The number of people in the United States who have received their first dose of a COVID-19 vaccine reached 27.1 million on Tuesday, surpassing the country's cumulative number of confirmed COVID-19 cases to date, according to a report from the U.S. Department of Health and Human Services.

In total, 8% of the U.S. population has received one or more vaccine doses.

Just over 26.5 million people have tested positive for COVID-19 in the U.S. since the start of the pandemic, according to data complied by Johns Hopkins University.

ABC News' Josh Margolin and Brian Hartman contributed to this report.

Despite older people being more likely to suffer severe illness or death, individuals between the ages of 20 and 49 were the ones responsible for 72.2% of COVID-19 spread in the United States last year, according to a new study from researchers at England's Imperial College London.

The study, which was published Tuesday by Science, the peer-reviewed academic journal of the American Association for the Advancement of Science, also found that adults aged 35 to 49 were responsible for more than 41% of spread in the U.S. in 2020. Imperial College London researchers believe this was not the result of schools reopening, but more likely because of the amount of movement in the community.

Less than 5% of COVID-19 cases in the U.S. originated from children aged 9 and under, while less than 10% originated from kids aged 10 to 19, according to the study.

ABC News’ Eric Strauss contributed to this report.

Switzerland's drugs regulator has declined to authorize a COVID-19 vaccine developed by England's University of Oxford and British-Swedish pharmaceutical giant AstraZeneca because the data that’s been submitted and analyzed so far "are not yet sufficient."

"To obtain more information about safety, efficacy and quality, additional data from new studies are needed," the Swiss Agency for Therapeutic Products, also known as Swissmedic, said in a statement Wednesday.

"The data currently available do not point to a positive decision regarding benefits and risks," the agency added. "To obtain a conclusive assessment, the applicant will among other things have to submit additional efficacy data from a Phase III trial under way in North and South America, and these will have to be analysed. As soon as the results have been received, a temporary authorisation according to the rolling procedure could be issued at very short notice."

Switzerland joins a growing list of European countries expressing skepticism over the amount of available trial data and the vaccine efficacy among older age groups. On Tuesday, health officials in both Belgium and France announced that, pending additional data, their countries won't recommend the Oxford/AstraZeneca vaccine to anyone over the age of 55. Last week, Germany's vaccine commission said the shot should not be administered to people older than 65, citing "insufficient data."

Meanwhile, the U.S. Food and Drug Administration has said it wants to see the results from a large, ongoing trial in the United States before authorizing the Oxford/AstraZeneca vaccine.

A COVID-19 vaccine developed by England's University of Oxford and British-Swedish pharmaceutical giant AstraZeneca not only protects people from serious disease and death but also may have a substantial effect on transmission of the virus, according to a new study that has not been peer-reviewed.

In a preprint published Tuesday, which is currently under peer review at the medical journal The Lancet, University of Oxford researchers said an analysis of further data from ongoing clinical trials of the two-dose vaccine shows a 67% reduction in positive swabs among those who received a single dose.

It's the first study to document evidence that any COVID-19 vaccine can reduce transmission of the virus.

Researchers said the analysis also reveals the vaccine efficacy is higher at longer prime-boost intervals and that a single dose of the vaccine is 76% effective from 22- to up to 90-days post vaccination, according to the manuscript.