If the new cases do indeed represent "community-level sustained transmission" of the virus in Asia, it would be the second region of the world to have wide spread of H1N1 swine flu. That would meet the official WHO criteria for moving from the current level 5 pandemic alert to the ultimate level 6 alert.
Aside from the emotional shock, the formal pandemic declaration won't mean a lot to the U.S., Anne Schuchat, MD, the CDC's interim deputy director for science, said at a news conference.
"We have seen sustained spread of this virus in the U.S. and are acting aggressively, so a change from level 5 to level 6 will have less effect on us than on other regions that have done less to deal with virus," she said.
Declaration of a pandemic does not mean that the virus has become more deadly, only that it is spreading more widely around the globe.
While H1N1 swine flu does appear more dangerous than seasonal flu bugs -- especially for older children, teens, and young adults -- the vast majority of cases have been relatively mild.
"From the virus strains we have tested we see no indication of a change to more virulent strain. But viruses do change and we will continue looking at this," Schuchat said.
Even so, the U.S. has recorded its sixth swine flu fatality, a 55-year-old assistant principal at a school in the New York City borough of Queens. The city has closed eight schools in Queens and Brooklyn; news reports suggest that 40 other New York schools have high absentee rates.
Despite the worrisome news from New York, Schuchat said the swine flu is now spreading most quickly in the Pacific Northwest and in the Southwest. Case counts -- 5,123 as of today -- are "the tip of the iceberg."
"The way this virus is spreading in the U.S., we are not out of the woods and disease is continuing," Schuchat said.
More than 200 Americans have been hospitalized. Most of those hospitalized have been young people between that ages of 5 and 24, and very few have been over 65. The reason for this is not clear.
That's the same disease pattern seen worldwide. In a speech today to the World Health Organization's annual meeting of health ministers, WHO Director-General Margaret Chan, MD, said that while the WHO has not yet declared a pandemic, H1N1 swine flu is expected to spread rapidly to new countries.