100,000 people. The US COVID-19 death rate is approximately 62 per 100,000.

“What is needed is clarity of thought and the ability to mobilize resources and put them to good use,” Mohan said. “I suppose a good governance structure makes the difference between what can be done and what cannot be done.”

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