This US winter season's flu vaccine has been just 23 percent effective at preventing doctor visits for people of all ages, according to health authorities' early estimates out Thursday.

The flu vaccine for 2014-2015 is not the worst ever -- the past decade has ranged from 10 percent to 60 percent effective -- but its record is worrying enough that the Centers for Disease Control and Prevention have urged doctors to ramp up the use of antiviral medications in people who fall ill with influenza.

Its lack of punch is being blamed on multiple strains of the H3N2 virus that are circulating and making people sick, but that were not included in this season's vaccine.

"Physicians should be aware that all hospitalized patients and all outpatients at high risk for serious complications should be treated as soon as possible with one of three available influenza antiviral medications if influenza is suspected, regardless of a patient's vaccination status and without waiting for confirmatory testing," said Joe Bresee, branch chief in the CDC's Influenza Division.

The flu vaccine is generally most effective in young, healthy people under 65.

This season, vaccine effectiveness has been highest -- 26 percent -- in those aged six months through 17 years.

Vaccine effectiveness was just 12 percent for ages 18 to 49 years and 14 percent for people age 50 years and older, the CDC said.

The flu season is usually about 13 weeks long. Officials say the current one is being classed as "moderately severe" and is now just over halfway from ending.

The CDC is still recommending that people get a flu shot because it may help prevent severe infections and complications that lead to hospitalization or even death.