A nurse practitioner prepares an influenza vaccination at a retail pharmacy clinic in Miami in October, at the start of flu season. (Joe Raedle/Getty Images)

This season’s flu vaccine reduces the need to go to the doctor’s office by about half, according to figures released Thursday. But in a reminder of how deadly the respiratory virus can be, federal health officials estimate that as many as 16,000 people have died of flu this season, more than what would be expected so far in a relatively mild season.

Unlike last winter, when flu killed and hospitalized more people in the United States than any seasonal influenza in decades, this season’s flu is considered significantly less harsh, according to officials at the Centers for Disease Control and Prevention. The vaccine also works better overall than last year and is even more effective — about 61 percent — in children, who are among the groups most vulnerable to flu-related complications.

But the vaccine has not provided any measurable protection for older adults, with only an 8 percent reduction in the need for medical attention for flu, according to preliminary estimates.

“That’s a difficult one to explain,” said Brendan Flannery, one of the scientists at the Centers for Disease Control and Prevention’s influenza division. It’s possible that’s because there isn’t that much flu in older adults now. “We don’t see as much disease in our study group of 50 and older population,” he said. There is also a lot of variation in the data, he said. It’s too early to “get a good bead on the protection in adults,” he said. “That’s really where we need to monitor.”

When effectiveness is about 50 percent, officials say there is a large decrease in illness, hospitalizations and deaths from flu.

Most illnesses now are being caused by a flu strain that leads to fewer hospitalizations and deaths than the kind of flu that dominated a year ago. This season’s strain tends to affect children more than other age groups.

Preliminary figures suggest that influenza infections are likely to result in a “substantial number” of deaths this season, Flannery said. At least 28 children in New York City and 21 states have died of flu. During past seasons, approximately 80 percent of pediatric flu deaths have been in children who were not vaccinated.

Several more weeks of influenza are likely, and officials say it is not too late to get a flu shot.

“The number of deaths is relatively high for a season that we consider to be low severity,” Flannery said. “Flu is still increasing in lots of different places,” he said. There have been fewer deaths among older adults this season, compared with last winter. But “it’s still a concern,” he said. “It still can go up.”

This is the first time agency officials are providing overall illness and death estimates while flu season is still underway. In the past, these estimates were calculated when the season was over.

So far this winter, preliminary figures estimate that influenza infection has sickened between 13 million and 15 million people, sent between 6 million and 7 million people to the doctor, hospitalized 155,00 to 186,000, and resulted in 9,600 to 15,900 deaths. Flu activity is widespread across the United States. Officials said it’s too early to know what the final numbers will be.

By comparison, influenza killed about 80,000 people in the 2017-2018 season. In recent years, flu-related deaths have ranged from about 12,000 to 56,000.

The CDC has estimated the burden of flu since 2010 using a mathematical model that is based on rates of laboratory-confirmed influenza-associated hospitalizations collected through a surveillance network that covers about 27 million people, or about 8.5 percent of the U.S. population.

Preliminary figures released Thursday suggest the vaccine is 47 percent effective in reducing a person’s risk of becoming sick enough to need to see a doctor. A final estimate for last season showed the vaccine was about 40 percent effective. Even in a good year, the flu vaccine is never as effective as most other vaccines. When the vaccine is well matched to circulating viruses, its effectiveness is about 60 percent. (The measles, mumps and rubella vaccine, by comparison, is about 97 percent effective with two doses.)

The preliminary estimate of overall effectiveness in the United States is lower than the 72 percent effectiveness in the same vaccine in Canada. Flannery said one reason could be that flu was more active earlier in Canada, allowing officials there to gather more data. By comparison, officials did not have as much data in the United States, particularly among those 50 and older.

“When there’s not as much flu activity, it’s difficult to get a good estimate,” he said.

But experts say focusing only on that measure misses too many other benefits of the vaccine that are less well-known. Flu’s impact on the body goes well beyond the acute respiratory infection.

The vaccine can reduce flu illness and hospitalizations, protect pregnant women during and after pregnancy, and be lifesaving in children. For older people with chronic health conditions, getting a flu shot is as effective in preventing a heart attack as quitting smoking, taking cholesterol-lowering drugs or taking blood pressure medications, recent studies show.

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