The leader of an international team helping Mexico face down the swine flu outbreak said it should soon learn whether the epidemic is really stabilizing in Mexico, but that many key questions about how the disease kills still need to be answered.

Dr. Steve Waterman, the head of a team from the Centers for Disease Control and Prevention, also warned against taking false comfort from the fact that only one person has died outside Mexico, saying more deaths are likely as the epidemic evolves.

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"That is the big question: Is it stabilizing or not? And it is too early to say, but I think we are getting systems in place where we are going to be able to get a handle on this soon," Waterman, standing amid CDC doctors and specialists at the Mexico City nerve center where officials are confronting the outbreak, said Friday.

Mexican officials have been cautiously optimistic that the worst is over here, even as the government took additional protective measures Friday by beginning a five-day shutdown of all nonessential government and private business.

In Washington, even President Barack Obama voiced hope Friday that the new virus may turn out to be no more harmful than the average seasonal flu.

"It may turn out that H1N1 runs its course like ordinary flus, in which case we will have prepared and we won't need all these preparations," Obama said, using the flu's scientific name.

In New York City, which has the most confirmed swine flu cases in the United States with 49, swine flu has not spread far beyond cases linked to one Roman Catholic school.

The U.S. case count rose to 155 on Friday, based on federal and state tallies, although state laboratory operators believe the number is higher because they are not testing all suspected cases.

Worldwide, the total confirmed cases were 653, with the real number also believed to be much larger. The virus has also been detected in Canada, New Zealand, China, South Korea, Israel and eight European nations.

China was suspending flights from Mexico to Shanghai because a case of swine flu was confirmed in a passenger on a flight from Mexico, China's state-run Xinhua News Agency reported. Hundreds of Hong Kong hotel guests and workers were quarantined after a tourist from Mexico tested positive for swine flu, Asia's first confirmed case.

Waterman, whose team is working with Mexican officials, said the scientists are trying to determine the mortality rate of the virus, and don't yet know where it started or why. But he and other experts said it appeared the outbreak could have been far more deadly, particularly in the teeming streets of Mexico's capital.

"The virus has been circulating for over a month in a city of 20 million of high population density. It could have been much worse," said CDC epidemiologist Marc-Alain Widdowson.

Waterman agreed that the virus does not appear to match the ferocity of past killers. "Most people think it is unlikely this is going to be as virulent as the 1918 epidemic. From what we know so far, it doesn't seem like it is as virulent," he said.

The two CDC doctors spoke during a tour of Mexico's Intelligence Unit for Health Emergencies, the operations center of the country's response to the disease. Teams of doctors and scientists sat at laptops monitoring the outbreak in real time. Plasma screens enabled frequent video conference calls with leaders from the Atlanta-based CDC, the World Health Organization and other institutions.

Hugo Lopez-Gatell Ramirez, deputy director general of epidemiology at the center, underscored the importance of getting fast and effective care. He said that among the 16 confirmed swine flu deaths in Mexico, the average time the victims waited before going to a doctor was seven days. For those who were sickened but recovered, the average wait was three days.

Lopez-Gatell said that even before the swine flu outbreak, Mexican authorities had been monitoring a higher-than-usual number of flu cases and an unusual phenomenon in which otherwise healthy young adults were falling ill with pneumonia in greater numbers. There had been 15 flu outbreaks in this year's flu season, as opposed to the 5 or 6 that Mexico normally sees.

He said that put Mexico on guard and led to a fast reaction when unexplained illnesses began in March. Despite some international criticism of the Mexican response, Lopez-Gatell said no mistakes were made.

"We would have done everything the same if we had it to do over again," he said.

Asked why the swine flu death rate is so high in Mexico while only one person is known to have died elsewhere, Waterman said that is one of the key questions they are trying to answer. One of the main reasons, they believe, is that there are a lot more people in Mexico who are sick than in other countries.

"The reason they haven't had any deaths, if the mortality rate is 1 percent and you only have 20 cases, you haven't had time to see that mortality yet," he said. He said many infected Mexicans may also have sought help too late to be treated successfully.

Friday was Mexico's Labor Day, which is normally a raucous day in the capital as the Paseo de la Reforma boulevard fills with hundreds of thousands of boisterous marchers headed to the central Zocalo square. This time, only a few tourists wandered down its broad sidewalks, lined with shuttered shops, banks and office towers.

"I'm going crazy in my house with this confinement," retiree Rocio Lara said of the shutdown of most businesses and other gathering places. "There is nowhere to go, nowhere to spend your time."