To date, there have been 17,855 confirmed and probable cases reported and 45 deaths from the illness in all 50 states, the District and Puerto Rico, though deaths were confined to 15 states.
Alaska has the smallest number of cases with 11 and no deaths; Wisconsin has the most with 3,008 and one death. Texas followed with 2,049 cases and three deaths. Illinois ranked third (1,983 cases, five deaths), followed by New York (1,160 cases, 13 deaths).
There have been 33 cases is the District, 90 in Virginia with one death, and 139 cases in Maryland.
The specter of swine flu continues to rattle nerves.
After noting a sudden and "alarming" number of absences in the city's schools, officials in Philadelphia launched a publicity campaign to prevent flu panic, publishing a rationale explaining why schools should remain open and practical flu prevention techniques. School officials in Boston have shut 15 schools "due to swine flu fears."
The CDC and other federal agencies remained straightforward about the situation.
"WHO's decision to raise the pandemic alert level to Phase 6 is a reflection of the spread of the virus, not the severity of illness caused by the virus. It's uncertain at this time how serious or severe this novel H1N1 pandemic will be in terms of how many people infected will develop serious complications or die," the CDC said in an advisory.
The agency called its experience with the virus "limited," pointing out that many people may have little or no immunity to it. Currently there is no vaccine, though $1 billion in federal funds have been designated to develop an antigen.
So far, swine flu has been found in 74 countries, resulting in 144 deaths.
The WHO announcement was no surprise to Health and Human Services Secretary Kathleen Sebelius, who said her agency was on the case.
"Once we saw how fast this virus was spreading, we activated our pandemic plans and started doing all the things we needed to do to keep the public as safe and secure as possible," she said. "What this declaration does do is remind the world that flu viruses like H1N1 need to be taken seriously. Although we have not seen large numbers of severe cases in this country so far, things could possibly be very different in the fall."
An immunization campaign could start in late September, Mrs. Sebelius said.
Homeland Security Secretary Janet Napolitano was also adamant that her shop was primed for a pandemic.
"We acted aggressively to stay ahead of the virus as it spread across the country. Now our challenge is to prepare for a possible return in the fall," she said. "We are working with our scientists to test and prepare a possible vaccine. And we are working with governments around the world to share what we know and learn from what is happening in their countries."
Dr. Richard P. Wenzel, chairman of the Department of Internal Medicine at the Virginia Commonwealth University School of Medicine, gave a favorable review of the status quo.
"It's absolutely important that the feds have called this a public health emergency, because what this does is heighten our scenario playing and improve communications," he said. "Our level of preparedness should go up so that we can say, 'If this happens then this is what our response will be.' And we have everything going in terms of hotlines established, communications across public health authorities, drugs, masks, all the supplies, including laboratory availability."
He also recommended down-to-earth prevention.
"It is important to use common sense, always wash hands, protect yourself from people who have respiratory illness and fever, and be alert to what's happening in your own region," Dr. Wenzel said, adding that young adults appear to be the most at risk with this strain of the virus.