GENEVA (Reuters) - The spike in cases reported from China reflects reclassifying a backlog of suspect cases using patients’ chest images and not necessarily the “tip of an iceberg” of a wider epidemic, a top World Health Organization official said on Thursday.

Mike Ryan, head of WHO’s health emergencies program, said that more than 13,300 cases reported in Hubei province overnight came after a change to include results from quicker computerized tomography (CT) scans that reveal lung infections, rather than relying on laboratory tests.

“Crucially we understand that most of these cases relate to a period going back over days and weeks and are retrospectively reported as cases, sometimes back to the beginning of the outbreak itself,” he told a news conference at WHO headquarters.

“We’ve seen this spike in the number of cases reported in China, but this does not represent a significant change in the trajectory of the outbreak,” he said.

In addition China has reported a total of 46,550 lab-confirmed cases since the outbreak began in December, WHO figures show.

No significant shifts in mortality or severity patterns had been detected, Ryan said.

Conducting household studies and developing serology tests that determine the level of antibodies in a community to a virus would deepen understanding of the epidemic’s extent, he said.

“But this idea that this iceberg is absolutely massive and in some way we’re only detecting 1 or 2 or 5 percent, this is all based on modeling. This is based on certain assumptions,” he said. “And those assumptions and those speculations are as valid as speculations in the other direction.”

Cases are not rising dramatically outside China apart from among passengers on a cruise liner now quarantined off the Japanese port of Yokohama, Ryan said.

“Outside cases on the Diamond Princess cruise ship we are not seeing dramatic increases in transmission outside China.”

A further 44 cases were reported on the vessel on Thursday, raising the total to 219 though authorities said some elderly people would finally to be allowed to disembark on Friday.

Ryan said that members of a WHO-led mission were expected to start arriving in China over the coming weekend, joining an advance team led by Dr. Bruce Aylward already in Beijing to help investigate the outbreak.

“In terms of the international mission, the advance team and their Chinese counterparts have now finalised the scope of work and design of the mission,” he said, declining to give details.