Young and middle-aged adults responsible for most COVID spread

The coronavirus pandemic in the U.S. has been chiefly driven by young and middle-aged people, while killing mostly older people.

Driving the news: Adults aged 20-49 were responsible for the vast majority of virus transmission last year, even after schools reopened in the fall, according to a new study published in Science.

Why it matters: The U.S. vaccination effort is racing to keep up with the spread of new, more transmissible variants of the virus. Millions more Americans could be infected before a substantial portion of the population is vaccinated, making transmission patterns deeply relevant.

• The notion that non-vulnerable people can go about their normal lives, while vulnerable people self-isolate, has not borne out in the U.S.

By the numbers: Three-quarters of new infections originated from adults 20-49 until mid-August of last year. Adults 35-49 contributed the most to spread.

• In October, after a large portion of U.S. students returned to school, this age group was still responsible for about 72% of new infections.
• The study estimates that school reopenings increased total infections by about 26% as of October, and deaths by about 6% — because children and teenagers spread the virus to adults, who are "more transmission efficient."

Between the lines: Young and middle-aged adults "naturally have most contacts to other adults aged 20 and above, which are more susceptible" to the virus, and they were more mobile from April on, per the study.

The bottom line: "This suggests that additional interventions to adults aged 20-49, including rapid mass vaccination if vaccines prove to block transmission, could bring resurgent COVID-19 epidemics under control," the authors conclude.