So far there seem to have been around 159 fatalities among perhaps 2000 cases in Mexico, although only a small number of these have been confirmed in the laboratory as influenza infections. Although this suggests a scary fatality rate of around 10%, there has so far been just one death outside of Mexico – of a 23-month-old Mexican child in Texas, who had recently been in Mexico. The "case fatality" of the virus is not yet known.
This is the key point that scientists are now investigating. Travellers returning to their home countries after acquiring the virus in Mexico have experienced mild influenza-like illness. What might explain this apparent discrepancy in disease outcome?
A matter of numbers
One simple answer is that there have been many more mild cases in Mexico than counted so far and that the true case fatality rate is more like 1 to 2%. This would be akin to that seen during previous pandemics such as the 1957 (H2N2) Asian influenza or the 1968 (H3N2) Hong Kong flu. Even with the 1918 Spanish influenza, which killed tens of millions, the case fatality rate is estimated at between only 2.5 and 5%.
In other words, as cases outside of Mexico increase, we may expect a small number of fatalities from what is otherwise a rather mild virus. Increased epidemiology – for example, monitoring the spread of the virus and the strain type in the affected areas in Mexico – will shed light on this issue.
Another possibility is that the Mexican fatal cases have been due to co-infections with other pathogens that together lead to a worse outcome, or that somehow the indigenous population of Mexico is genetically predisposed to fare badly with this particular influenza strain. There is no evidence to support either of these possibilities, but then so far we have few clinical details of any of the cases.