As the U.S. appeared to downplay rising alarm about the global H1N1 flu outbreak Friday, residents in Mexico City were under a government-ordered lockdown which made the world's second-largest city look like a ghost town.

While the Centers for Disease Control and Prevention said the current illness is an "unusual" combination of human and swine flu genes, it lacks the genetic power of last century's pandemic, which was one of the worst in history.

The 1918 outbreak, an H1N1 strain which struck healthy young adults, killed up to 50 million worldwide.

"We do not see the markers for virulence that were seen in the 1918 virus," Dr. Nancy Cox, the CDC's flu chief, said in Atlanta on Friday.

Echoing that sentiment, U.S. President Barack Obama said scientists aren't yet sure if the current outbreak is actually worse than "ordinary flus," which strike the public every year.

While the new flu "is relatively mild on the front end, it could come back in a more virulent form during the actual flu season," Obama warned in Washington, following a cabinet meeting dominated by discussions about the illness.

However, in Mexico City, the danger remained very real.

The massive metropolis is considered ground zero for the outbreak, and so far, 15 people have died from the infection and another 343 have been infected.

While the city's streets are usually crowded and bustling this time of year with May Day revellers, the streets were nearly deserted Friday as millions obeyed an order to stay home for five days.

Only essential services like hospitals and supermarkets will remain open over the break, which is aimed at ending the spread of the illness.

Meanwhile, the World Health Organization said there is "no doubt" scientists can quickly develop a vaccine for the H1N1 flu virus as news broke Friday that the virus had spread to France.

Marie-Paule Kieny, director of the WHO's initiative for vaccine research, said samples needed to make a vaccine could be ready to send to manufacturers by mid-May.

However, he added that it would take four to six months for the first doses to be made public.

"We have no doubt that making a successful vaccine is possible in a relatively short period of time," Kieny said Friday in Geneva.

Kieny said it's unlikely the seasonal vaccine will be effective against the H1N1 virus.

Anne Schuchat, an interim deputy director at the CDC, said officials must plan for vaccines a long time in advance. She said officials are working on creating a vaccine as a precautionary measure.

"We haven't made the decision yet that we will manufacture large numbers of doses of vaccine and we haven't made the decision yet that we will use a vaccine if we produce a lot of it," Schuchat told reporters Friday.

"Those are decisions that need to be done in a deliberative, careful, thoughtful way... it's really scientific expertise that helps with those types of decisions."

The WHO said the H1N1 flu has been confirmed in 14 countries around the world.

Gregory Hartl, a spokesperson for the WHO, Officials in Hong Kong, Denmark and France also independently reported cases of H1N1 flu Friday.

In Mexico, there have been 15 confirmed deaths. In the U.S., there has been one death linked to the outbreak.

In total, here is the breakdown of confirmed cases worldwide from the WHO:

• Mexico: 343
• U.S.: 149
• Canada: 51 cases
• Spain: 13 cases
• United Kingdom: 11 cases
• Germany: 5 cases
• New Zealand: 3 cases
• France: 2 cases
• Israel: 2 cases
• Netherlands: 1 case
• Switzerland: 1 case
• Denmark: 1 case
• Hong Kong: 1 case
• Austria: 1 case

In Hong Kong, officials said the victim was a Mexican citizen who developed a fever Thursday after arriving in the Chinese territory via Shanghai.

The patient has reportedly been isolated in hospital and is in stable condition.

As a sign of increasing concern about the outbreak, a Washington-bound flight from Germany had to be diverted after a passenger complained of flu-like symptoms.

The plane, with 245 passengers and 14 crew, landed in Boston at the request of the CDC on Friday afternoon.

A 53-year-old female passenger was taken off the plane and taken to hospital and the flight continued onto Washington.

It isn't yet known if the woman has the H1N1 flu.

WHO on defensive

Meanwhile, the chief epidemiologist in Mexico, where H1N1 flu was first reported, said the WHO responded too slowly to warnings about the health crisis.

Dr. Miguel Angel Lezana is calling for an investigation of the WHO, saying his centre alerted the Pan American Health Organization about the problem as early as April 16.

Eight days later, the WHO said it was "very, very concerned" about the threat of a pandemic.

"It seems it should have been more immediate," Lezana, director of the National Epidemiology Center, told The Associated Press in a recent interview.

In response, the WHO said Friday that they learned of cases of "suspicious influenza" on April 9 from Mexico.

The WHO said it responded quickly on April 24 when U.S. and Canadian laboratories identified the virus as the H1N1 flu.

"We moved into operation within a matter of hours," WHO spokesman Thomas Abraham told reporters.

To stop the spread of the virus, Mexico has ordered a suspension of nonessential federal government and private business activity from May 1-5.

The WHO raised its alert level to Phase 5 Wednesday, meaning the probability of a pandemic is "high to certain," as the virus spreads around the planet.

However, Abraham said Friday that there were no meetings of the emergency committee scheduled for today.

The committee can recommend whether to raise the WHO's pandemic alert level.

With files from The Associated Press