Young and middle-aged adults are driving the spread of the novel coronavirus across the United States, accounting for about 75% of new infections both before and after many students returned to in-person classes last October, according to a new study published in Science.  

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Study details

For the study, researchers from the United Kingdom's Imperial College London examined mobility data pulled from the cellphones of more than 10 million Americans between February 2020 and October 2020 using Foursquare's location technology, Pilgrim, and another service called Emodo. According to Yahoo! Life, the age-segregated dataset enabled the researchers to track where people went, including shops, parks, universities, and elsewhere.

Then, the researchers compared that data with publicly available information on age-segregated coronavirus infection and fatalities rates.

Key findings

The researchers found that the "majority of [coronavirus] infections" originate from adults between the ages of 20 and 49. In fact, young and middle-aged adults were the only age cohorts who sustained coronavirus transmission with reproduction numbers—the average number of other people to whom an infected person spreads the virus—well above 1.

Furthermore, the researchers found that these age cohorts drove the majority of new coronavirus infections both before and after many students returned to in-person classes last October. Specifically, the researchers found that young and middle-aged adults drove 34.7% and 41.4% of new infections, respectively, through mid-August, before many schools reopened, and for 34% and 38.2% of new infections, respectively, in October, after many schools had reopened.

Overall, the study indicated that school reopenings in October increased the overall number of infections by about 26% and fatalities by about 6%, primarily because children and adolescents were spreading the virus to adults, who are "more transmission efficient."

Comments and conclusions

In a press release, Oliver Ratmann, a study co-author and a lecturer in statistics at Imperial College London, said the "study is important because we demonstrate that adults aged 20 to 49 (years) are the only age groups that have consistently sustained [coronavirus] spread across the [United States], despite large variations in the scale and timing of local epidemics."

According to Samir Bhatt, another co-author of the study and an associate professor of geostatistics at Imperial College London, these findings suggest that "due to work, school, and general activity, the middle ages generally move a lot compared to the elderly or very young."

However, Bhatt cautioned that it's "difficult to disentangle" precisely what is occurring "aside from the fact they generally move more" since the study focused on only "the signal in mobility, not the factors that drive that mobility."

Separately, Amesh Adalja, senior scholar at the Johns Hopkins Center for Health Security, agreed that it's "not completely clear what's going on," but he hypothesized that it could be that this age group is "working more outside the home," more likely to interact with others," and "may be more risk-tolerant" than other age groups.

Ultimately, Ratmann said the findings demonstrate the importance of "nonpharmaceutical interventions," such as mask-wearing, to curb the spread of the novel coronavirus while vaccines continue to be rolled out. "At least where highly transmissible variants have not established, additional interventions targeting the 20 to 49 age group could bring resurgent epidemics under control and avert deaths," he said (Owens, "Vitals," Axios, 2/3; Miller, Yahoo Life, 2/2; Dunleavy, UPI, 2/2; Monod et al., Science, 2/1).