LOS ANGELES AND MEXICO CITY — The number of swine flu cases continued its slow climb, reaching 263 in the United States and at least 937 in 19 countries worldwide, but both Mexican and U.S. authorities expressed cautious optimism Sunday that the outbreak may not be as severe as originally feared.
U.S. officials continued to express confidence that the H1N1 virus was not unusually virulent, but they cautioned that the number of cases and deaths would rise. In Mexico, however, officials said the disease was on the decline.
"What I can say is that we're seeing encouraging signs," Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention, said on ABC News' "This Week With George Stephanopoulos." "That makes us all very happy."
"When we get a virus we look to see: Does it relate to any other viruses?" Besser said. "And then we look for things that are called virulence factors, those things that in the past have been linked to more severe disease. And what we've found is that we're not seeing the factors that were associated with the 1918 pandemic" that claimed millions of lives.
That 1918 virus, however, began mildly in the spring, but it returned in the fall in a more lethal form. Flu season is imminent in the Southern Hemisphere. Its severity and the way it spreads will help U.S. officials decide how to shape an H1N1 vaccine, and how much to produce.
Besser, who also appeared on CBS News' "Face the Nation," said on the show, "It's a rapidly evolving situation, and it's still one cloaked in uncertainty."
But, he said, "we're starting to see encouraging signs."
Nevertheless, in a telephone news conference, the CDC's Dr. Anne Schuchat cautioned against over-optimism. "We believe we are just in the upswing here," she said. "I expect the numbers to jump quite a bit in the next few days" and for there to be more deaths.
"Virtually all of the United States probably has this virus circulating now," she added. "That doesn't mean that everybody's infected, but within the communities, the virus has arrived."
The strain has been found in at least 34 states. The only swine flu death in the United States has been a Mexican boy who was visiting Texas.
A few more schools across the nation were ordered closed Sunday, including 24 in a Detroit suburb after a high school student developed symptoms, and 14 in New Mexico after the state's first swine flu case was confirmed.
