Since February, when the virus spread picked up in the U.S., through October, adults aged 20 to 49 likely caused 76% of new transmissions, researchers at Imperial College in London estimated based on cell phone location data from more than 10 million Americans and case rates from around the country.
The older end of that age group, those aged 35 to 49, drove slightly more of the spread, an estimated 41%. Just below that were people between the ages of 20 to 34, who were responsible for about 35%.
"Over time, the share of age groups among reported deaths has been remarkably constant, suggesting that young adults are unlikely to have been the primary source of resurgent epidemics since summer 2020, and that instead changes in mobility and behavior among the broader group of adults aged 20-49 underlie resurgent COVID-19 in the US in 2020," the researchers wrote in the study, published in the journal Science.
Meanwhile, people aged 50 to 64 accounted for 15% of the spread, and children and teens just 6%. The study data includes when students were still all in school, before most states moved to stay-at-home orders in March, the months when they were closed, and the restart of in-person classes for many children that fall. Children are typically major spreaders of viruses like the flu, but when they went back to school — with most areas requiring masks and reduced class sizes — they did not appear to cause a rise in COVID-19 transmission. A recent study from the Centers for Disease Control similarly found that in-person school is not driving the spread.
"Unlike pandemic flu, these adults accounted after school reopening in October 2020 for an estimated 72.2% of SARS-CoV-2 infections in the US locations considered, whereas less than 5% originated from children aged 0-9 and less than 10% from teens aged 10-19."
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Based on their findings, the researchers say that transmission would go down in the U.S. if the 20- to 49-year-old age group is prioritized for vaccinations.
"This study indicates that in locations where novel highly transmissible SARS-CoV-2 lineages have not yet established, additional interventions among adults aged 20-49, such as mass vaccination with transmission-blocking vaccines, could bring resurgent COVID-19 epidemics under control and avert deaths," they added.
Currently, that age group is the lowest priority for vaccination in most states in the U.S. unless they are frontline health care workers or teachers. Some areas, like New York, are now opening up their vaccine eligibility to occupations that may overlap with that age group, like grocery store workers, police and, as of Wednesday, restaurant workers. They may also be able to get the vaccine if they have preexisting conditions in places like New Jersey, where people with cancer, diabetes, heart diseases, obesity and smokers are now eligible.
As of Feb. 3, more than 27,154,950 Americans have received at least one dose of either Pfizer or Moderna's COVID-19 vaccine, and 6,436,931 people have been fully vaccinated with two doses.