New research found that more than 70% of COVID-19 infections in the U.S. last year originated with people between the ages of 20 and 49.

British researchers at the Imperial College London gathered the information by tracking cell phone data on more than 10 million Americans. Their findings suggest that healthcare experts target this group for vaccination to help control COVID-19.

According to the New York Post, people in that age category accounted for 72% of the cases after schools reopened in October, while less than 5 % came from children and less than 10% came from teenagers.

The study found that children going back to school probably did not drive the number of cases up in the fall surge, according to the Daily Mail. Instead, when normal life resumed, it appeared that when young adults went back to work and college and middle-aged people went back to work, the number of cases soared.

“We find adults aged 20 to 49 are a main driver of the COVID-19 epidemic in the United States and are the only age groups contributing disproportionally to onward spread, relative to their population size,” said study co-author Dr. Melodie Monod.

The researchers said that their findings show that more intervention is needed in this age category to control the spread of the virus.

“Thus, at least where highly transmissible variants have not been established, additional interventions targeting the 20-49 age group could bring resurgent epidemics under control and avert deaths,” said Dr. Oliver Ratmann, of Imperial College London.