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(Bloomberg) -- This season’s flu vaccine is just 23 percent effective against the illness, further evidence that the inoculation was mismatched to the viral strains that eventually infected Americans.

The vaccine’s effectiveness “is relatively low compared with previous seasons, when circulating viruses and vaccine viruses were well matched,” the Centers for Disease Control and Prevention said in an early estimate published today. The agency was able to make an estimate early in the 2014-2015 flu season because the flu has reached an epidemic level faster than in recent years.

Since the CDC began conducting effectiveness studies in 2004-2005, estimates have ranged from a 10 percent to a 60 percent reduction in a person’s risk of having to go to the doctor because of the flu, the agency said. Effectiveness is from 50 percent to 60 percent when the viruses circulating and those in the vaccine are well matched, the CDC said.

For the week ended Jan. 3, 7 percent of all deaths reported through a CDC monitoring system were attributed to influenza or pneumonia, above the 6.9 percent threshold for an epidemic at this point in the season, according to the agency’s website. Twenty-six children have died from the flu this season.

To allow manufacturers sufficient time to produce and distribute a vaccine, health officials must decide before the season begins which viruses to include. H3N2 viruses have been the most common this season and about 70 percent of them have been different from the strains in the inoculation.

Patient Age

Vaccine effectiveness varies by age. The vaccine has kept children with H3N2 viruses out of the doctor’s office 26 percent of the time, the CDC said. For adults 49 and younger, the inoculation has been 12 percent effective and for people 50 and older, 14 percent.

The low effectiveness “underscores the need for additional prevention and treatment efforts this season, including the appropriate use of influenza antiviral medications for treatment,” the CDC said.

Antiviral medications include Roche Holding AG’s Tamiflu. Manufacturers of antiviral treatments have said they have sufficient supply. Some areas of high demand have experienced spot shortages, so patients may need to check multiple pharmacies, the agency said.

To contact the reporter on this story: Anna Edney in Washington at aedney@bloomberg.net To contact the editors responsible for this story: Crayton Harrison at tharrison5@bloomberg.net Tony Robinson, Andrew Pollack

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