Thoughts on the novel influenza A(H1N1) outbreak

It has been almost two months since Mexican health officials detected a surge in cases of influenza-like illness (ILI) in Mexico city, heralding the start of our first global influenza A pandemic of the 21st century. Just to be clear, it is not officially a pandemic yet. For that, the World Health Organization (WHO) will have to acknowledge that at least one other country outside the “North America region” (to which USA, Canada and Mexico belong) is seeing sustained community transmission of the novel influenza A(H1N1) virus. But a quick glance at the map of H1N1 confirmed cases (reproduced below) provided by WHO will demonstrate the incredible spread of the virus within just a couple of months.



Three important interlinked questions that have not been definitively answered are:
1. How serious is an infection by the novel virus?
2. What will be the course of the pandemic?
3. What should we do about the pandemic?
We will try to touch on the first question today.

Many people hypnotized by the WHO “scorecard” will conclude that, as of 15th May 2009, the case fatality rate (CFR) of this virus is 0.3% (corresponding to 65 deaths in 7,520 confirmed cases) – a rate higher than any previously documented pandemic except for the Spanish flu of 1918-1919. Others argue that these figures are misleading – confirmed cases only represent a small fraction of the actual number of H1N1 cases. Dr Daniel Jernigan, deputy director of US Centers for Disease Control and Prevention (CDC)’s influenza division, had expressed his opinion in a news conference yesterday that the number of confirmed cases reported to US CDC was a gross underestimate – perhaps by more than a factor of 10. This would put the CFR of influenza A(H1N1) virus more in the league of seasonal rather than pandemic influenza. Not that this is a figure to be sneezed at, of course – more people are projected to die from causes related to influenza in Singapore each year than from the combined mortality of all the diseases listed on our ministry’s weekly infectious disease bulletin.

The scenario painted by Dr Jernigan also makes more sense from the epidemiological viewpoint. There is just no way that “merely” 6,000 or so cases in Mexico and US can result in the export of so many cases so quickly to so many countries in the rest of the world.

But there are disturbing possibilities that lie ahead that we will try to address in future posts. Namely, the chance that a second wave of the pandemic virus will result in more severe disease and higher mortality; and the chance that this new virus will re-assort with current existing influenza viruses, resulting in a “monster hybrid” that will either be resistant to tamiflu, or will have the virulence of avian influenza (whose CFR shockingly exceeds 60% to date), or both.