MIAMI (AFP) - This US winter season's flu vaccine has been just 23 per cent 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 per cent to 60 per cent effective - but its record is worrying enough that the Centres for Disease Control and Prevention (CDC) 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 hospitalised 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 per cent - in those aged six months through 17 years.

Vaccine effectiveness was just 12 per cent for ages 18 to 49 years and 14 per cent 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 hospitalisation or even death.