The flu vaccine seems effective in reducing the risk of flu by 23 per cent, based on an early estimate from the U.S. among people going to the doctor for respiratory illness.
Canadian and U.S. health officials have said the most common type of flu circulating this season is H3N2, which is not well-matched with the seasonal flu vaccine.
On Thursday, the U.S. Centers for Disease Control and Prevention released an estimate based on 2,321 American children and adults who got the flu vaccine between November and January. The CDC said vaccinated people had a 23 per cent lower chance of winding up at the doctor with the flu.
The poor effectiveness likely reflects the fact that more than two-thirds of circulating flu viruses are genetically different or "drifted" from seasonal flu vaccines, the CDC said in its Morbidity and Mortality Weekly Report.
The effectiveness was highest among children aged six months to 17 at 26 per cent. Effectiveness fell to about 12 per cent among people aged 18 to 49 and 14 per cent for those aged 50 and older.
In Canada, the flu vaccine could be working even more poorly, with "little or no protection."
"About 98 per cent of the viruses are mismatched that have been characterized in Canada, whereas in the U.S. its closer to about 68 per cent, or about two-thirds are mismatched. So it's not a good omen," said Dr. Danuta Skowronski of the British Columbia Centre for Disease Control.
There are other ways to protect yourself and others besides the flu shot, such as handwashing and staying home when sick, said Dr. Michael Gardam, director of infection prevention and control at Toronto's University Health Network.
"This year you could go off and get your flu shot and who cares? It really is not providing a great deal of protection," Gardam said.
The U.S. estimates are consistent with what the CDC said was likely based on lab studies, said Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Nashville.
"These estimates of vaccine efficacy are doleful," Schaffner said.
Despite the poor match, the CDC concluded flu vaccines are the best tool for prevention currently available. The researchers said there could also be a change in viruses circulating late in the season when the vaccine could help more.
Since the CDC started doing flu vaccine studies in 2004, overall effectiveness has ranged from 10 per cent to 60 per cent.
Vaccine effectiveness studies estimate how protective a vaccine is by comparing the odds of being vaccinated among those with positive flu test results to those with negative flu test results.