The flu has gotten off to an early and fatal start in the Bay Area and the rest of California, though whether this season will go down as more severe or longer lasting than usual remains to be seen, public health and infectious disease experts said.

As of Dec. 16, the date of the most recent report from the California Department of Public Health, 10 people under age 65 had died from influenza-related illness statewide. Typically, only one or two deaths, and sometimes none at all, have been reported in the same time frame. The state does not track flu-related deaths among people age 65 and older.

The higher-than-usual number of fatalities — plus other reports of increased influenza activity — does not necessarily mean this flu season will be worse, experts said. It’s possible the season is starting ahead of schedule, but otherwise will be normal.

“The flu season usually lasts about 12 weeks. Whether this will be a typical 12-week season or it will be a longer cycle — well, it kind of looks like it’s following the usual curve,” said Dr. Shelley Gordon, an infectious disease specialist with California Pacific Medical Center in San Francisco.

What has some experts concerned, though, are reports that this year’s flu vaccine is not offering good protection against the strain that’s circulating most widely: Type A, subtype H3N2.

H3N2 “tends to be the strain of virus that most impacts the elderly, that causes the most complications, and up until this point the vaccine results have been quite disappointing,” said Dr. Randy Bergen, clinical lead for Kaiser Permanente’s flu vaccination program in Northern California. “Those things make us concerned that we’re going to have a lot of sick people.”

Early reports in the United States are mimicking what public health officials saw in the Southern Hemisphere, where H3N2 was also the predominant strain. By the end of their flu season in September, several countries had reported the largest influenza outbreak since 2009, when the swine flu was a global threat.

In Australia, hospital admissions for influenza were more than double what is reported in a normal season, according to officials there. Deaths more than tripled, but some of that increase may have been due to discrepancies in how fatalities are counted.

The increase in deaths in Australia was not necessarily because the flu virus in circulation was more severe. It was just infecting more people, experts said.

“They just had a lot of cases — the most they’ve had since 2009, which was a pandemic year,” said Dr. David Relman, an infectious disease specialist at Stanford Health Care.

One reason for the high rate of illness was the lack of vaccine protection, public health officials said. Australian officials believe that the H3N2 strain mutated in a way that weakened the impact of the vaccine.

The vaccine — which protects against three or four different influenza strains — reduced the overall risk of flu infection by about 33 percent. But for the H3N2 strain, officials believe it reduced the risk by only about 10 percent.

Infectious disease experts do not believe the vaccine will work any better in the United States, because the same composition is being used. But that doesn’t mean it’s useless, doctors said.

It can still prevent people from being infected by other strains in circulation. And even a small percentage of protection could keep some people from contracting H3N2 — or lessen their symptoms if they do get sick.

“Do not be discouraged, do not think that it’s too late in the season or the vaccine is just no good. Neither is the case,” Relman said. “There is still more than enough time to protect yourself and the vaccine is much, much better than nothing.”

Nationwide, the flu season also seems to be starting a bit early, with elevated levels of hospitalizations and positive lab tests being reported in most states, according to the Centers for Disease Control and Prevention.

The Bay Area tends to experience a later flu season than the rest of the country, with reports of illness spiking in January. What will happen in 2018 is hard to predict, said Dr. Kim Erlich, medical director of infection prevention and control at Mills-Peninsula Medical Center in Burlingame.

“When it starts early, it often ends early, so there’s kind of a silver lining. But every year is different, and influenza is wildly unpredictable,” Erlich said. “We just sort of batten down the hatches and ride it out.”

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com Twitter: @erinallday

Getting vaccinated

This year’s flu vaccine is far from perfect, but it will still offer some protection. And it’s not too late to get vaccinated. To find a flu shot nearby, go to Vaccinefinder.org.