TO MANY, the swine flu panic of 2009 was an overreaction. As the bug spread from Mexico, health officials began to fear the worst. The World Health Organisation declared it to be a pandemic. Airports and schools were shut down, and manufacturers were ordered to work double-time to produce a suitable vaccine.

Though the strains of A/H1N1 virus that caused the panic did not prove particularly deadly outside Mexico, officials gave warning that this might change. It is common for viruses that originate in animals and go on to afflict humans to evolve by recombining their genetic material with that of other strains. This can make them more virulent. In the case of A/H1N1, however, the bug remained mostly benign, so popular attention has faded. That is a mistake, argues a study published this week in Science.

As part of a long-running research project (beefed up since last year's outbreak), a group at the University of Hong Kong has been monitoring the viruses of pigs slaughtered in the territory's main abattoir. Malik Peiris and his colleagues have found strong evidence that the A/H1N1 virus afflicting humans is indeed recombining in pigs. They saw the mingling of A/H1N1 with two other types of swine influenza virus. These were the North American triple-reassortant viruses and the Eurasian avian-like swine viruses. They did not, however, observe reassortment with human seasonal influenza viruses, something they had worried might happen.

What all this means is unclear. It is possible that the new recombinations will make the virus even less dangerous. But the opposite is also possible. It would make sense, therefore, to keep a sharp eye on other pig populations around the world, just in case.