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This year’s flu season is off to a fast start and early indications suggest that it may be more severe than the previous season.

Four states currently have widespread flu activity, according to the Centers for Disease Control and Prevention. Last winter there were none at this time.

And it may be spreading fast. Dr. William Schaffner, who is involved in the CDC’s flu surveillance network in Tennessee, has noticed cases of influenza picking up "dramatically" in the last week.

Even worse, it appears the dominant strain will be H3N2, which can produce more severe illness, said Schaffner, an infectious disease specialist at the Vanderbilt University Medical Center.

To get an idea of what a flu season in the U.S. will look like, scientists like Schaffner observe what happens in Australia, which experiences winter and the flu while Americans are having summer. In the summer of 2017, the flu was diagnosed in more Australians than the previous season — 168,337 versus 91,000 — with H3N2 predominant.

How effective is the 2017 flu vaccine?

Unfortunately, the flu vaccine in Australia, which is the same one available this fall and winter in the United States, was only 10 percent effective in preventing illness from H3N2.

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“Typically in years when the predominant strain is H3N2, there are more hospitalizations, more severe disease and people tend to get sicker,” said Dr. Michael Ison, a professor of infectious disease and organ transplantation at Northwestern University’s Feinberg School of Medicine.

Related: Here's one reason flu shots are so lousy

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, suspects the vaccine won’t work any better in the U.S.

“It is possible that we will experience low vaccine effectiveness against influenza A (H3N2) viruses and a relatively severe influenza season if they predominate,” Fauci and colleagues wrote in an article published recently in the New England Journal of Medicine.

However, while vaccinated people can still get sick, generally they get a milder and less dangerous form of the illness.

Related: Most kids who died of flu weren't vaccinated, study finds

Also, the vaccine protects against other flu strains.

“The protection against the others is much higher — 50 to 70 percent,” Schaffner said.

Even if the flu vaccine is just 10 percent effect against H3N2, a flu shot is especially important for adults aged 50 and older. Because the influenza virus triggers an inflammatory response in the body, in the two to eight weeks after recovering, older adults have a three to five times increased risk of having a heart attack and a two to three times increased risk of having a stroke, said Schaffner.

H3N2 is known to cause a much more substantial inflammatory response than some of the other strains.

Related: Death of teen athlete a reminder flu can kill anyone

“We think of the influenza virus as attacking the throat, the bronchial tubes and the lungs,” he explained. “But it affects the whole body. It can involve the blood vessels, particularly those going to the heart and the brain. If they are inflamed they can clog up and that can produce a heart attack or a stroke.”

However imperfect the flu shots are, “current influenza vaccines remain a valuable public health tool, and it is always better to get vaccinated," Fauci and colleagues wrote in the Journal.

The CDC estimates that the flu shot prevented 40,000 deaths in the United States between the 2005-2006 and 2013-2014 seasons.