But a public health and infectious disease expert from Vanderbilt University, Dr. William Schaffner, said the test results were “going to change, I think in a substantial way, the image of this outbreak in Mexico.”

If the outbreak is much smaller than initially thought, Dr. Schaffner said, “It would, I think, enable the world’s public health community to take a deep breath and continue to track the outbreak and reduce the tendency, as the W.H.O. has been doing, to notch up on its pandemic scale.”

If the testing also shows that the disease has caused fewer deaths than the 170 or so suspected, Dr. Schaffner said, it might resolve a question that has been puzzling health experts since the outbreak began: why did the disease appear to be so much more severe in Mexico than in the United States? In the United States, cases have been mild and there has been only one death, that of a 23-month-old child from Mexico. Meanwhile, the disease continued to spread to other countries and was confirmed in more American states on Friday. The disease is expected to drop off during the summer, because flu viruses do not thrive in heat and humidity, but it could rebound in the fall and winter. The World Health Organization said that the flu vaccine given to millions of people for the most recent flu season appeared ineffective against the A(H1N1) strain, but that health officials were talking to manufacturers about creating a new swine-flu vaccine, which would take four to six months to produce.

Dr. Marie-Paule Kieny, director of the Initiative for Vaccine Research at the World Health Organization, said that unless the numbers of cases decreased significantly, “it seems mostly likely that the manufacturers will proceed and we will certainly support them.”

Officials at the Centers for Disease Control said a decision had not yet been made about whether to manufacture a vaccine, but President Obama said that the government would support it.

New cases were reported in Denmark, France, Russia, Hong Kong and South Korea on Friday, but they were not confirmed by the health organization. The United States reported 141 confirmed cases in 19 states, up from 109 cases in 11 states on Thursday.

Concerns about the disease are having an increasing impact. On Friday, a United Airlines flight with 245 passengers heading from Munich to Dulles Airport in Washington landed in Boston instead because a female passenger had flu symptoms and the airline thought she needed prompt attention, a United Airlines spokesman said.