Young adults and middle-aged people were responsible for the vast majority of COVID-19‘s spread in the US last year, a new study suggests.
People between ages 20 and 49 drove just shy of 72 percent of transmissions between February and October, according to a new research by Imperial College London scientists.
Those aged 20 to 34 contributed about 35 percent of the spread, but older adults were responsible for an even larger share of the spread. People between ages 35 and 49 accounted for more than 41 percent of transmissions.
The study found that young children going back to school in the fall was probably not a major driver of the increase in infections seen in the fall. The US was seeing more than 50,000 new cases a day by October, compared to about 25,000 a day in mid-June.
Instead, as more Americans went back to relatively ‘normal life’ – kids back to school, young adults back to work and college, middle-aged people back to work – movement in general led to a sharp rise in COVID-19 cases.
Encouragingly though, students going back to school did not lead to more deaths, likely because the virus is rarely deadly for children and young adults, who have relatively little contact with at-risk elderly people.
And while the US has been scrambling to vaccinate health care workers and elderly people against COVID-19, the authors of the new study suggest that vaccinating young adults and middle aged people who drive the spread could be among the best ways to head off future coronavirus outbreaks.
But these groups are at the back of the line, and likely won’t get their first doses until late-spring unless they are essential workers.
Americans between ages 20 and 34 were responsible for nearly 35% of COVID-19 transmission in the US, and people between ages 35 and 49 drove nearly half of the spread
‘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 Imperial College London researchers wrote.
Only four percent of the spread of coronavirus was traced to children between ages 10 and 19, and just 2.1 percent of transmissions were linked to kids nine and under.
The researchers also created models for how the disease might have looked if schools remained closed.
Keeping schools closed would not have dramatically changed the trajectory of the pandemic, according to their study.
Over the summer, anxious parents worried that sending their kids back to school in the fall would be a catastrophic outbreak waiting to happen.
US COVID-19 cases rose sharply in the fall after more people started returning to school – but the Imperial College London experts say class restarting alone does not explain the increase in transmissions
Surging infections among young people did not lead to a rise in deaths in their age groups – but did fuel higher daily deaths among older and more at-risk people, the researchers say
But it’s now clear that children don’t pass the virus amongst themselves particularly often, and even when they get infected, they usually have mild or no symptoms (although there are exceptions).
However, kids can ‘seed’ infections among adults, who are more likely to pass the virus on to others, the Imperial College researchers found.
‘Since children and teens seed infections in older age groups that are more transmission efficient, as of October 2020, school opening is associated with an estimated 25.7 percent…increase of COVID-19 infections and a 5.9 percent…increase in COVID-19 attributable deaths,’ they wrote in the article published Tuesday in the journal Science.
How well schools kept up with COVID-19 cases in their classes was also key. The study found that the effects on overall community and cases were more pronounced when there were actually six-fold more COVID-19 infections than school reports documented.
Even so, school reopenings did not not influence the spread of coronavirus beyond what would be expected – only young and middle-aged adults outdid themselves.
Imperial College researchers say mass vaccinations for younger people could prevent ‘resurgences’ of COVID-19 and keep infections and deaths down – but the US is only vaccinating about 1.3 million people a day, and young Americans are at the back of the line
‘These findings indicate that adults aged 20-34 and 35-49 continue to be the only age groups that contribute disproportionately to COVID19 spread relative to their size in the population and that the impact of school reopening on resurgent COVID-19 is mitigated most effectively by strengthening disease control to adults aged 20-49,’ the researchers wrote.
As states and schools reopened and people went back to work and class, who was getting diagnosed with COVID-19 changed, but who was dying didn’t.
Previous CDC research showed that the average age of a person who caught coronavirus in May was 46.
By May, the average age dropped to 38. It came after the share of infections happening in 20- to 29-year-olds rose to 20 percent between March and August.
But the division of deaths among age groups has remained ‘remarkably’ stable, the Imperial College researchers found.
Combined with what they saw in the transmission patterns, this suggests a few factors driving upward trends in COVID-19 last year.
College students are more likely attend classes in person and to be ‘risk tolerant’: willing to to risk infection to go to a party without a mask, for example.
However, the researchers found that 20- to 34-year-olds on their own really only disproportionately drove the spread of coronavirus in some regions of the country: the South, Southwest and Western states.
Without the inclusion of 35- to 49-year-olds, the rise in new infections isn’t quite explained.
That, in turn, suggested to the researchers that it wasn’t just the behavior of particularly young people, but the general increase in movement that boosted the spread of coronavirus last fall.
‘Changes in mobility and behavior among the broader group of adults aged 20-49 underlie resurgent COVID-19 in the US in 2020,’ the study authors wrote.
In effect, they believe that his group is generally mixing in with others outside their households more often – whether for work, school or social purposes – and that these interactions drive up transmissions.
They also suggested that ‘mass vaccination’ of 20- to 49-year-olds could ‘bring resurgent COVID-19 epidemics under control and avert deaths.’