International influenza vaccine experts are apparently not convinced that Canadian researchers have found a true link between getting a seasonal flu shot and catching swine flu.

The consensus that emerged from a World Health Organization teleconference Friday on the controversial data seemed to be that the Canadian findings are likely due to some confounding factor or factors in the data themselves and may not reflect a real increased risk, according to a WHO official who helped pull together the meeting.

'Are there some effects that are just not being detected that are really behind this?' Because it is an unexpected finding.'— David Wood

"From a WHO point of view, the fact that the findings are not replicated in other countries I think is reassuring for us that this is an outlier, if you like, the unexpected findings that are coming out of Canada," said David Wood, co-ordinator of the quality, safety and standards team of WHO's department of immunization, vaccines and biologicals.

"Most people are still looking at this as some sort of undetected confounding in the data, that for some reason is giving the results that are there."

In an interview from Geneva, Wood was diplomatic. But when pressed, he admitted most experts on the call didn't seem to believe that the unpublished study, based on data from British Columbia, Quebec and Ontario, had found a true link between getting a seasonal flu vaccine and having an increased risk of coming down with a mild case of H1N1 flu.

"Well, yeah," he said. "It's a totally unexpected finding."

"So I think people do then try to think: 'Well, why is this happening? Are there some effects that are just not being detected that are really behind this?' Because it is an unexpected finding. That's the way people tend to think."

Provinces changed flu plans

The work, which, it is said, is being considered for publication by a medical journal, contributed to decisions by most provinces and territories to stagger or delay their seasonal flu shot efforts this fall.

Instead of launching full-fledged seasonal flu vaccine programs in October, most have announced they will offer seasonal shots during the month only to seniors — who aren't currently at high risk from the pandemic H1N1 virus — and residents of long-term-care facilities.

After pandemic vaccination efforts are completed, most of those provinces plan to offer seasonal vaccine more broadly.

A couple of jurisdictions — Quebec and Nunavut — will wait until after they've completed their pandemic vaccination efforts before offering seasonal flu shots. At the other end of the spectrum, New Brunswick is going ahead with its regular seasonal flu shot campaign before offering pandemic flu shots.

Statistical flaws?

The Canadian findings, which are reportedly mirrored in data from Manitoba as well, suggest that people who got a flu shot last fall were twice as likely as people who didn't to contract swine flu. But the association, if it is real, is to mild disease. There is no evidence that people who got seasonal flu shots are more prone to develop severe illness if they catch the new H1N1 virus.

Scientists from the United States, Britain and Australia have looked at their data, but didn't see the same effect. A number of scientists have speculated that the Canadian data may have some built-in confounders — factors that can produce false results.

For instance, if people who get flu shots are also more likely to seek a diagnosis of swine flu if they get sick, that could make it seem like more of them got the illness when in fact what happened is that more of their illnesses were recorded.

But if the Canadian results are due to some statistical flaws or selection biases, no one on the 4½-hour teleconference was able to put a finger on what exactly the problem is, Wood acknowledged.

And he admitted there may not be a satisfactory answer to that puzzle in the foreseeable future.

"It didn't seem very likely that we're going to be able to … suddenly come up with the magic explanation as to why the Canadian data are different to others," Wood said. "In the short term, this is really probably as far as we're going to get."

New studies will likely be needed to get a definitive answer, he said.

More studies needed

Experts say there will need to be prospective studies — following people who get a flu shot forward —rather than the retrospective studies that produced the unusual findings. The evidence from retrospective studies isn't considered as high quality as that garnered from prospective studies.

In the meantime, a summary of the situation will be presented to the WHO's Strategic Advisory Group of Experts on immunization, also known as the SAGE. The group, which meets later this month, makes recommendations for the WHO on vaccination policy.

Wood said he couldn't prejudge what the committee will decide, but said for the moment it doesn't seem like the WHO needs to ask countries to change their vaccination programs for this fall.

"The fact that it's just been seen in Canada at the moment, I don't think that that's going to force global policy changes," he said.