From the moment last spring when swine flu swept out of Mexico and triggered the world’s first pandemic for 40 years, medical experts have emphasised the unpredictability of the influenza virus. Their refrain has been: “Expect the unexpected.”

So far, however, the new strain of H1N1 has more or less followed the script that epidemiologists wrote for it three months ago. The virus has stuck to the normal seasonal pattern of flu, so current outbreaks are more intense in the southern hemisphere where it is winter.