People who regularly catch flu or have repeatedly been vaccinated against flu may have more immunity to the H1N1 virus.

Scientists in California have discovered that the virus has similarities to seasonal flu and that the human immune system recognises these so-called markers.

This explains why the over-50s are less likely to become infected and cases are less severe.
Children on the other hand have not been exposed to similar infections and therefore have less immunity.

Although the elderly may have greater resistance to the disease, health authorities still recommend they become vaccinated.

Photo: flu vaccine in production - ANP