Where is China's Vice President Xi Jinping?

As Xi Jinping, the man tipped to lead China for the next decade, mysteriously stayed out of the public eye for a 10th straight day Tuesday, the official silence about his whereabouts only intensified speculation about his future.

Analysts can suggest no reason why Mr. Xi should not be made head of the ruling Communist Party at a long-awaited set-piece meeting expected next month. But his unexplained absence from public life at such a sensitive moment, in the run-up to a once-in-a-decade leadership change, is puzzling outsiders.

“There might be a simple and reasonable explanation, but they won’t explain it,” says Sidney Rittenberg, a former communist, familiar with Chinese leaders for more than half a century. The Chinese government “is still awkward in its relations with the outside world,” he adds.

Xi, who was anointed five years ago as heir apparent to Hu Jintao, head of the Communist Party and president of the nation, surprised observers when he canceled a meeting with US Secretary of State Hillary Rodham Clinton on Sept. 5 at the last moment.

A US official said at the time that Chinese counterparts had explained privately that Xi was suffering from back trouble. Xi also skipped a meeting with the Singaporean prime minister that day, and yesterday begged off a previously announced appointment with the Danish premier.

If Xi is indeed avoiding public appearances because of back pain, that would explain the authorities’ refusal to make any comment, says Zhang Jian, a professor of politics at Peking University.

“Leaders’ health has long been a very sensitive issue,” he says. “Bad health can be used as a weapon against you by your enemies. It would not be good for Xi Jinping to be labeled a ‘sick man’ just before such an important power transition.”

Overshadowed by scandal 

The run-up to the 18th Communist Party Congress, which is due to unveil the next generation of men who will lead China, has been overshadowed by scandal. Bo Xilai, a contender for one of the top nine party posts, has been detained and stripped of his rank during an investigation into the murder of a British businessman, Neil Heywood. Mr. Bo’s wife was convicted last month of the murder.

The party has still not announced an opening date for the congress, which is expected to be held in Beijing next month. The last congress in 2007 was announced two months in advance.

Analysts say, however, that it is highly unlikely that Xi’s extended disappearance indicates that he is in any political trouble so late in the carefully staged process of his elevation.

“It doesn’t seem that anything cataclysmic has happened in political life,” says Mr. Rittenberg. “If it had, something of that magnitude would have affected more than one person.”

If the cloak of secrecy thrown around the personal lives of China’s top leaders is almost impenetrable, says Zhang Lifan, an independent political commentator, it simply reflects “a Communist Party tradition of being opaque.”

“It’s not just leaders’ health,” Mr. Zhang argues. “The whole government policy-making process is non-transparent and the public just hears the results.”