Rates of swine flu are levelling off in the northern hemisphere as summer progresses. The question now is whether we’ll have a second wave this winter, and if so how bad it will be. Two evolutionary virologists writing in JAMA are cautiously reassuring (JAMA 2009;302:679-80). Looking back over the 14 or so pandemics since 1510, they say that pandemic flu has never been able to infect the entire population at once, and although it tends to recur after a first wave, it eventually adopts the familiar seasonal flu pattern.

As for A/H1N1, they say its modest transmission efficiency and its arrival in the northern hemisphere’s early summer, as well as the degree of pre-existing population immunity from previous flu viruses and vaccines, …