MEXICO CITY - Mexico's top health official said Thursday the number of new swine flu cases is stabilizing in the country at the epicentre of the outbreak.

Health secretary Jose Angel Cordova told a news conference he hoped the trend will continue and that a vaccine would be available in six months.

The World Health Organization's flu chief, reacting to similar comments from other Mexican officials, cautioned that case numbers often go up and down, and said the WHO had yet to see concrete evidence that swine flu, believed to have killed 168 people in Mexico, was levelling off.

"It's a mixed pattern out there," Dr. Keiji Fukuda said. "What's happening in one part of the country is not necessarily what's happening in another part of the country."

The Mexican health secretary's comments followed similarly hopeful remarks from the mayor of Mexico City, who said statistics indicated "we are entering a period of stabilization."

On Wednesday, Mexican President Felipe Calderon ordered citizens to stay home, businesses to close and government services to be suspended for five days beginning Friday.

Calderon said only essential businesses such as supermarkets, hospitals and pharmacies should stay open, and only critical government workers such as police and soldiers would be on duty from Friday through Tuesday.

School had already been cancelled countrywide through Tuesday. The steps are aimed at stopping further spread of the virus even though the WHO has suggested countries should focus on minimizing its effects, not containing its spread.

"There is no safer place to protect yourself against catching swine flu than in your house," Calderon said Wednesday night in a televised address. He defended the government against criticism that it had been slow to act against signs of a new and dangerous virus.

Calderon said authorities would use the five-day partial shutdown in Mexico to consider whether to extend emergency measures or ease some restrictions. The dates include a weekend and two holidays, Labour Day and Cinco de Mayo, minimizing the added disruption.

Even before the shutdown went into full effect, a surprised radio reporter exclaimed that traffic was unusually light Thursday. Businessmen in surgical masks trudged in for their last day of work, passing beggars who kept their masks on too. Even the capital's legendary smog seemed to be easing.

Several countries have banned travel to or from Mexico, and some countries have urged their citizens to avoid the United States and Canada as well. Health officials said such bans would do little to stop the virus.

Medical detectives have not pinpointed where the outbreak began. Scientists believe that somewhere in the world, months or even a year ago, a pig virus jumped to a human and mutated, and has been spreading between humans ever since.

China has tried to squash any suggestion it's the source of the swine flu after some Mexican officials suggested it sprang from China or elsewhere in Asia. A Mexican health official has also suggested the virus could have been brought to Mexico from Pakistan or Bangladesh.

By March 9, the first symptoms were showing up in the Mexican state of Veracruz, where pig farming is a key industry in mountain hamlets and where small clinics provide the only local health care.

The earliest confirmed case was a five-year-old Veracruz boy, one of hundreds of people in the town of La Gloria whose flu symptoms left them struggling to breathe. People from La Gloria kept going to jobs in Mexico City despite their illnesses, and could have infected people there.

Days later, a door-to-door tax inspector was hospitalized with acute respiratory problems in the neighbouring state of Oaxaca, infecting 16 hospital workers before she became Mexico's first confirmed death.

Mexico's health care system has become the target of widespread anger and distrust. In case after case, patients have complained of being misdiagnosed, turned away by doctors and denied access to drugs.