“COVID-19 primarily spread by young adults:” Largest contact tracing study

By Rahul Vaimal, Associate Editor
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Children can spread coronavirus among themselves efficiently, but young adults are the primary source of the spread of coronavirus, reveals a study.

The research was based on a major contact tracing effort involving more than 3 million people in India and is the largest contact tracing study yet. It reveals that most patients with COVID-19 have never infected anyone else. The researchers found that 70% of infected individuals did not infect any of their contacts, while 8% of patients accounted for 60% of the new infections observed.

The research also contradicts the commonly held belief that the coronavirus is unlikely to be contracted by children.

“We find otherwise. They are getting infected in significant numbers,” study leader Ramanan Laxminarayan of the Center for Disease Dynamics, Economics and Policy in New Delhi, said.

“While the role of children in transmission has been debated, we identify high prevalence of infection among children who were contacts of cases around their own age,” the team wrote in their report, published in the journal Science.

And that’s even though schools in India have been closed since March, noted Laxminarayan.

Infection from same age individuals

The study found that children and young adults were much more likely to contract coronavirus from individuals their own age. This is applicable to all as people have a higher risk of contracting the coronavirus from someone their own age in all age groups. The average likelihood of coronavirus capture ranged from 4.7% for low-risk contacts to 10.7% for high-risk contacts.

High & Low risk 

The research categorizes contact into high and low risk. High-risk contacts were listed as those who shared a household with an infected individual, had contact at a distance of less than one meter with no protective equipment and traveled in the same shared mode of transport within three rows of them.

Over 10 percent of such contacts tested positive, meaning they had a one in ten chance of being infected. On the other hand, people who shared the same space with an infected person without meeting these high risk communication requirements were classified as low risk. Among them, under 5 percent tested positive.

Travel risk

The highest risk interaction is reported when there is close proximity to the infected person in shared transport for six hours or longer. Almost eight out of ten interactions tested positive in such situations. From a prevention point of view, it would be necessary to understand whether wearing a mask could minimize the risk of shared transport, but this data is not yet available.

Shared household

The next highest risk was living in a shared household. Yet there was a much smaller risk of infection than in the case of co-travelers. The study found that fewer than 10 percent of contacts tested positive in shared households.

The research included the efforts of universal contact tracing undertaken in two large south Indian states – Tamil Nadu and Andhra Pradesh. From March to August, authorities monitored and tested more than 575,000 people exposed to almost 85,000 confirmed coronavirus events.

“We were surprised to find that just 8% of the infected primary cases were responsible for 60% of the contacts that were infected. That’s a hugely disproportionate effect,” he said.

Most of the index cases, the first patient in a chain of transmission, were adults aged 20 to 45, the team found. “The young to middle adult age group is the one that is coming into contact with people. They are the people most likely to be outside the household. They are the ones taking the disease from one place to the other,” Laxminarayan said.

Effectiveness of lockdown

The research also showed that lockdowns worked. Restrictions decreased the transmission rate, the team discovered. Over time, case fatality rates, one indicator of the death rate, also dropped. Laxminarayan said, “Those who tested positive in May and June were 13 percent less likely to die than those tested in March and April.”