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The flu vaccine is working better than doctors feared, but still is not especially effective, health officials said Thursday.

Overall, this year’s vaccine is 36 percent effective, meaning it reduces the number of doctor visits by 36 percent, the Centers for Disease Control and Prevention reports.

“Early estimates indicate that influenza vaccines have reduced the risk of medically attended influenza-related illness by about one-third in vaccinated persons so far this season,” the CDC says in its first report on how well the vaccine is working.

It’s only 25 percent effective against the most common strain circulating this year, the H3N2 strain, CDC’s Dr. Alicia Fry and colleagues found.

But the good news is that it works better in kids. The vaccine provided 59 percent protection against H3N2 in children 8 and younger and did not appear to provide any protection at all for older children.

That’s better than initial reports from Australia and Canada had indicated. Studies in those two countries showed the vaccine only prevented 10 percent or 17 percent of H3N2 cases.

“CDC continues to recommend influenza vaccination because the vaccine can still prevent some infections with currently circulating influenza viruses, which are expected to continue circulating for several weeks,” the agency said in its report.

“Even with current vaccine effectiveness estimates, vaccination will still prevent influenza illness, including thousands of hospital­izations and deaths.”

Vaccine effectiveness is tricky to measure, because there is no way to know how many vaccinated people never got ill at all. The CDC uses data from the U.S. Influenza Vaccine Effectiveness Network — five study sites across the U.S.

For Thursday’s report, they looked at the cases of 4,562 kids and adults who showed up with flu-like illness at the five sites.

It's not always the flu

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Most did not even have flu — just 38 percent of them tested positive for flu. There are hundreds of viruses that can cause flu-like symptoms. A different CDC team looked at wider reports and found that only about a quarter of people showing up to the doctor’s office with flu-like symptoms actually had influenza.

Of those who did have influenza, 43 percent had been vaccinated. And 53 percent of those who did not have flu had been vaccinated.

“The risk for H3N2 associated medically attended influenza illness was reduced by more than half (59 percent) among vaccinated children,” the team noted.

This study does not include data on how well the vaccine prevented deaths of serious flu requiring hospitalization, the CDC pointed out. That data will come later.

The vaccine was more effective against H1N1, which is also circulating. It protected an estimated 67 percent of people who got it from flu bad enough to go to the doctor. It protected 42 percent of people against influenza B.

Most importantly, remember that your best defense is getting the #flu vaccine. Getting vaccinated prevents spread to others, and reduces the duration and severity of illness. For more information, visit https://t.co/jHSZeaEKSy — U.S. Surgeon General (@Surgeon_General) February 15, 2018

The low efficacy was not the result of a mistake, CDC acting director Dr. Anne Schuchat said. "Each year we work hard to predict which strains will be circulating," she told a news conference.

"The strains that are circulating are the strains we predicted."

Flu vaccines are a cocktail — they protect against either three or four strains of flu. People must get a new vaccine every year to be protected.

Plus, there are different formulations. Some are stronger for older people with weaker immune systems and some are made using newer technology.

The CDC says it is still too soon to tell if one formulation works better than another. So all the different vaccines on the market are lumped in together in vaccine effectiveness estimates.

“Even with current vaccine effectiveness estimates, vaccination will still prevent influenza illness, including thousands of hospital­izations and deaths.”

Even when it doesn’t work well, the flu vaccine can save many lives.

“During the 2014–15 season, when vaccine efficacy against medically attended illness caused by any influenza virus was less than 20 percent, vaccination was estimated to prevent 11,000–144,000 influenza-associated hospitalizations and 300–4,000 influenza-associated deaths,” the CDC team wrote.

"Getting vaccinated does not mean that you can’t or you won’t get sick, but it can reduce the duration and severity of illness and prevent the spread to others," Surgeon-General Dr. Jerome Adams told a news conference.

"Some protection is better than no protection at all."

It’s not clear why the H3N2 component of the vaccine works so poorly.

“Immune responses to vaccination differ by age and previous infection or vaccination history and can affect vaccine protection; higher vaccine efficacy against H3N2 viruses among young children suggests that vaccination might provide better protec­tion against circulating H3N2 viruses to this age group.”

"There is still time. Go get a flu shot."

Previous reports have shown that the old-fashioned technology used to make most flu vaccines — it involves growing the virus in eggs before making it into vaccines — might cause parts of the H3N2 strain to mutate, making it a poor match for the circulating flu viruses.

The CDC says that flu kills from 12,000 to 59,000 people every year. This year so far, 84 children have died from influenza, CDC said Friday.

Federal health officials urged Americans repeatedly to get a flu vaccine.

"Three out of four children who died of flu had not gotten the flu vaccine," Schuchat said.

"There is still time. Go get a flu shot," Health and Human Services Secretary Alex Azar said.

"Although we can’t predict how much longer this season will last, we may be looking at several more weeks of intense flu activity," Schuchat added.