This flu season ranks as California's deadliest in four years, according to state public health officials, who say it's now mostly over.
The flu killed at least 355 people younger than 65 since the flu season began in late September, officials said. That's at least three times the number of Californians in that age group who died during the flu season last year.
Public health experts for months have attributed the spike to a resurgence in the 2009 "swine flu" strain, known as H1N1, which at the time killed 539 Californians under age 65.
"Otherwise healthy young adults who usually do pretty well with flu strains don't do so well with H1N1," said Dr. Sara Cody, Santa Clara County's health officer.
State health officials, who expect the flu season to officially end by mid-May, say the majority of those younger than 65 who died in the last seven months were between 40 and 64 years old. And at least seven were under age 18.
Experts say the H1N1 flu bug hits younger generations harder because they often go without vaccinations or may have not acquired immunity through previous exposure.
State and local health officials say they don't track flu deaths in people 65 and over because flu deaths are so much more common in that age group. So tracking flu deaths in people under 65 is a better indicator of the severity of influenza in any given year.
Why is the swine flu so deadly?
Some scientists think the genetic structure of H1N1 targets the lungs, while other common strains tend to attack the upper respiratory system. A mutation in an amino acid might allow H1N1 to bind more effectively to lung cells, making people more susceptible to pneumonia and death.
After catching the H1N1 flu virus in 2009, which took him five to six weeks to shake, Oakland accountant Randy Thornally picked it up again in mid-November, even though he'd been vaccinated two weeks earlier.
Memories of the 2009 virus -- difficulty breathing and excessive coughing that kept him up all night -- taught him a lesson. Instead of waiting weeks to see his doctor, he got treated right away.
"Is it bad luck?" asked Thornally, 63, of his two bouts with H1N1. "I don't know."
While Los Angeles County has led the state with 64 reported flu-related deaths, Santa Clara County reported 19, the highest in the nine-county Bay Area, followed by Alameda County with nine and Contra Costa County with seven.
Several possible flu-related deaths are still being investigated, and many of those who died were believed to be unvaccinated, state health officials said.
Fewer than a third of 18- to 49-year-olds in the United States get an annual flu vaccine, said Jason McDonald, a spokesman with the U.S. Centers for Disease Control and Prevention. And just under half of all Americans get a flu shot each year.
McDonald said the H1N1 virus was the dominant strain from 1918 through 1956. It resurfaced in 1977 and again in 2009 in slightly different forms. That's why anyone age 58 or older by now has probably built up an immunity, he said.
Dr. Erica Pan, Alameda County's deputy health officer, said flu strains never really disappear.
"H1N1 has been circulating every year, and it's been in our vaccines ever year since 2009," Pan said. "It causes infection every year, but it did not dominate the way it did this year."
State lab tests indicate that the influenza season began exploding in late December, several weeks earlier than last year and six weeks earlier than the 2011-12 season. And by late January, the flu season had peaked.
"Flu is unpredictable," said Paul Leung, immunization program manager at the Contra Costa County Public Health Department, who added that while flu cases have declined significantly, they're "not completely absent."
California generally sees an increase in cases in late December or early January and often peaks in February or March. But Alameda County's Pan noted that the state's last swine-flu pandemic started in April 2009 and ended 16 months later in August 2010, after at least two waves of the virus hit Californians.
It's why Leung and Pan urge anyone who still has not received this year's vaccine -- which expires June 1 -- to get one immediately. The shot will last at least through September, they said, about the time that next year's vaccine will become available. Because flu viruses constantly change, Leung and Pan said, the 2014-15 vaccine will be different from this year's.
Under the new health care law, flu vaccines are now free to anyone with health insurance. For those without insurance, the shot is available at local pharmacies but not for long.
Bay Area News Group writer Lisa M. Krieger contributed to this report.
BAY AREA FLU-RELATED DEATHS
At least 65 people under age 65 have died from the flu in the nine-county Bay Area and two Central Coast counties since September.
Alameda County: 9
Contra Costa County: 7
Marin County: 2
Monterey County: 5
Napa County: 1
San Francisco County: 4
San Mateo County: 6
Santa Clara County: 19
Santa Cruz County: 3
Solano County: 3
Sonoma County: 6
Source: California Department of Public Health, county health departments.