For months, public health officials have said that younger adults have been big drivers of the spread of COVID-19 in the U.S. But new research suggests there’s another age group that may be fueling new cases of the virus: people ages 35 to 49.

That’s the conclusion of a just-released study from the U.K.’s Imperial College London. The study, which was published in the journal Science, analyzed mobility data from the cellphones of more than 10 million Americans between early February and late October 2020. Among other things, the data helped researchers determine where people went, like restaurants and grocery stores. The researchers then compared that data to COVID-19 case and mortality rates by age.

The researchers concluded in the study that the “majority of COVID-19 infections” originate from people between the ages of 20 and 49, but people between 35 and 49 were responsible for 41.1 percent of new cases of the virus. Those in their late thirties and forties driving the spread of the virus was consistent across the country, the researchers said in the study, but “estimated contributions” of people between 20 and 34 were higher in the Southern, Southwestern and Western regions of the United States.

“This study provides evidence that the resurgent COVID-19 epidemics in the U.S. in 2020 have been driven by adults aged 20-49, and in particular adults aged 35-49, before and after school reopening,” the researchers wrote. “These adults accounted after school reopening in October 2020 for an estimated 72.2 percent of SARS-CoV-2 infections in the U.S.”

The findings overlap with research shared by the CDC in September. That study analyzed data from all 50 states and the District of Columbia and found that 20 percent of COVID-19 cases between March and August 2020 in the U.S. were in 20- to 29-year-olds — the highest percentage of all age groups. The report also found that there was a drop in the median age of people infected with COVID-19 from 46 years old in May to 38 years old in August because of this.

So, what’s going on here? The latest data suggests that “due to work, school and general activity, the middle ages generally move a lot compared to the elderly or very young,” study co-author Dr. Samir Bhatt, an associate professor in geostatistics in the Department of Infectious Disease Epidemiology at Imperial College London, tells Yahoo Life. That, he says, increases their risk of infection — and of passing the virus on to others. But, Bhatt adds, it’s “difficult to disentangle” exactly what’s happening “aside from the fact they generally move more.”

People of middle age also may have more contacts than those who are younger and older, study co-author Oliver Ratmann, a lecturer in statistics at Imperial College London, tells Yahoo Life. And, he adds, those contacts can include people who are “highly susceptible to COVID-19 infection.”

Still, Bhatt says, “the exact reason is speculation at this point — our work only looks at the signal in mobility, not the factors that drive that mobility.”

Infectious disease expert Dr. Amesh A. Adalja, senior scholar at the Johns Hopkins Center for Health Security, tells Yahoo Life that the data is not shocking. “We’ve seen for a while that younger groups ages 20 to 49 are driving the spread,” he says. While Adalja says that it’s “not completely clear what’s going on,” he has some theories. More contacts than younger or older groups is one.

“It may be that they are working more outside the home and interacting with individuals than other ages,” he says. And given that this age group is typically at lower risk of severe complications from COVID-19, they “may be more risk-tolerant” and laxer on preventing the spread of the virus, Adalja says.

Ratmann says that his findings suggest that “nonpharmaceutical interventions” like mask-wearing and clear guidance on preventing the spread of the virus are “so important” for controlling the spread of COVID-19. “Even with strong interventions and guidance, parts of the population will still need to move for a range of reasons,” he says. “Therefore, even while the vulnerable are being vaccinated, measures need to remain in place to control the parts of the population still spreading infection widely.”

Adalja says the study and other data raise the possibility that vaccinating middle-aged people sooner may help control the spread of the virus. “It’s clear from this data where public health targeting needs to be,” he says. “There could be a theoretical advantage of vaccinating the group of individuals to decrease spread.”

For the latest coronavirus news and updates, follow along at According to experts, people over 60 and those who are immunocompromised continue to be the most at risk. If you have questions, please reference the CDC’s and WHO’s resource guides.

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