LONDON - Health experts say extraordinary measures against H1N1 flu -- most notably quarantines imposed by China, where entire planeloads of passengers were isolated if one traveler had symptoms -- have failed to contain the disease.

Despite initially declaring success, Beijing now acknowledges its H1N1 flu outbreak is much larger than official numbers show.

China's official count of some 63,000 reported illnesses with 53 deaths dwarfs estimates of millions of cases with nearly 4,000 deaths in the United States, a nation with about a third of China's population.

Dr. Michael O'Leary, WHO's top representative in China, says there has been a dramatic spike in Chinese H1N1 flu cases recently and those reported by the government are only "minimum numbers."

"We have new cases occurring all the time," he told The Associated Press last week. "There's always more deaths than we could possibly know about."

He said there is little data to prove interventions like mass quarantines and school closures slow down disease transmission. "To draw a causal link ... is not always possible," O'Leary said, adding that WHO expected a disease as contagious as H1N1 flu to spread regardless of what measures countries impose.

China's Health Minister Chen Zhu defended his country's aggressive quarantine policy, telling the AP on Wednesday that the measures helped slow the spread of the virus long enough for China to develop a vaccine, which authorities are now scrambling to administer.

"With initial efforts of containment, actually we not only reduced the impact of the first wave to China, but we also won time for us to prepare the vaccine," Chen said in an interview on the sidelines of a meeting of the Global Forum for Health Research in Havana.

He said China was vaccinating 1.5 million people a day against H1N1 flu as part of a massive effort to try to reach as many as 90 million people -- about 7 percent of the country's population -- by the end of the year.

"We know this is not enough for a population of 1.3 billion, but at least for the vulnerable people, for the students, people with underlying basic diseases and ... for pregnant women, we have vaccines," Chen said.

China has acknowledged H1N1 flu is now widespread despite its aggressive attempts at containment.

Earlier this month, Feng Zijian, head of China's Center for Disease Control and Prevention, said the country's reported figures are only "a very small portion" of the total number of cases.

He said China is now focusing on confirming severe cases and no longer tests every person with a fever for H1N1 flu. He said the official figures were based on cases confirmed in outbreaks or at monitoring sites like hospitals.

Other nations that have carried out draconian H1N1 flu policies indicate they have little effect in containing the disease.

Ukraine, which reported more than 250,000 suspected cases last week, closed all schools and universities, and advised people not to travel and to stay away from public places. In Mongolia, all bus travel has been suspended and gatherings of more than 40 people have been banned. Still, both countries are now facing major H1N1 flu outbreaks.

Argentina, Singapore, Malaysia and Egypt have also enacted radical H1N1 flu prevention measures -- and all have been gripped by widespread outbreaks.

When WHO declared H1N1 flu to be a pandemic in June, it described the virus as "unstoppable." It advised countries not to close their borders or impose mass quarantines, warning such measures would be useless since people often spread flu viruses before developing any symptoms.

China is no exception, scientists say.

"China did not keep the virus out. They failed," said Dr. Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota.

He said he believes the actual number of H1N1 flu cases is "far in excess of what China is reporting," based on the center's own network of official and unofficial sources in the country.

Some experts say the relatively small size of China's reported outbreak is suspicious given that neighboring regions are battling huge epidemics. Last week, WHO said Mongolia, which borders China, was reporting its health system was being crushed by H1N1 flu cases.

In Hong Kong, a city of 7 million on China's southern coastline, authorities have reported 40 H1N1 flu deaths, compared to the 30 reported in China.

"The issue in China has to do with surveillance," said Sandra Mounier-Jack, a flu expert at the London School of Hygiene and Tropical Medicine. Because H1N1 flu symptoms are so vague, many cases are being missed in China, as they are everywhere, she said.

WHO and the U.S. Centers for Disease Control and Prevention gave up counting H1N1 flu cases months ago when the virus became widespread.

Past disease outbreaks also give experts reason to question China's numbers. In 2003, China covered up an epidemic of Severe Acute Respiratory Syndrome, which ultimately killed about 800 people when it spread worldwide.