Governments and health authorities worldwide went on the alert over the weekend for a possible influenza pandemic as the death toll from a new strain of swine flu in Mexico reached 81.

Janet Napolitano, homeland security secretary, yesterday declared a “public health emergency” in the US as about 20 people there were confirmed to have been infected, though none is seriously ill.

The World Health Organisation in Geneva declared “a public health emergency of international concern”.

Keiji Fukuda, WHO assistant director-general for health security, said its emergency committee would decide tomorrow whether to raise the official pandemic threat level above “phase three”, where it has been for several years in response to the threat from H5N1 bird flu in Asia.

A pandemic would seriously harm the prospects of the world economy recovering from recession, with the travel and leisure sectors particularly vulnerable – as the 2003 Sars outbreak showed. Authorities in several Asian countries were yesterday starting to screen travellers at airports and border crossings.

The Mexican authorities declared a state of alert in and around Mexico City, the sprawling capital, with about 20m inhabitants where the outbreak is centred. They cancelled all public events over the next few days, including ministerial speaking engagements, large-scale meetings, rock concerts in the city and even football matches. The Mexican Football Federation confirmed that yesterday's two big games in Mexico City would be played before empty terraces.