Canadian research suggested this week that the seasonal flu vaccination may increase the risk of catching the H1N1 pandemic strain, but government officials said such a pattern has not been found in the U.S., the Centers for Disease Control and Prevention said.
The unpublished work appears to suggest that people who had been vaccinated against last year's seasonal flu were about twice as likely as others to catch the pandemic strain when it appeared this spring.
But statistics from the CDC do not show a similar risk.
"It is difficult to speak about a study that has yet to be published," said CDC spokesman Joe Quimby, a senior press officer.
But, he added, "it is important to note that scientists at the Centers for Disease Control and Prevention have not seen this effect in systems we have reviewed in the U.S."
"We continue to urge people to receive both the seasonal flu vaccine and the 2009 H1N1 vaccine," Quimby said.
Likewise, an official of the World Health Organization (WHO) said that investigators in countries other than Canada had not found a similar risk increase when they looked at their own data.
"We are in contact with other countries and are having them look at their own data to see if they could have similar observations, and none has been able to find anything like that," said Marie-Paule Kieny, director of the World Health Organization's vaccine research initiative.
"The reason why this may be different in Canada and in this particular study than in other places of the world is not yet identified," she told reporters during a telephone press conference.
Kieny said experts are now trying to decide whether the effect is real or a result of "study bias."
The Canadian research -- drawn from studies in three Canadian provinces during the spring pandemic outbreak -- has been submitted for publication to a scholarly journal, and the lead authors were not immediately available for comment.
But co-author Dr. Danuta Skowronski, of the British Columbia Centre for Disease Control in Vancouver, told the Canadian Press she wanted more scrutiny on the Canadian research in question.
"Good scientists know that methods can influence results," she said. "And if there are methodologic flaws, we need to be assured that every stone was turned over to make sure what we're reporting is valid."
Skowronski said the findings could be real, due to chance, or arise from some sort of bias in the research.
Meanwhile, at least one Canadian province is changing its flu vaccination strategy in the wake of the report.
Dr. Arlene King, the chief medical officer of health for Ontario, said people over 65 -- those at least risk for the pandemic flu strain -- will be offered the seasonal flu vaccine as usual in October.
Others will be advised to wait until after they've had their H1N1 shot, King said. The pandemic vaccine is expected to be available in Canada in November.
Ontario, Canada's most populous province, offers the seasonal flu vaccine to all comers free of charge, a policy that has yielded public health benefits.