Ana Martinez, a medical assistant at the Sea Mar Community Health Center in Seattle, gives a patient a flu shot last month. (Ted S. Warren/AP)

This year's flu season has now sent more people to the hospital with the illness than in nearly a decade, federal health officials said Friday. Nationwide, during the fourth week of January, 16 more children died of the virus.

So far this season, influenza has caused the deaths of at least 53 children, rivaling the death toll of the especially severe 2014-2015 flu season. Eighty percent of the children had not had a flu shot.

The unrelenting pace of infection is unusual, officials said. High levels of flu activity are widespread across the country, and over the past week, the rate of patients seeking care for flulike symptoms again rose sharply — nearing rates seen during the swine-flu pandemic in 2009, according to the latest report from the Centers for Disease Control and Prevention.

“As of this week, overall hospitalizations are now the highest we've seen” in nearly a decade, Acting Director Anne Schuchat told a briefing for reporters. Schuchat was placed in charge of the agency Wednesday after Brenda Fitzgerald resigned from the top job because of conflicts over her financial interests.

In her remarks, Schuchat said the personnel change will not detract from CDC’s mission to protect the health, safety and security of Americans, including from the flu, which she called “one of the biggest health threats facing the country.”

For the week ending Jan. 27, a new crop of states reported high numbers of patients going to doctor offices, hospital emergency rooms and urgent-care clinics seeking treatment for flulike symptoms, said Daniel Jernigan, head of CDC’s influenza division.

Of the children who died, about half had no underlying medical conditions.

Schuchat urged parents to be on the lookout for worrisome symptoms that could indicate severe illness. They include: very high persistent fever, difficulty breathing, rapid heartbeat and significant fatigue or confusion. In addition, a child who appears to be improving but suddenly gets worse could signal a secondary bacterial infection.

Officials said they are particularly concerned about the season's hospitalization rate — 51.4 per 100,000 people — which is already significantly higher than the 43.5 rate for the same period during the 2014-2015 season.

That season was also characterized by the same predominant flu strain known as H3N2, which typically results in the most complications, especially for the very young, the elderly and people with certain chronic health conditions.

In California, hospitalization rates are four times higher than what they were in 2014-2015; in Oregon and Minnesota, hospitalization rates are double what they were back then.

If trends continue, Jernigan said, the toll from this season could well exceed that of 2014-2015, which ended with 56,000 flu-related deaths, 710,000 people hospitalized and 16 million who sought care from a clinician or hospital. There are early indications that flu activity may be peaking in the West, “but we are by no means out of the woods,” said Schuchat, noting that the disease continues to increase in the East and remains largely unchanged in the South.

Officials still are urging those who have not yet been vaccinated to get flu shots. The flu season has not yet peaked, and all indications suggest many more weeks of significant flu activity are ahead. The country is now in its 10th week of the flu season; the average duration is 16 weeks, but seasons can last for 20 weeks.

Officials said they don’t yet know how effective this year’s vaccines ultimately will be against the three strains of circulating flu. In seasons when the predominant strain is H3N2, vaccine effectiveness tends to be lower than that against H1N1 and influenza B viruses, which are also showing up this season.

Officials have said they expect this season’s vaccine to be about 32 percent effective against circulating H3N2 viruses. But a study earlier this week by Canadian researchers suggests the H3N2 component of the vaccine is about 17 percent effective in preventing infection. In Australia, where flu activity is often a clue for what to expect in North America, interim reports suggest that vaccine effectiveness was 10 percent. Australia’s approach for vaccination is different from that of the U.S., and experts say that makes a difference in measuring vaccine effectiveness.

Schuchat said CDC officials plan to provide an interim estimate of vaccine effectiveness in the United States in several weeks. “We expect effectiveness against H3N2 will be low, but we don’t have that final number yet,” she said.

One reason the vaccine may be less effective on this strain has to do with the way most vaccines are manufactured: by growing them in eggs. “The problem with this H3N2 virus especially is that when you grow it in eggs, it makes changes to the virus that are different to the ones out there in circulation,” Jernigan said, which could be a factor in vaccine effectiveness.

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