H1N1 flu strain has returned with a vengeance

Jan. 29--When the state released its most recent report Friday on the flu situation in California, one number stood out as particularly alarming: 95 deaths. That's the total of deaths reported to the state since the flu season started in October.

By this time last year, only nine deaths had been blamed on the flu.

But this season may not be as deadly as the report would have people believe, public health experts say. As striking as 95 deaths might seem, that number is just one piece of a larger flu puzzle that's complicated and still largely unsolved.

"This year seems to be shaping up to be a more severe flu season, but until it's over we're not going to know how much more severe," said Dr. Cora Hoover, director of communicable disease control and prevention for the San Francisco Public Health Department.

"If we just look at deaths, that's really the tip of the iceberg," she said. "Most of the iceberg is illness that is disruptive and keeps people away from normal activities for a few days, but it's not, thank goodness, life-changing."

Here's what we know about this year's flu season:

The main virus in circulation is a strain of influenza known as Type A, subtype H1N1 -- known informally as the swine flu. It's the same strain that caused the 2009 outbreak that began in Mexico in April that year and swept around the globe in just a few months.

Mystery H1N1 surge

The biggest mystery about the current flu season may be why H1N1 has resurfaced. It's been circulating every year since 2009, but other strains have been dominating until this year.

No one knows why H1N1 is surging now.

The strain is known to target people younger than 65, which is why the death toll for this year seems unusually high -- California only collects data on flu deaths in that younger population. Older people are usually much more susceptible to the flu, and tracking their deaths would be a monumental, and expensive, task.

Because the state doesn't keep strict records on total deaths from the flu, it's impossible to say whether this season is more or less deadly than any other season. In fact, records of deaths in adults under 65 only go back two years -- the state began requiring counties to report those cases in 2011 -- so no one can say just how bad a season this is even for middle-aged or young adults.

Statewide, outpatient visits and hospitalizations for the flu are higher than usual for this time of year, although that may be because the season is starting, and peaking, earlier than usual.

In last week's state flu report, outpatient visits fell slightly from the previous week -- possibly the first sign that the season has peaked, said Dr. Gil Chavez, deputy director of the Center for Infectious Diseases with the California Department of Public Health.

Even so, the state's looking at another several weeks of flu activity, and it's not unusual for a second, smaller wave of flu -- often from a Type B strain of the virus -- to hit in March or even April.

Vaccine still relevant

The point being, people should still get vaccinated if they haven't already, said Dr. Lyn Finelli, head of flu surveillance and outbreak response at the Centers for Disease Control and Prevention.

"The peak is the peak, so that means only half of the flu season has taken place," Finelli said. "We think it's going to continue to go on for several more weeks, if not four or six weeks."

The flu season technically starts in October, when the CDC and states begin collecting surveillance data.

This year, the first signs of flu came hard and fast in mid- to late-December, with a sudden uptick in outpatient doctor visits and hospitalizations. On Christmas Eve, the CDC issued an alert to public health departments: analyses of specimens taken from people who were sick showed that the dominant strain in circulation was H1N1.

That was an early and important clue to state and regional public health officials, who knew from the 2009 pandemic that the H1N1 strain behaves differently from other types of influenza in that it tends to target people younger than 65.

It's not necessarily a more severe strain of flu, infectious disease experts say.

"In fact, it's thought to be less virulent than other strains, and mortality is actually very low," said Dr. Charles Chiu, head of the viral diagnostics laboratory at UCSF. "The best estimates are mortality of about 0.02 percent. Your usual seasonal influenza is probably around 0.2 percent -- 10 times that. It's just that this one has a predilection for young and middle-aged adults."

This year in California, counties began reporting the first fatalities in under-65 people in early January, and since then, the number of deaths confirmed by the state has climbed every week.

Theories abound as to why H1N1 tends to afflict younger-than-expected adults, but no one has a clear answer. The most likely explanation is that older adults were exposed to a similar strain -- another type of H1N1 -- that was the primary flu in circulation from about 1918 to 1957. After 1957, a different strain took over.

Infectious disease experts believe the exposure gave people born in 1957 or earlier some leftover protection against H1N1. They're less likely to become infected at all, and when they do get sick, they'll probably have less severe symptoms.

If the theory is correct, that means people in their mid- to late-50s also probably have some protection, which seems to be supported by data on death and infection rates this year.

"When we had the pandemic in 2009, we saw that people over age of 50 or so, born in '57 and earlier, had immunity," said Finelli with the CDC. "They were exposed to a similar strain in their very early years, which seems to be very important to immunity -- the first ones you're exposed to, you build the best immunity to."

The rest of the population, meanwhile, has no natural immunity to this strain of H1N1 because it's never been exposed to it.

During the 2009 pandemic, the virus hit school-age children and young adults especially hard, with infection rates of nearly 70 percent, according to the CDC. Interestingly, that means many of those children -- now between ages 8 and 25, or so -- have some immunity to H1N1.

It's the adults from 25 to 50 who are getting sick now. They also tend to have the lowest vaccination rates -- as few as 20 percent of working-age adults get flu vaccines, according to some reports -- which puts them further at risk.

95 flu deaths

Of the 95 deaths in California so far, three were children under age 10, but most have been adults in their 20s, 30s or 40s.

Most, but not all, of the deaths have been people with some other condition that put them at additional risk for getting a severe case of flu. Those risks, some of which are unique to H1N1, include pregnancy, heart disease and lung disease.

Surprisingly, simply being obese, even if a person has no other underlying health problems, is also a risk factor for serious illness from the H1N1 strain.

"There seems to be some immunologic function related to obesity," said Dr. Shelley Gordon, chairwoman of infection control at California Pacific Medical Center. "We saw it during the 2009 pandemic, and it seems to be repeating this year. At least at CPMC, the really bad cases in our intensive care unit have all been obese."

Doctors at Bay Area hospitals said they've already started to see a slight decrease in patient visits due to the flu, reflecting similar data statewide. That may mean the flu season is starting to wind down.

But Chiu and other infectious disease experts said it's likely that H1N1 will be a regular player in the annual flu season.

"It's very likely from now on we'll be seeing 2009 H1N1 come back year after year after year," Chiu said. "It's concerning because it has this predilection for young adults. But that just means it's really important to emphasize vaccination."

Erin Allday is a San Francisco Chronicle staff writer. E-mail: eallday@sfchronicle.com

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