Nearly half of all adults hospitalized nationwide for flu so far this season have been obese, much higher than in other recent flu seasons, according to the Centers for Disease Control and Prevention.
Forty-six percent of adults hospitalized for influenza as of Jan. 4 were obese.
Joseph Bresee, a CDC influenza expert, cautioned that the number may change over time but agrees that it is high.
"I think we're seeing the same sort of pattern emerge we saw in 2009," he said.
The reported percentage of obese patients hospitalized is "a high number," said Zack Moore, a medical epidemiologist with the North Carolina Division of Public Health in Raleigh, N.C. "In previous years it's usually in the 20s and 30% range."
This season's flu is also hospitalizing more pregnant women than normal. So far this season 22% of hospitalized patients are pregnant, whereas 4.6% is the norm, Bresee said.
Researchers aren't entirely sure why these groups may be so strongly at risk, but believe it might be linked to immunological effects, said Bresee. Both obesity and pregnancy are known to alter the immune response. They also both tend to result in respiratory restrictions.
The most common complication that sends people with the flu to the hospital is pneumonia," said Lisa Winson, infectious disease physician with San Francisco General Hospital. Some of her patients have needed "to go to the intensive care unit and be on a ventilator."
In children, severe flu can result in encephalitis, when the infection reaches their brain. That can cause inflammation, seizures and convulsions.
Her hospital has not begun to restrict visitors coming to see patients, but is considering it. Several hospitals nationwide have put visitor restrictions in place. "Our nurses are screening visitors," she said.
The flu has been seen in every state, with widespread activity in 35 states, CDC reported Friday. That's 10 more than the previous week.
This year is pretty typical for the flu, which often peaks in January or February. It's still not too late to get vaccinated, Bresee said. Immunity begins to build very quickly, though it's not fully in effect for two to three weeks.
The flu's highest activity levels are in 14 states, mostly in the South and Southwest, according to CDC.
Flu season is by no means over, Bresee said. "I expect this will go on for quite a few more weeks longer. The average flu season is 10 to 12 weeks, and we're only a few weeks into it now."
The vast majority of flu cases in the nation are with the H1N1 flu strain, which caused an international pandemic during the 2009-2010 flu season. The CDC estimated that 8,870 to 18,300 H1N1-related deaths occurred during that outbreak in the USA.
This year's flu strain seems to affect people between the ages of 18 and 64 most strongly. Sixty-one percent of people hospitalized for the flu were in that age range, even though the people most likely to fall ill with the flu were under 4 or 65 and older.
The most commonly reported underlying medical conditions among adults hospitalized with the flu were obesity, metabolic disorders, cardiovascular disease and asthma, according to CDC.
Overall, this year's flu strain seems to be hitting younger people, pregnant women, people with chronic disease and the obese hardest.
Nationally, 10 children have died from the flu so far, according to the CDC.
States are only required to report children's deaths from the flu, so there are no national statistics for adults. However, North Carolina began requiring doctors to report adult deaths in 2009, so its statistics offer a glimpse into what groups the flu is affecting most strongly.
"This year we're seeing a lot of deaths in young adults and middle-aged adults more than older adults," said Moore.
So far this flu season, 21 adults in the state have died. Only two of them were over 65.
That compares with a total of 59 adults who died in North Carolina during last year's flu season, the vast majority of whom were over 65.
"So the big difference this year is not so much the numbers as the groups affected," Moore said.
The H1N1 flu variant first emerged in 2009. No flu variants similar to it had circulated since 1958, when it killed an estimated 69,800 people in the United States.
"That's probably why we don't see it in the elderly as much because people born before then have some residual protection. But you can't rely on it - they still need to be vaccinated," Moore said.
While H1N1 has continued to circulate worldwide since 2009, this is the first North American flu season since then in which it's been the main circulating strain.
Elizabeth Weise, USATODAY