An interesting article published in the Proceedings of the National Academy of Sciences correlates the La Niña conditions in the equatorial area of the Pacific with flu pandemics. The authors propose that the La Niña conditions (which upwell colder water to the surface, changes migration patterns of migratory birds. Since birds are one host for the influenza virus, these changed migration patterns may change how the birds interact with other species moving new influenza subtypes into different parts of the world.
We are, however, a long way from predicting a pandemic based upon a La Niña event. Right now, the authors can only correlate pandemics and the Pacific circulation patterns in just four cases: the Spanish Flu of 1918, the Asian Flu of 1957, The Hong Kong Flu of 1958, and the Swine Flu of 2009. It’s hard to make a case for causality based on these four data points, but the authors do lay out a compelling argument.
Whenever I critique a correlation vs. causation argument, I first determine if the causality is even possible. I often make the argument that MMR vaccinations cause broken arms in 12-16 year old girls, because a small, but significant proportion of kids being vaccinated get broken arms (or car wrecks, gastroenteritis, an itchy nose, and angry comments to their parents). However, there is no physiological, biological, or scientific reason why a broken arm might result from a vaccination.
In this article, the authors do make a scientifically plausible case that avian migration patterns do change in La Niña events, so they are on the path to providing outstanding evidence to support this hypothesis.