The analysis of age-specific mobility data, which tracks the location and movement of people based on information from cellphones and other mobile devices, revealed that 65 of every 100 COVID-19 infections in the country through the end of October originated in adults age 20 to 49.
The biggest increases in disease spread were linked with adults 35 to 49 years old after the reopening of schools in many states, researchers said.
"We find adults aged 20 to 49 [years] are a main driver of the COVID-19 epidemic in the United States and are the only age groups contributing disproportionately to onward spread, relative to their population size," study co-author Melodie Monod said in a press release.
"While children and teens contribute more to COVID-19 spread since school closure mandates have been lifted in fall 2020, we find these dynamics have not changed substantially since school reopening," said Monod, a professor of natural sciences at Imperial College London, where the study was conducted.
Although younger adults are less likely to suffer severe illness following infection with COVID-19 compared to older adults, mass vaccination campaigns in this age group could help contain spread and bring an end to the pandemic, according to Monod and her colleagues.
To date, adults 65 years old and older, and healthcare workers, have been given priority for vaccination in the United States, based on guidance from the Centers for Disease Control and Prevention.
The Imperial College London researchers based their findings on mobility data for more than 10 million adults in the United States.
Across the country as a whole, from March through October, mobility trends indicated substantial initial declines in visits to stores, restaurants and other venues outside the home, followed by a subsequent rebound for all age groups, the researchers said.
However, as of October, adults age 20 to 49 were the only groups sustaining COVID-19 transmission with reproduction numbers well above 1, they said. A reproduction number reflects the average number of people to whom an infected person spreads the virus.
"This study is important because we demonstrate that adults aged 20 to 49 [years] are the only age groups that have consistently sustained COVID-19 spread across the U.S., despite large variations in the scale and timing of local epidemics," study co-author Oliver Ratmann said.
"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," said Ratmann, who is also a professor of natural sciences at Imperial College London.