Many of the patients with COVID-19 now arriving at the hospital are not just unvaccinated — they are much younger than 50, a stark departure from the frail, older patients seen when the pandemic first surged last year. In Baton Rouge, La., young adults with none of the usual risk factors for severe forms of the disease — such as obesity or diabetes — are also arriving in E.R.s, desperately ill. It isn’t clear why they are so sick. 

Physicians working in COVID hot spots across the nation say that the patients in their hospitals are not like the patients they saw last year. Almost always unvaccinated, the new arrivals tend to be younger, many in their 20s or 30s. And they seem sicker than younger patients were last year, deteriorating more rapidly. Doctors have coined a new phrase to describe them: “younger, sicker, quicker.” Many physicians treating them suspect that the Delta variant of the coronavirus, which now accounts for more than 80 percent of new infections nationwide, is playing a role. 

Studies done in a handful of other countries suggest that the variant may cause more severe disease, but there is no definitive data showing that the new variant is somehow worse for young adults. Some experts believe the shift in patient demographics is strictly a result of lower vaccination rates in this group. As of Sunday, more than 80 percent of Americans ages 65 to 74 were fully vaccinated, compared with fewer than half of those ages 18 to 39, according to figures from the Centers for Disease Control and Prevention. 

The vaccines are powerfully effective against severe illness and death after infection with any variant of the virus, including Delta. A vast majority of hospitalised patients nationwide — roughly 97 percent — are unvaccinated. “I don’t think there’s good evidence yet about whether it causes more severe disease,” Dr. Adam Ratner, associate professor of paediatrics and microbiology at the New York University Grossman School of Medicine, said of the Delta variant. “This may be behavioural — a combination of the fact that we’re opening things up, and in some places they’re wide open and there are no masks, which is different from a year or 15, 16 months ago,” he added. But recently the Delta variant has offered up a series of unpleasant surprises to scientists, and questions about the variant’s virulence and capacity to cause more severe disease are taking on new urgency. 

An internal C.D.C. document obtained last week by The New York Times described the Delta variant as being as contagious as chickenpox and said it “may cause more severe disease than Alpha or ancestral strains.” People ages 65 and over represented half of all hospitalised patients at the end of January, while adults under 50 represented 22 percent, according to the C.D.C. Now, older people make up just over a quarter of hospitalised patients, while those ages 18 to 49 account for 41 percent. “Something about this virus is different in this age group,” said Dr. Catherine O’Neal, chief medical officer at Our Lady of the Lake Regional Medical Center in Baton Rouge, La. “We always saw some people who we just said, ‘Why the heck did this get them?’ But that was rare. Now we’re seeing it more commonly.” “I think it is a new COVID,” she added. Research in Singapore, which is to be published in The Lancet, concluded that patients with the Delta variant faced higher odds of requiring oxygen, needing intensive care, or dying. And a study in India, which was also posted online and not yet peer-reviewed, found that in the second wave of infection, when the Delta variant was dominant, patients faced a greater risk of dying, especially those under the age of 45.