People in their 20s and 30 seem to be at high risk for getting very ill, in part because this year's main flu strain last circulated before they were born so they haven't built up antibodies to it.
This year's flu season appears to have peaked, but there are still plenty of weeks of sickness to come, medical experts say.
"We still have pretty high activity in most parts of the country; 41 states are reporting widespread flu," said Michael Jhung, a flu expert with the Centers for Disease Control and Prevention in Atlanta.
The percentage of people who went to doctors or clinics for flu-like symptoms last week was 3.4%, down from 3.6% the week before and 4.4% the week before that, according to the CDC's weekly FluView report.
"Since it's gone down several weeks in a row, we may have peaked," Jhung said.
However, there are pockets of the country that are still on the upswing, he said. Because the flu started this year in the South first, it's declining most quickly there.
"In the Northwest they're still seeing very high activity," Jhung said. "In the Northeast they're seeing increasing activity."
Although fewer people may be getting sick, this is the time in the flu season when more patients are hospitalized and more deaths begin to occur, as people who got sick earlier become more ill.
"You get infected; you get sick for a week; then you go to the doctor and then the hospital. There's always a lag of a few weeks," Jhung said.
So far 28 children have died from the flu this season, the CDC reported.
"I think a lot of people forget how serious flu can be until they're reminded of some severe outcomes in people they know," Jhung said.
Young adults seem to be at high risk. Patricia Cary, who directs the emergency department at Mount Sinai Roosevelt Hospital in New York, says she's been surprised "that we are seeing the occasional patient who is desperately ill with the flu —i n particular, young adults."
This year's predominant flu strain is H1N1, "which is similar to a flu strain from the 1950s and 1960s. So older adults have probably 'seen' it before and developed antibodies," Cary said.
People in their 20s and 30s, especially if they haven't been vaccinated, are sometimes having "serious respiratory illness because the strain is new to them," she said.
Cary's take-home message is, "It still makes sense to get vaccinated, even at this late date."
There is no national figure for the number of flu-associated deaths among adults. Instead, CDC tracks the cause of death in 122 cities across the United States. Last week 8.1% of adult deaths were due to pneumonia or influenza.
In the San Francisco Bay Area 27 people had died as of Thursday, local health departments reported. San Diego County reported 12 flu-related deaths as of Thursday.
Pregnant women and the obese are at high risk for complications, according to CDC data. Among adults who have been hospitalized for flu-related complications, 43% were obese.
Among women of child-bearing age who were hospitalized due to the flu, 23% were pregnant.