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 what’s in the seasonal flu vaccine.
On Thursday, the U.S. Centers for Disease Control and Prevention released an estimate that suggests more than three-quarters of American children and adults who got the flu vaccine between November and January still fell sick enough with the flu to seek medical care.
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.
"I would qualify these findings as showing little or no protection. And I don't think we should expect the Canadian estimates to be higher, given that we have a greater proportion of drift variants [viruses]," said Dr. Danuta Skowronski of the British Columbia Centre for Disease Control.
She and colleagues across Canada are working on early estimates of vaccine effectiveness for this season.
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.
Since the CDC started doing flu vaccine studies in 2004, overall effectiveness has ranged from 10 per cent to 60 per cent.