Health experts say the one thing that's predictable about the flu is its lack of predictability.
They can't forecast which strain will strike hardest, when it will arrive, how long it will stay, or how deadly it will be.
That couldn't be more true this year.
H1N1 is the flu virus that since December has killed at least 95 people statewide and more than 20 in the central San Joaquin Valley.
The ferocity of the virus has made this the most severe flu season in five years, local health officials say. And the Valley's dry winter could be contributing to the flu's spread, one expert suspects.
H1N1 — the same strain as the 2009 H1N1 influenza pandemic that spread worldwide — has been circulating in the air for the past five years. A combination of avian, human and swine viruses, it was dubbed "swine flu" when it was first detected. Flu experts thought it would be part of the mix of viruses again this year, but no one foresaw it breaking out of the pack to take control this winter.
Nationwide for the week of Jan. 12-18, the virus was 96.8% of the influenza A viruses tested. H3 viruses were a small minority at 3.2%, according to federal Centers for Disease Control and Prevention.
Flu experts admit to being caught off guard: "It's fair to stay if you called 10 or 12 flu experts, if they're being honest, they would all say, 'We're surprised at the extent of H1N1 this year,' " said Dr. William Schaffner, a professor of preventive medicine and an infectious diseases specialist at the Vanderbilt University School of Medicine in Tennessee.
State officials said they're stumped to explain the resurgence of the virus. "We really don't know what causes different viruses to emerge and one to predominate over another," said Dr. James Watt, the California Department of Public Health's chief of the division of Communicable Disease Control at the Center for Infectious Diseases. "Each year multiple viruses are circulating, usually four, and sometimes one dominates and sometimes another."
But H1N1 is behind the surge in flu deaths in the state. On Friday, California health officials said 95 flu deaths have been reported and another 51 were under investigation and likely to be confirmed this week. Last year at the same time, the state had recorded only nine flu-related deaths, and by the end of last season only 106 deaths had been recorded.
Valley health officials have said H1N1 has been confirmed in all but one of 22 flu-related deaths reported this season.
Fresno County has had 13 flu deaths, with 12 of them caused by H1N1. Kings and Merced counties have had three deaths each, Madera County has had two and Tulare County has had one.
Schaffner said California's drought could be spurring the spread of influenza.
The flu doesn't strike every part of the country simultaneously and weather patterns could be a factor. Flu season has been mild in New England, which has been slammed by frigid temperatures and snow, he said. But in dry California, it's a different story. Flu is widespread here.
Low humidity helps in the spread of the flu virus, Schaffner said. When flu viruses are expelled through a cough, they float in the air surrounded by a small membrane of water vapor. In high humidity, the flu particle is heavier and drops to the ground, but in low humidity, the virus hangs longer in the air. The longer the virus is suspended, the more chances for it to be inhaled by others, he said.
Outpatient visits to doctors' offices and hospitalizations are exceeding expected levels, state officials said Friday. And although there was a slight decrease from the previous week, that's no guarantee flu is at its peak, they said.
This flu season, however, is nowhere near the levels reached during the 2009 pandemic, Watt said. Then, thousands died nationwide — 607 people in California.
Disease levels for H1N1 this time around likely are lower because so many people got flu shots during the pandemic, Watt said. And since then, the vaccine has included the virus, giving people a chance to develop some immunity. This year's vaccine provides protection, and county health departments have vaccine, he said.
This year, as in 2009, what remains a mystery is why so many of the severe flu cases and deaths are younger adults.
In Fresno County, most of the deaths have been men and women in their 30s, 40s and 50s. The average age: 48.7 years. None of the deaths have been children. Statewide, three deaths have been children younger than 10. Riverside, San Mateo and Los Angeles counties have reported one child death each.
One possibility why younger adults are at greater risk: They don't get flu shots — at least not at the rate of children or the elderly. According to the CDC, in 2012-13, less than a third of adults ages 18 to 49 got the vaccine and less than half of those ages 50 to 64. In comparison, more than two-thirds of seniors 65 and older got flu shots and more than three-quarters of babies got the vaccine.
It's not clear how many of those who have died this year had gotten flu shots. But of those with known vaccine records, 80% of those who died were unvaccinated this flu season, state health officials said.
Some experts also theorize that elderly people, exposed to viruses in years past, have some baseline immunity to H1N1, Watt said.
The state's reported flu deaths, however, don't capture the entire story. No one keeps track of how many elderly have died. Only flu-related deaths of people younger than 65 are recorded in California. Nationwide, only pediatric deaths are tallied.
Since the 2009 pandemic, California has required hospitals to report the deaths of younger adults and children. The flu tends to be more severe for people with chronic health conditions and many of the elderly have health problems, so the state decided that measuring flu-related deaths of younger adults would be a better marker of influenza activity and severity, said Dr. Gil Chavez, the state's epidemiologist and deputy director of the Center for Infectious Diseases.
Of the younger adults who have died this flu season, 80% to 90% had some health problem that put them at higher risk of death, Chavez said. People who are smokers and obese are at higher risk of complications, as well as pregnant women and people with lung, heart and immune system problems.
But it's still unknown why there's a higher level of fatalities among even healthy younger adults, Watt said.
And influenza raises a lot of other questions. Very little is known about the vast majority of infections, because most are mild cases that never come to the attention of doctors, Watt said. "We're just looking at the tip of the iceberg with the very severe cases."
There could be a new virus to fight next flu season.
Flu viruses, unlike measles for example, mutate easily to create variations in strains. In contrast, the measles virus has been stable genetically for years worldwide, Schaffner said. "The measles in my childhood is the same as you would find in Nigeria today."
Change is the only constant with influenza, Schaffner said. "The quip is: Flu is fickle."
Flu deaths in the central San Joaquin Valley
As of Friday:
Fresno County: 13
Kings County: 3
Madera County: 2
Merced County: 3
Tulare County: 1
Source: County health departments
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