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Will We Suffer Two Flu Seasons This Year?

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With so much fire aimed at a swine flu vaccine that's only beginning to circulate, there's been little talk of potential shortcomings in the seasonal flu vaccine. But in this season of discontent, the annual shot aimed at quelling "normal" flu may have a bad year.

To start with, the medical world has been vexed by "the Canada Problem," a Canadian study that concluded that people under the age of 50 vaccinated for seasonal flu will then be twice as likely to catch swine flu. The study, which assessed results obtained by 2,000 Canadians in three big provinces, has not been published.

Its authors say it is undergoing prepublication peer review at a scientific journal -- but most Canadian provinces have revised their plans and will inject only the elderly and residents of long-term care facilities with seasonal shots until after the country's swine flu vaccines have been rolled out in November.

Perhaps worse, there's a chance that we will experience two flu seasons this year. Swine flu could be followed by the North American debut of a new strain of H3N2, the nastier of the two seasonal flu A strains -- to which few would have immunity. The World Health Organization recently reported that H3N2 accounts for almost half of new flu cases in China, where the virus is endemic in pigs. The new strain that is spreading seems not to be the H3N2 targeted by this year's seasonal flu.

In theory then, the seasonal shot could make people more likely to catch swine flu while not protecting against a later wave of H3N2.

The Canadian study was recently the subject of a high-level global teleconference. As detailed in a report by Helen Branswell--the world's top flu reporter--authorities have decided to proceed with seasonal flu vaccinations everywhere but in Canada on the assumption that the study is somehow flawed.

A prime problem is that no other health body or researcher has found a link between seasonal flu vaccine exposure and a greater likelihood of catching swine flu. (A study in the British Medical Journal found the opposite effect, but tracked only 60 Mexican patients.)

Count the U.S. Centers for Disease Control unimpressed. "We have not seen such evidence in cases here in the U.S. and we recommend seasonal and H1N1 vaccination in those groups recommended to receive them," said a spokesman. (The CDC lists recommended recipients for seasonal and swine flu.)

What's disturbing is that no one has been able to find any inconsistencies in the Canadian study's methodology. As Branswell reported after speaking with David Wood, coordinator of the quality, safety, and standards team of WHO's department of immunization, vaccines, and biologicals: "But if the Canadian results are due to some statistical flaws or selection biases, no one on the 4 ½ hour teleconference was able to put a finger on what exactly the problem is, Wood acknowledged."

Wood, said Branswell, "admitted there may not be a satisfactory answer to that puzzle in the foreseeable future."

So how might a flu vaccine backfire in a season involving rival strains that require separate shots to thwart them? Robert Roos, news editor at the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota, wrote a detailed explanation in August of a longstanding phenomenon known as original antigenic sin.

Like an army using the tactics from its last war against a new foe, the human immune system seems to respond to a new flu strain by cooking up the same antibodies it used against the first one. Researchers at Emory University, Roos said, came up with some evidence that among closely related flu strains, the second one can thrive, presumably because it doesn't perceive a new threat.

Does this explain 'the Canada Problem?" Not necessarily. The Emory team felt that our immune systems would meet the challenge of swine flu after a seasonal vaccination because Novel H1N1 is sufficiently different from either of the Influenza A seasonal flu strains in that vaccine that our bodies will detect the new viral enemy.

To make sure, the government is running trials at the National Institute of Allergy & Infectious Diseases (NIAID) -- but they won't be completed until next May and June. For now, Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, told Branswell that preliminary data from a U.S. study shows no lessening of effect when seasonal and swine flu vaccines are taken at the same time.

Further confusing the situation is the question of how much seasonal flu we can expect to encounter this year. With swine flu dominating the world for many months, there seemed to be little space for the old-timers.

But with the upstart version of H3N2 spreading around the world, it's anyone's guess whether it will break out here after swine flu runs out of steam. Some fear H3N2 might prey on senior citizens, who hold some immunity to swine flu but who would have none to the new strain.

It's enough, at least, to give us all a headache. And that's a real flu symptom.