Monday, June 22, 2009

H1N1 (aka American flu) - mortality rates, real or not?

Minister of Health Khaw yesterday at his press conference quoted a mortality estimates of H1N1 of 0.37%. This is already a much lower estimated than was initially computed. And even less than what I had earlier roughly estimated (~0.55%) from the Wikipedia data.

But are the numbers a good reflection of the real risk of H1N1?

We won't really know because everyone has a different way of computing these numbers depending on whether they want them up or down. The crude estimates would be to take the total reported deaths divided by the total reported cases. If you take the current CDC figures for US it is 87/21449 = 0.4%, not far from Minister Khaw's figures.

Are these figures believable? Well they are pretty much what we have at the moment. It is probably quite an overestimate I believe. Why do I say this? Well.... reported case numbers do tend to be an underestimate, because many who develop flu symptoms may not be tested for H1N1 at all. On the other hand the deaths numbers are probably an overestimate because many H1N1 cases who die have many other concurrent problems and their deaths though associated with H1N1 may not be directly attributed to H1N1. So in all likelihood the numerator is an overestimate while the denominator is an underestimate, making it very likely the actual risk of dying is much less than the ~0.37% quoted by Minister Khaw.

Also the numbers have been inflated because the early death rates in Mexico and even the US were very high.... This has come down substantially. If you look at data from UK, Australia and Canada,...those with health care standards on par with the US and were involved later, the death rates are much lower - UK (1/2773 = 0.036%), Australia (1/2733 = 0.036%) and Canada (16/6457 = 0.25%).

Peter Doshi published a paper in the American Journal of Public Health last year criticizing the ways that scientists have traditionally computed fatality risks of pandemics. He is of the opinion the the risks have generally been inflated, for various reasons.

I quote from his paper:

"The notion that pandemic influenza’s fundamental property is excess mortality is difficult to reconcile with the recorded influenza death data over the past century. There are many possible explanations, one of which may be the tendency to generalize the exception—the 1918—1919 pandemic. In 1918, doctors lacked intensive care units, respirators,respirators, antiviral agents, and antibiotics, an important fact in light of historical evidence of interactions between influenza and secondary bacterial respiratory pathogens (e.g., Haemophilus influenzae) as a significant cause of death during the pandemic.

It is also important to recognize that commercial interests may be inflating the perceived impact of influenza and other infectious “pandemics.” There is a clear need for more evidence-based accounts of influenza in the context of historical epidemiology and current social and medical advances."

He concludes (2008):
"Whatever the reasons for the misconceptions, should the trends observed over the 20th century continue to hold in the 21st, the next influenza pandemic may be far from a catastrophic event."


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