Flu vaccines work by introducing a killed version of circulating virus strains, which trains the body's immune system to recognize and attack similar invaders in the future.

This specific H1N1 virus rapidly circulating around the world, then, is not a new phenomenon but simply a new strain of influenza.

The new H1N1 virus rapidly crowded out other circulating influenza viruses and appears to have displaced the older H1N1 virus.