A study of 1,300 Northern California Kaiser patients who tested positive for the coronavirus last month found that nearly a third were hospitalized and almost 1 in 10 ended up in intensive care — and nearly as many young and middle-aged adults were admitted as people age 60 and over, according to results published online Friday.

The analysis is among the first large studies of people diagnosed with COVID-19 in the United States, and the first in California. More than 16,200 Kaiser patients across 21 hospitals in Northern California were tested for the coronavirus in March, and about 8% came up positive.

The study results were published in a brief letter in the Journal of the American Medical Association.

The purpose of the study was to help hospital administrators better understand the type of care patients with COVID-19 may need, which can help them prepare how many hospital beds, ventilators and other resources they may need for future outbreaks, the study authors said.

“The whole point was to try to understand on a population level what resources are needed during this pandemic,” said Dr. Laura Myers, lead author of the study and a Kaiser Permanente Division of Research research scientist.

But the age breakdowns in who was hospitalized were striking, Myers added. Across Kaiser’s Northern California medical centers, 377 people were hospitalized with the coronavirus in March, or 29% of all those who tested positive.

People age 60 to 69 made up a quarter of those patients — the single largest group. But people age 18 to to 59 made up 46% of all hospitalizations, compared to 54% for those 60 and older.

“We found that the most common age group was in their 60s, but there were patients hospitalized in their 30s and their 90s,” Myers said. “We really emphasize that this is a disease that is affecting all ages.”

About a third of hospitalized patients eventually were treated in the intensive care unit and almost all of them required mechanical ventilation. About half of the ICU patients died, the study found. Among all who were hospitalized, about 15% died. Fifty-six people remained hospitalized at the time the study was published.

Nearly 44% of hospitalized patients had been previously diagnosed with high blood pressure and 31% had diabetes. The most common symptoms they experienced when seen in the emergency department were shortness of breath, fever and a cough.

The Kaiser results align roughly with a study published in late March by the Centers for Disease Control and Prevention, which found that about 30% of all COVID-19 patients were hospitalized and 5% to 12% were treated in intensive care. The CDC study also showed serious illness requiring hospitalization across all age groups.

Earlier studies in China and Italy had reported that older adults were more at risk of serious illness than younger people.

“This is not exclusively a disease of people older than 65 as was suggested by the data out of China,” said Warner Greene, a senior investigator with the Gladstone Institutes who was not involved with the Kaiser research.

Myers added that the Kaiser results, along with other research of U.S. COVID-19 patients, suggests that public health policy, especially when it comes to social distancing measures, should not focus narrowly on older adults.

“On a public health scale we have to target the whole population, and not just patients in their 60s, or patients over 65, because that’s kind of an arbitrary cutoff,” she said. “Everyone is being affected.”

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com