The flu has reached epidemic levels in the United States, with 15 children dead so far this season, the federal Centres for Disease Control and Prevention reported on Tuesday.
But it's too early to know if this year's strain is more severe than usual, CDC says.
Read: Flu (influenza)
Every state will likely have flu cases within the next few weeks, and more deaths are expected, said Dr. Michael Jhung, a medical officer in CDC's influenza division.
Every state affected by flu
"We are in the middle of flu season," Jhung said. "It's a safe bet that we are going to see flu activity continue to increase for a few more weeks. We are going to see every state in the country affected by flu."
Flu reaches epidemic levels in the United States every year, Jhung said. Whether this flu season will be more severe or milder than previous ones won't be known until April or May, he noted.
The number of children's deaths from flu varies by year, Jhung added. "In some years we see as few as 30, in other years we have seen over 170. To have 15 reported at this point in the season is, unfortunately, not unexpected," he said.
Whether more children are dying of flu this year won't be known until the end of the flu season, around March or April, Jhung said.
The South, Midwest and Western states have been especially hard hit this flu season. At least six children have died in Tennessee and four in Minnesota, according to published reports.
The predominant flu strain this season is the H3N2 virus, the CDC says. This virus is not well-matched to this year's flu vaccine, but what part this mismatch is playing in flu deaths isn't known, Jhung said.
The CDC recommends that everyone 6 months and older get a flu shot. It's not too late to get vaccinated, Jhung said. More than one type of flu is circulating, and the vaccine protects against at least three strains of circulating virus, he added.
"If you encounter one of those viruses where there is a very good match, then you will be well-protected," he said.
"Even if there isn't a great match, the vaccine still provides protection against the virus that's circulating."
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People at risk of flu-related complications include young children, especially those younger than 2 years; people over 65; pregnant women; and people with chronic health problems, such as asthma, heart disease and weakened immune systems, according to the CDC.
Common flu symptoms can include fever, chills, cough, sore throat, muscle aches and fatigue. Vomiting and diarrhoea are seen more often in children with flu than adults.
Most people recover from flu anywhere from a few days to a bit less than two weeks. But others suffer life-threatening complications, such as pneumonia, according to the CDC.
Parents should take flu seriously, and get medical help if they feel their child is very sick, Jhung said.
"People should have a low threshold and contact the health care system, whether that means going to the emergency room or calling their doctor," he said. "That should be done early."
Recognise warning signs
Warning signs might include a cough that disrupts sleep, a fever that doesn't come down with treatment, or increased shortness of breath, according to the U.S. Food and Drug Administration.
Adults and children with bad flu can be treated with antiviral medications, such as Tamiflu (oseltamivir) and Relenza (inhaled zanamivir), Jhung said.
"Those work best when they are given very quickly. So if you do have signs and symptoms of flu, reach out to a health care provider and get evaluated," he said.
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Flu seasons are unpredictable, according to the CDC. Each year, on average, 5 percent to 20 percent of the U.S. population gets the flu and more than 200,000 people are hospitalised from complications.
During a 30-year period, from 1976 to 2006, estimates of flu-related deaths in the United States ranged from a low of about 3,000 to a high of about 49,000 people, the agency said.
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