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Flu Shot Gave Modest Protection Against A Strain but None Against B This Year

Posted on: Thursday, 17 April 2008, 21:00 CDT

By Helen Branswell, Medical Reporter, THE CANADIAN PRESS

TORONTO - This year's flu shot performed better than expected against some strains but failed to protect against the viruses that have been sending Canadians to their beds at the tail end of flu season, new research reveals.

An interim look of the efficacy of the vaccine by U.S. researchers showed it had zero impact on the main family of influenza B viruses circulating this winter, the so-called B/Florida-like viruses. That line of viruses has caused substantial illness in Canada in recent weeks.

"What we saw with B this time is that there was really a terrible match," said Dr. Edward Belongia, an infectious diseases epidemiologist with the Marshfield Clinic Research Foundation in Marshfield, Wis.

"It was just a completely different lineage of B. And it didn't work at all."

Belongia was lead author of the interim report, published Thursday by the U.S. Centers for Disease Control in its journal Morbidity and Mortality Weekly Report.

On the flip side, a mismatch in another component of the vaccine, the one that protects against influenza A viruses of the H3N2 subtype, didn't turn out to be as bad as experts feared. The vaccine included an A/Solomon Islands virus, but a different virus, called A/Brisbane emerged as the main H3N2 threat.

When H3N2 became the dominant type of influenza virus causing disease in the United States in early February, public health experts there worried that the vaccine might be offering suboptimal protection against the Brisbane viruses.

"And I think that was widely interpreted as saying: Well the (H3N2 component of the) vaccine really didn't work. But in fact, that's not the case at all. It did work to some reasonable degree," said Belongia, who led the research effort.

The Marshfield study showed that the H3N2 part of the vaccine was 58 per cent effective, meaning people who got a shot were 58 per cent less likely than those who didn't to be infected with influenza and be sick enough to seek a doctor's care.

The study suggested the vaccine efficacy overall was 44 per cent. That suggests that if 100,000 people got a flu shot and 100,000 did not, one would expect to see 10,000 flu cases in the unvaccinated group, compared to 4,000 in the group that took the shot, according to the CDC.

Clinical trials have suggested that on average flu shots are between 70 and 90 per cent effective at preventing infection in healthy adults.

Flu vaccine is made to protect against three strains of viruses: the influenza A subtypes H1N1 and H3N2 and one type of influenza B virus.

The study couldn't offer an assessment of the efficacy of the H1N1 component of the vaccine, because the researchers didn't see any cases caused by those viruses. But lab testing has shown the H1N1 viruses circulating in North America were in the same family as the H1N1 component of the vaccine.

Flu vaccine has to be reformulated every year because influenza viruses mutate constantly. In addition to the viruses the vaccine protects against, other families of viruses are also making the rounds. How well the vaccine protects against them depends on how closely related they are to the vaccine strains.

Strains are selected by experts who gather at the World Health Organization months in advance of flu season to make best guesses about which virus families within each of those three types are likely to be major causes of disease the following winter.

While the word "guess" might suggest a haphazard process, it is anything but. The experts meet for days, studying the characteristics of viruses from around the globe. The meeting is held months before flu season because of the time it takes for pharmaceutical companies to make and distribute vaccine.

Many years, the experts chose well. In 16 of the past 19 years, the vaccine was well matched, the CDC says. But when the strain selection panel gathered in February 2007 to make choices for the 2007-2008 flu shot, they missed the mark for both the H3N2 and influenza B viruses.

That has had differing impacts in Canada and the U.S., because the flu season has taken divergent paths north and south of the border.

In the U.S., three quarters of confirmed flu infections have been caused by influenza A viruses, Dr. Dan Jernigan, deputy director of the CDC's influenza division said Thursday. And H3N2 viruses made up about three quarters of all the influenza A cases. B viruses were responsible for about 25 per cent of confirmed infections.

In Canada, H3N2 has been a minor player this flu season, accounting for only 13 per cent of confirmed flu cases. As of April 5 - the most recent data available - B viruses caused 44 per cent and H1N1 viruses accounted for 43 per cent of confirmed flu cases.

The B activity in Canada surged late in the flu season, and some of those cases may well be in people who got a flu shot, said Dr. Andrew Simor, head of microbiology at Toronto's Sunnybrook Health Sciences Centre.

"There's no question that there is still flu activity around. We're certainly seeing that, both in terms of individual cases and, as well, outbreaks that are continuing to be reported," said Simor, who was not involved in the U.S. research.

He said the late season surge is likely due in part to the fact that the flu season got off to a late start. "But in addition, I'm sure that the reduced efficacy of the vaccine this year compared to previous year . . . is also contributing to ongoing cases."

Experts know the failure of the B strain this year could shake confidence in the value of flu shots. But Dr. Danuta Skowronski, a flu expert with the B.C. Centre for Disease Control, said this study and others have shown it is a worthwhile measure, especially for people for whom influenza poses a serious risk.

"So for people who are at risk of serious outcomes, it underscores the importance that they get the vaccine to protect themselves. Because it can be life-saving to them," she said from Vancouver.

Source: Canadian Press

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