WASHINGTON, March 24 (Xinhua) -- Nearly 40 percent of the COVID-19 patients hospitalized in the United States were younger than 55, according to a recent report of the Centers for Disease Control and Prevention (CDC).

Leading experts told Xinhua that potentially greater activity in the community, increased frequency of international travel, and contact with greater numbers of coronavirus carriers may have played a role.

The CDC report looked at COVID-19 cases in the United States from Feb. 12 to March 16. A total of 4,226 COVID-19 cases had been reported in the United States as of March 16, with reports increasing to 500 or more cases per day beginning March 14.

Among 508 patients known to have been hospitalized, 38 percent were aged between 20 and 54, according to the report. Besides, 9 percent were aged over 85, 36 percent aged between 65 and 84, and 17 percent aged between 55 and 64.

"The reason for younger adults making up a large percentage of coronavirus hospitalizations in the U.S. remains a matter of speculation, such as potentially greater activity in the community, increased frequency of international travel and/or contact with greater numbers of individuals who may be infected and/or carriers of the coronavirus," said Kent Pinkerton, professor of the University of California, Davis, School of Medicine, in an interview with Xinhua.

Stanley Perlman, professor of microbiology and immunology at the University of Iowa, believed it may have something to do with which age population is most often infected in different countries.

For instance, South Korea also had a large number of younger people being infected. Once they are infected, aged individuals and those with co-morbidities always have the highest case-fatality rate, Perlman told Xinhua.

Robert Schooley, professor of medicine at the Division of Infectious Diseases and Global Public Health at the University of California, San Diego, said the epidemic is still young in the United States and those who are exposed first may be people who are out and about.

As the epidemic gets more mature and these younger people take it home to their parents and grandparents, the average age might change, he told Xinhua.

"Fewer young people live with their parents and grandparents in the United States than in China, and it is possible that this mixing of younger and older people happened earlier in the epidemic in China than in the United States," Schooley said.

"Time will tell if this difference persists," he added.

According to the CDC report, of all the patients, 53 percent of intensive care unit (ICU) admissions and 80 percent of deaths occurred among adults aged over 65 with the highest percentage of severe outcomes among persons aged over 85.

"These findings are similar to data from China, which indicated over 80 percent of deaths occurred among persons aged over 60," said the report.

"We are fully aware older individuals as well as those with a compromised immune system and/or a pre-existing health condition are likely to be more susceptible to coronavirus infection," Pinkerton told Xinhua.

Preliminary data also demonstrated that severe illness leading to hospitalization, including ICU admission and death, can occur in adults of any age with COVID-19.

The CDC report recommended social distancing for all ages to slow the spread of the virus, protecting the health care system, and helping protect vulnerable older adults.

The number of COVID-19 cases in the United States topped 50,000 as of Tuesday afternoon, with 606 deaths, according to the Center for Systems Science and Engineering at Johns Hopkins University.