Bowing to pressure, the World Health Organization announced Friday that it would rewrite its rules for alerting the world to new diseases, meaning the swine flu circling the globe will probably never be declared a full-fledged pandemic.

Dr. Keiji Fukuda, the deputy director general making the W.H.O. announcement, said that he could not predict exactly what the new rules would be but that criteria would include a “substantial risk of harm to people,” not just the geographic spread of a relatively benign virus.

The six-point system was created in 2005 when the threat was H5N1 avian flu, which has a fatality rate of about 60 percent. But the system does not take into account a virus’s lethality, and in the current outbreak, some countries have complained that the warning system created panic and pressure for border closings, even though the strain was less deadly.

Asked if the W.H.O. could damage its credibility by changing the rules in mid-outbreak, Dr. Fukuda said: “There’s nothing like reality for telling you whether something is working or not. Rigidly adhering to something that is not working would not be very helpful.”