A sign advertising flu shots hangs in a Walgreens Pharmacy. Public health officials are encouraging residents to get flu shots as an aggressive strain of the H1N1 "swine flu" has killed 15 people in the San Francisco Bay Area. (Justin Sullivan/Getty Images)

The H1N1 virus responsible for the 2009 global pandemic is back. State health officials from across the country say the resurgence is resulting in a dramatic rise in flu deaths in young and middle-aged adults and in children this season.

While the reported death tolls so far are only a fraction of what they were four years ago, they are significantly higher than last year. The Centers for Disease Control and Prevention said the flu has been killing at epidemic levels since mid-January.

With one month to six weeks to go in the flu season, which typically ends in March or April, the CDC said the number of people visiting doctors and hospitals for flu-like symptoms is declining overall, but some states are continuing to see high levels of flu activity or even increases in activity. Although the flu usually disproportionately affects the very old and the very young, this season 60 percent of those hospitalized for influenza have been age 18 to 64.

“These severe flu outcomes are a reminder that flu can be a very serious disease for anyone, including young, previously healthy adults,” CDC spokesman Jason McDonald said.

H1N1, which is also known as the “swine flu” because it was originally a respiratory illness in pigs, has been popping up in some patients seasonally for the past few years, but this is the first flu season since the 2009 pandemic in which it has been circulating so widely.

The outbreak has been especially severe in California. There have been 243 deaths of residents younger than 65 so far this year. An additional 41 cases were reported but have not been confirmed. In the 2012-13 season, there were 26 deaths by this time, and in the 2011-12 season there were nine deaths. In the 2009-10 season, there were 527 deaths.

Surveillance reports from the health departments in Virginia and Maryland show that the flu is widespread in the region, but the two states and the District of Columbia do not track adult deaths from the flu. The District has seen a surge in flu cases in the past month, with 90 percent being H1N1. Virginia reported that one child died from flu this season, while Maryland and the District had no child deaths.

In the San Francisco Bay Area, some hospitals have been so inundated with patients complaining of flu-like symptoms that triage tents have been set up on their lawns to prevent them from spreading the virus to others in the medical centers. In Sacramento, intensive care units are overflowing with those with breathing issues, water in their lungs, organ failure or other complications from the flu.

Online, residents are swapping stories via social media of people who have died of the flu, and doctors and public officials are seizing on the panic to urge the unvaccinated to get a flu shot immediately. (It takes about two weeks after the shot for the antibodies to develop.)

The death of Nancy Pinnella, a 47-year-old sales manager who worked at Sacramento’s News10, an ABC affiliate, has served as a cautionary tale to many. Pinnella left work Jan. 21 saying that she wasn’t feeling well, was hospitalized the next day and died three days later. Family members told News10 that Pinnella was in great health before she got the flu and did not get a flu shot.

Her story has resulted in an outpouring of sympathy from around the world. California’s first lady, Anne Gust Brown, wife of Gov. Jerry Brown (D), tweeted that she went to CVS and got her first flu shot ever “after reading the heartbreaking story of Nancy Pinnella.”

North Carolina also appears to be looking at a possible record year for flu deaths. The number of deaths stands at 64. Last year, the state had 59 deaths the whole season, and in 2012 it had only nine.

View Graphic Watch the spread of the flu across the U.S. each week.

In a study of Duke University Medical Center patients published this month, researchers found that those hospitalized for the flu between Nov. 1 and Jan. 8 were much younger — with an average age of 28.5 years — and more likely to have serious complications than those who had H1N1 in the past. About 40 percent of the patients this year ended up needing intensive care, compared with 20 percent in 2009.

“We don’t know why, but it is worrisome,” said Jelena Catania, an infectious diseases fellow at Duke and a co-author of the study, published in the American Journal of Respiratory and Critical Care Medicine.

Laurie Forlano, deputy state epidemiologist for Virginia, said that although it’s too early to draw any conclusions, there are indications that the population being affected in her state is also skewing toward young adults. She said that H1N1 was included in this year’s flu vaccine, so some of those who are coming down with the flu may not have gotten the vaccine. The vaccine’s efficacy rate is usually in the 50 to 70 percent range.

“It’s never perfect, but for some people, getting the vaccine is a matter of life and death,” she said.

Scientists have been working on a universal flu vaccine, which would provide long-term protection and remove the need to get one every year, but even the most optimistic say such a product is years away.

Meanwhile, the severity of this year’s flu is renewing the controversy over mandatory flu vaccinations.

In Rhode Island, the state has proposed a regulation that would require annual flu vaccines for children up to age 5 and would require those with exemptions to stay out of day care during outbreaks. Opponents, which include the American Civil Liberties Union, say parents should have the right to choose the best medical treatment for their children. A similar debate took place in New York City in December, when the board of health voted in favor of a mandatory vaccine for children younger than 6.

The reemergence of H1N1 in the United States comes as even more virulent strains that are combinations of several genetic strains begin to appear around the world.

In recent months, the World Health Organization has been tracking more than 300 cases, mostly in China, of people infected by a dangerous avian influenza strain, H7N9. A quarter of those infected are estimated to have died, but so far the WHO says there is no evidence of sustained human-to-human transmission.

This month, there was more alarming news: Chinese officials, writing in the journal Lancet, said they identified yet another brand-new bird flu, H10N8, in a 73-year-old woman in Nanchang, a city in the southeastern part of the country. Researchers hypothesize that the woman, the first known death from this strain, may have contracted the virus while at a poultry market. The scientists warned that the virus could become extremely dangerous if it developed the ability to be transmitted from human to human.

“The pandemic potential of this novel virus should not be underestimated,” the researchers concluded.