In Mexico, the study compared death rates from diarrhoea and vomiting before the vaccine was introduced with the death rates in the year after the programme began.

The approval follows an expedited procedure which was based mainly on mock-ups of how the vaccines were expected to behave if bird flu rather than swine flu had sparked a pandemic.

Travelers from polio-affected areas should have a full course of vaccination against polio before leaving, with a minimum one dose of OPV before departure.