Story Summary

• An unpublished Canadian study suggests that people who had last year’s seasonal flu shot were about twice as likely to catch the H1N1 (pandemic) flu this spring.
• No other countries have shown any link between the two events: a person’s taking the 2008 seasonal flu and then contracting 2009 H1N1.
• The World Health Organization (WHO) has not found any evidence like that.
• The CDC has not.
• The province of Ontario is changing its flu advice based on this unpublished study, recommending that people who need both wait for the pandemic flu vaccine and take that first, then the regular flu shot.
• Why are unpublished studies often are not discussed? There can be methodological design problems, study bias or what are called “confounding” factors. “A” and “C” matter in a way no one could anticipate and that may, or may not, affect the findings. However, in large studies which have the potential for a major impact on how a diagnosis or treatment is managed, there can be advance notice of the findings. Some of the largest studies may be ended before the planned point because the evidence is so compelling for or against, it would not be responsible to with-hold the findings.
• To be fair, one confounding factor may be that there is something specific to Canada which could lead to results others do not find. There is no way to hazard a guess as to what that might be but the point is that’s how confounding factors work. They make study findings messier business that researchers like—but that’s the real world. Confounding.
• WHO and the CDC continue to recommend that people get seasonal and pandemic flu vaccines, if they are eligible and if their doctors know of no reason why they should not be vaccinated.

Medpage Today

“In unpublished data, Canadian researchers have suggested that seasonal flu vaccination may increase the risk of catching the H1N1 pandemic strain, but such a pattern has not been found in the U.S., the CDC said.”

…”But the CDC said U.S. data do not show a similar risk.”

…”Likewise, an official of the World Health Organization 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, PhD, director of the 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.”

…”Arlene King, MD, 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.”

Source: Medpage Today, September 28, 2009

Citation: Study unpublished. No citation is available unless it is.