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.”

Speaking in Geneva, Dr. Fukuda added, “We’re trying to walk a fine line between not raising panic and not being complacent.”

The W.H.O., starting in April, quickly raised its alert level to 4 and then 5 as the virus spread in North America. But even as the virus infected people in Britain, Spain and Japan, the agency did not go to Level 6, which signifies spread to a new continent. Dr. Fukuda argued that there was still no proof of “community spread,” meaning beyond travelers, schools and contacts. Some experts were skeptical.

While acquiescing, he noted that experts hashed out these issues in 2005. Geographic spread is easy to detect, but severity is highly subjective. Death rates are impossible to calculate before many people are infected; if they turn out to be high, precious time has been lost. Viruses can mutate, becoming more lethal, and even a less lethal strain can kill many people in poor nations with young, malnourished and AIDS-infected populations.

Dr. Fukuda also said the new virus has been confirmed in 42 countries and had killed 86 people. About half of those hospitalized are young and healthy with no underlying conditions, he said. In the United States, such conditions are more common among the 300 now hospitalized.