A new flu strain can spread around the globe in two months. Credit: © GettyImages

A new mathematical model of influenza evolution could be a step towards the development of more effective flu vaccines.

Every year new flu strains replace old ones, rendering vaccines obsolete. The model could help researchers to predict key features of successful strains and act to contain them.

The model "allows us to vary the biology of the microbe and the human host, and see how these changes affect viral evolution", explains Robin Bush, one of the researchers, at the University of California, Irvine.

Bush and her colleagues pooled information on disease spread, the build-up of human immunity and genetic data from virus family trees gathered over the past 20 years1.

Understanding how flu changes is vital: it kills thousands of people every year, particularly the young and elderly. Vaccination can limit the number and severity of these infections, but every few decades a new strain emerges that can kill many more - for example, up to 40 million people died in the 1918 Spanish flu pandemic.

"Statistically speaking, we're due another pandemic fairly soon," says Neil Ferguson, the mathematical biologist at Imperial College in London who developed the model. Now that viruses can hitch a ride on passenger jets, a new infection could spread around the globe in two months, he warns. Vaccine development struggles to keep up.

But models can be limited in their predictions, warns Alan Hay, a vaccine adviser to the World Health Organization (WHO) who works at Britain's National Institute for Medical Research in London. They lack data, he says, on how small genetic changes in the influenza virus make humans more or less immune to different viral attacks.

But any research that may speed the development of vaccine design is welcome. Aided by 112 institutions in 83 countries, the WHO monitors flu outbreaks around the world and issues vaccine guidance accordingly. It makes predictions for winter outbreaks the preceding spring. In the interim, the virus can already have changed.

New for old

The new model also successfully predicts the curious 'new for old' policy of the influenza virus. One might assume that the number of strains would increase over time for a virus to maximize its chances of survival. In reality, new strains wipe out old ones, so the number of strains is stable over time.

This occurs because, while recovering from a bout of flu, we are temporarily immune to attack by a second flu strain. "The strains have to compete for susceptible people to infect," so one strain dies out, explains Ferguson.