The existing formula is simple. When vaccinating against influenza, inoculate those most susceptible to the diseases wrath. Such vulnerable types include the elderly (who are the most likely to die if infected) and infants (whose immune systems are not fully developed). This seems a reasonable policy, and it is the one that has long been promulgated by Americas Centres for Disease Control (CDC). Only recently has it been extended to include children up to the age of 18, on the basis that they are more likely than other people to catch flu in the first place, through enforced socialising at school — even though they are at little risk of dying from it.

According to Jan Medlock of Clemson University in South Carolina, and Alison Galvani of Yale, however, vaccinating those most at risk of bad effects is not the right way to deal with the disease. In a report published this week in Science, they argue that even with the extension of vaccination to school-age children, the existing policy of protecting the individual is still playing down the real public-health value of vaccines-namely that they create a so-called herd immunity which helps to break the diseases chain of transmission.

They argue that it would be better to concentrate on vaccinating those most likely to spread the virus-both schoolchildren and people between the ages of 30 and 40, who are likely to be the parents of those children, and who are, at the moment, at the bottom of the vaccination priority list. That, at least, is the outcome of their mathematical model of how influenza spreads. Indeed, it is almost all of the outcomes. For in order to obtain a robust result, Drs Medlock and Galvani considered two different sorts of epidemic and five different definitions of an optimal conclusion.

... contd.