H1N1's true toll not shown by death tally

Are you confused by theswine flunumbers? Wondering why public health officials are making such a fuss about a virus that has so far killed so few people?

You aren't alone.

After all, seasonal flu kills between 4,000 to 8,000 Canadians and between 250,000 and 500,000 people worldwide each year. Yet as of late last week, seven months into the currentoutbreak, the H1N1 influenza A virus had killed 161 Canadians and an estimated 6,260 people around the globe.

Critics of Canada's pandemic response point to the discrepancy between those sets of numbers and question the full-court press.

'Nobody has seen a flu season like this on the ground level' Dr. Kumanan Wilson, public health policy expert

But as tempting as it is to compare those two sets of figures and conclude that H1N1 is much ado about nada, you shouldn't. The two sets of numbers count different things, experts say.

"You might as well compare the number of flu deaths with the number of Subarus sold in Canada,"said Jordan Ellenberg, an associate professor of mathematics at the University of Wisconsin who explained the problem in an article published online in Slate Magazine earlier this year.

"If you want to compare the number of confirmed deaths to seasonal flu to the number of confirmed deaths from H1N1, OK, you can do that," hesaid in an interview. "But what you can't do is compare the number of certified deaths on one side to the best estimate of the full number of deaths on the other side."

Seasonal flu's deathtoll isan estimate

The tally of confirmed H1N1 deaths captures the few times someone who caught the bug died from it after testing positive for it. The seasonal flu numbers are estimates, mathematical calculations aimed at capturing all the deaths influenza had a hand in.

The frequent attempts to equate the twovex Kumanan Wilson.

Wilson is an expert in public health policy, as well as an internal medicine physician at the Ottawa Health Research Institute. He readily admits he never sees anyone die of seasonal flu a common claim that drives infectious-disease experts crazy.

Wilson is, however, seeing the destructive power of the current pandemicstrain of influenza.

"Nobody has seen a flu season like this on the ground level," he said. "If you talk to any frontline worker, they've never seen anything like this. And we keep getting told this is nothing."

Emergency departments "are filled. All the children's hospitals are filled. Family docs I talk to say, 'Oh my God, I've never seen so many flu cases,' " Wilson added.

Wilsonsaid it is "disingenuous" to criticize the response to the pandemic by comparingits low death tollwith the substantially higher estimates of seasonal flu deaths. "I feel it underplays the significance of this."

But what is the difference between counting confirmed flu deaths one by one and estimating seasonal flu death tolls? The answer lies in the way flu kills and the way statisticians try to capture that effect.

Infectionkills few directly

Influenza does kill some people directly.The phenomenonis sometimes seenin cases ofthe pandemic strain of H1N1flu: Some victims develop an aggressive and ultimately fatal viral pneumonia.

But generally that number is small. A 2007 study of looking at 10 years of influenza deaths in Canada reported that deaths directly attributed to flu made up only eight per cent of all influenza-related deaths in a given year.

In most cases, influenza contributes to or hastens death. A bout of flu can trigger a heart attack in someone with heart disease. But the cause of death is likely to be listed as a heart attack.

Other times flu weakens the immune system of, say, an 80-year-old. Bacteria seize the opportunity and a pneumonia develops. The cause of death may be bacterial pneumonia, but the pneumonia wouldn't have occurredhad the personnot caught the flu.

A lot of seasonal flu doesn't get captured in official reports of deaths and hospitalizations. Not everyone dies in hospital. Hospitals don't always test for influenza. And if they do, if the test isn't done early enough in the infection,it may come back negative.

The 2007 study, led by Dena Schanzer of the Public Health Agency of Canada, estimated 12.5 deaths were attributable to flu for every certified flu death from 1990 to 1999, on average.

Public health researchers come up with figures like that by plotting curves of annual flu outbreaks against what is known, from long-term study, about how many people die on average each week of the year. When you lay the two curves over each other, you see deaths rise when flu hits.

Some of the excess mortality, as it is called, may be due to other factors, like poor winter driving conditions. But some of the excess is due to influenza. And over the years, public health researchers have worked out ways to calculate the portion of those extra deaths for which flu is to blame.

Mathematical models

"We try to isolate the amount that we can attribute to influenza, using our mathematical models," explains Dr. Jeff Kwong of Toronto's Institute for Clinical Evaluative Sciences.

Some people question the tactic. But Ellenbergsaid it's a valid approach.

"If somebody gets in a car crash and they bring them to the hospital and they die, you would say, 'Hey, that person was killed in a car crash,' "he explained. "And if somebody was like 'No, when they were in the car they were alive. They died of blood loss in the hospital,'that would be a little ridiculous, right?"

Often, though, there can be considerable lag time between an outbreak and the attempts to quantify its impact. It takes time for vital statistics to be processed. Kwongsaid it may be 2011 or so before Canadian researchers can come up with a good estimate of what has happened this year with H1N1.

The U.S. Centers for Disease Control decided not to wait.

Last week they released new estimates of the toll H1N1 has taken in the United States. Their calculations, the methods for which are explained on theCDC's website, nearly quadrupled their estimate of how many Americans have died from the virus. They now peg it at 3,900.

Dr. Anne Schuchat, director of the CDC's centre for immunization and respiratory diseases, said the agency knew focusing on lab-confirmed cases was painting a "very incomplete" picture of the problem.

Canada's chief public health officer, Dr. David Butler-Jones, doesn't think the discrepancy will be as big here. "No test will capture 100 per cent [of cases]. But certainly we believe we're much closer to the actual number here in Canada."

But influenza expert Dr. Allison McGeersaid she isn't sure how many cases are being missed. She knows the tests aren't sensitive enough to pick up all cases. But are they missing 15 per cent or 35 per cent? She can't tell, she said.

Harvard epidemiologist Marc Lipsitch, a modelling expert, isn't sure how much it matters at this point.

"If all this effort goes to save hundreds or a few thousand lives, that's still hundreds or a few thousand lives that have been saved," he said. "If these are preventable deaths, which I think is pretty clear many of them are, we should try to prevent them.