This year's flu vaccine provided little protection to people 65 and older who got a shot, new data released by the U.S. Centers for Disease Control reveal.

The study estimates how well the vaccine did at preventing people from becoming sick enough to need to see a doctor —preventing "medically attended flu" in influenza science parlance. And the data suggest this year the vaccine didn't perform very well for older adults.

The overall effectiveness of this year's flu vaccine for older adults in the U.S. was estimated at 27 per cent. (Jeff McIntosh/Canadian Press)

It found that a flu shot cut an older adult's risk of contracting a case of influenza A H3N2 that needed medical care by only nine per cent.

When the scientists calculated the overall effectiveness — the vaccine's ability to prevent medically attended flu caused by both flu A and B viruses — the estimate was 27 per cent for older adults.

The findings are disappointing given the H3N2 component in this year's vaccine is well matched to the viruses circulating. Paradoxically, the B component of the vaccine isn't as closely matched to circulating viruses, yet it is performing better, according to the study.

Dr. Joe Bresee, a flu expert with the CDC, said older adults need to know that if they got a flu shot and they develop flu-like illness, they may have influenza and should see a doctor to determine if they should take antiviral drugs.

"We wanted to make sure that people knew that — especially if you're elderly to know that — because we still want to communicate the fact that if you do get sick, get antivirals. And don't assume just because you got the vaccine, that that flu-like illness you've got can't be flu," said Bresee, who heads the CDC flu division's epidemiology team.

This type of study may actually overestimate the effectiveness of the vaccine, some experts say. It cannot capture a picture of what percentage of people got a shot and got the flu but didn't see a doctor. Most people simply ride out a case of the flu in bed.

Vaccine effectiveness in elderly 'going to be quite low'

As well, it doesn't account for people who become so sick they go directly to hospital, which would not be uncommon in elderly people with influenza.

The findings are an interim estimate, based on data gathered during the middle of the flu season. The researchers doing this work are continuing to gather data and will run a final analysis at the end of the season.

The numbers may change slightly in the final analysis; in this run-through, the overall vaccine effectiveness figure dropped to 56 per cent from 62 per cent in an analysis conducted in January.

But Bresee suggested that the current figures are likely a pretty accurate reflection of the vaccine's performance in older adults.

"The nine [per cent] is going to change a little bit one way or the other," he said in an interview from Atlanta.

"But the nine's not going to become a 50, definitely, as we add data. I think we're confident that the effectiveness against that A strain in the elderly is going to be quite low."

Even in younger adults and children, the study suggests this year's vaccine offered modest protection. Bresee described it as "moderate" and said it's a word people in the flu world are starting to realize seems to apply to flu vaccine.

Even in young adults — the group which typically mounts the best immune response to flu vaccine — the performance of this year's vaccine was not stellar.

In people aged 17 to 49, it cut the risk of medically attended flu by 52 per cent overall. But for the H3N2 component, the protection was only 46 per cent in this age group.

Dr. Danuta Skowronski, a flu expert with the British Columbia Centre for Disease Control, said the findings are consistent with Canadian and European vaccine effectiveness estimates released in recent weeks.

For Canada the H3N2 effectiveness rate was 45 per cent, she noted, while European investigators pegged the effectiveness against that component at 42 per cent.