The vaccine was about 42 percent effective in preventing illness severe enough to send a patient to the doctor's office. But it was essentially ineffective protecting some age groups. That includes people 65 and older — the group that's hardest hit by flu, suffering the most deaths and hospitalizations.
The flu season that just ended was a long one that peaked in February and was considered moderately severe. But the flu-related hospitalization rate for older adults was the highest it's been since the severe 2014-2015 flu season.
Like that season, last winter was dominated by a kind of flu — Type A H3N2 — that tends to cause more deaths and serious illnesses than other seasonal flu viruses.
In four of the last seven flu seasons, flu vaccine was essentially ineffective in seniors, past studies suggest. The worst performances tend to be in H3N2-dominant seasons.
Health officials say flu vaccine still protects many people. And even if fares poorly against the dominant virus, it can do a good job against other circulating flu strains.
"While it is clear we need better flu vaccines, it's important that we not lose sight of the important benefits of vaccination with currently available vaccines," said Jill Ferdinands, a flu epidemiologist at the Centers for Disease Control and Prevention, in a statement.
The CDC calculates vaccine effectiveness from a sample of flu tests done on patients in five states.
Vaccines against some other infectious diseases are not considered successful unless they are at least 90 percent effective. But flu is particularly challenging. Over the last ten winters, overall flu vaccine effectiveness has averaged about 46 percent.
Last winter's vaccine did well in protecting young children, about 60 percent effective. And it did OK in older children and in adults ages 50 to 64. But it had no clear effect in adults 18 to 49, or among the oldest adults.
Results were presented at a meeting in Atlanta of the Advisory Committee on Immunization Practices, which provides vaccine advice to the CDC.