The results of the world's largest Covid-19 tracking study to date are in, revealing a number of important insights into the way the disease spreads.
Above all, the findings suggest that the majority of infected people did not appear to pass on the disease to any of their contacts, but a small number of infected individuals — so-called “superspreaders” — accounted for most of the new infections. Contrary to previous studies, it also hints that children and young adults play a surprisingly important role in the transmitting of the virus, namely within their own households.
Scientists in the US teamed up with public health officials in the southeast Indian states of Tamil Nadu and Andhra Pradesh to track the infection pathways and mortality rate across over half a million people, just under 85,000 of whom had confirmed cases of Covid-19. Reported in the journal Science, they found that 71 percent of infected individuals did not infect anyone, instead just 8 percent of infected individuals were behind up to 60 percent of new infections.
“Our study presents the largest empirical demonstration of superspreading that we are aware of in any infectious disease,” lead researcher Ramanan Laxminarayan from Princeton Environmental Institute said in a statement. “Superspreading events are the rule rather than the exception when one is looking at the spread of Covid-19, both in India and likely in all affected places.”
Across all ages, the chances of a person with Covid-19 passing it on to a close contact ranged from 2.6 percent in the community to 9 percent in their own household. The risk of transmission from an infected individual to another person was 10.7 percent for high-risk contacts, defined as having close social contact or direct physical contact with an infected person without protective measures. If people were in one of these high-risk contacts in a confined space for over 6 hours, such as a long bus journey, that rate could be as high as 79 percent.
However, it seems that young people and children, accounting for around one-third of cases, were especially key to transmitting the virus.
“Kids are very efficient transmitters in this setting, which is something that hasn’t been firmly established in previous studies,” Laxminarayan explained. “We found that reported cases and deaths have been more concentrated in younger cohorts than we expected based on observations in higher-income countries.”
There are a few limitations to consider with this research. This case study was only carried out in southeast India and, given the country's demographics and culture, the findings might not be directly applicable to other parts of the world. For starters, India has one of the youngest populations in the world, which could perhaps explain the trend of young people spreading the virus. Equally, multi-generation houses are common in India, with up to a third of all households containing both a senior citizen and a young person. This, too, could help to determine why young people have played a prominent role in spreading the virus there.
However, it’s not the first study to reach these findings. An earlier contact tracing report in Hong Kong found that just 20 percent of people with the infection was responsible for 80 percent of reported transmissions, while up to 70 percent of people with infection did not pass the virus on to anyone.
Furthermore, a case study by the US Centers for Disease Control and Prevention (CDC) in May found at least 52 people were infected at a choir practice in the US apparently due to a single superspreader who attended the event.