The high impact of an influenza pandemic

I applaud the CMAJ for its efforts to increase public and political awareness of the potential impact of an influenza pandemic in Canada. However, I would like to clarify one of the numbers that appeared in a recent news article.1

It is difficult to precisely quantify — or to exaggerate — the impact of the next influenza pandemic. That impact will depend on how virulent the virus is, how rapidly it spreads from person to person, and how effective and available prevention and control measures prove to be. Estimates based on previous pandemics (in 1918, 1957 and 1968) can be used as a guide, but global travel is far greater than ever before and will no doubt accelerate the speed of international spread, as with SARS.

Models have been developed to estimate the possible impacts of the next pandemic,2 but they are based on assumptions derived from the US experience, and their applicability to other health care settings or systems is limited. Nor do these models incorporate the use of antivirals or vaccines, should these become sufficiently available.

Estimates of impact can nevertheless be useful in showing the scale or magnitude of the crisis relative to that of other disasters and in increasing the awareness that is critical to preparation and planning required to minimize that impact. Assuming attack rates in the range of 15% to 35% during the next influenza pandemic, the Canadian Pandemic Influenza Plan (www.hc-sc.gc.ca/pphb-dgspsp/cpip-pclcpi/) incorporates the Meltzer model in estimating that 5–10 million Canadians could become clinically ill, such that they would be unable to attend work or other activities for at least half a day. Furthermore, an estimated 2–5 million Canadians would require outpatient care, between 30 000 and 140 000 would require admission to hospital, and between 10 000 and 60 000 could die. As such, although it is staggering to imagine and difficult to accept, the estimate of 50 000 cited in the CMAJ article1 refers not to the number in Canada who could become ill but to the number who could ultimately perish.

Because communities would be affected over a short period of time (6–8 weeks), simultaneously and possibly in 2 waves during the same season, a pandemic of influenza will be unlike any other catastrophe. The SARS outbreak was an important but limited rehearsal that has helped further refine our preparedness. Technology has improved substantially since avian influenza H5N1 first appeared among humans in 1997, and our ability to detect and respond to this virus and its pandemic potential is far better. Either way, it is clear that a pandemic of influenza has the potential to be exponentially worse than SARS in its capacity to cause human suffering — both illness and despair — let alone economic and social upheaval.

Given the warning signals repeatedly emanating from Southeast Asia, an all-out and unified international effort to mitigate this possibility should be undertaken now.

Danuta Skowronski Epidemiology Services BC Centre for Disease Control Society Vancouver, BC