Keiji Fukuda, the WHO's top influenza expert, said the UN agency's six-phase system for declaring a pandemic had sown confusion about the flu bug which was ultimately not as deadly as the widely-feared avian influenza.
"The reality is there is a huge amount of uncertainty (in a pandemic). I think we did not convey the uncertainty. That was interpreted by many as a non-transparent process," Fukuda said.
He was addressing a three-day meeting of 29 external flu experts called to review WHO's handling of the first influenza pandemic in 40 years.
Critics have said the WHO created panic about the swine flu virus, which turned out to be moderate in its effect, and caused governments to stockpile vaccines which went unused.
Some questioned its links to the pharmaceutical industry after companies like GlaxoSmithKline and Sanofi-Aventis made big profits from producing H1N1 vaccine.
H1N1, which emerged in Mexico and the United States almost exactly a year ago, has killed 17,770 people in 213 countries, according to the WHO, which declared a pandemic under way in June. Most victims were young, with an average age of 37, versus 75 for seasonal flu.
The WHO will need another year or two after the pandemic is declared over to determine a final death rate from the virus. The pandemic is still officially under way.
The separate but highly lethal H5N1 bird flu virus -- which has killed 60 percent of those infected since 2003 -- "injected a high level of fear about the next pandemic," Fukuda said.
It had been difficult to meet public demands for advice as the H1N1 virus spread quickly across borders, and blogs and other new media generated speculation and criticism, according to the WHO official.
"Populations around the world have very high expectations for immediate information," Fukuda said. "In many ways it is unforgiving out there."
One big surprise had been that only one dose of vaccine was needed to provide immunity, whereas most planning had been built around two doses being required, he said.
This meant that some countries were left struggling with an oversupply of unused vaccines while poorer ones had little or no access to supplies.
"Confusion about phases and level of severity remains a very vexing issue," Fukuda said, referring to the WHO's six-level scale for pandemics which takes into account the geographic spread of a virus but not its severity.
The WHO tried to come up with quantitative basis for measuring the pandemic's severity using death rates, but this proved difficult as countries provided different levels of information. Many lack even basic birth and death registries.
"Many countries don't have the actual capacity to determine reliably the severity of the virus," said Dr. Martin Cetron of the US Centers for Disease Control and Prevention, one of the experts taking part in the review.
WHO Director-General Margaret Chan said her agency welcomed a frank, critical review of its handling of the pandemic to help it prepare for future public health emergencies.
"We want to know what worked well. We want to know what went wrong and, ideally, why. We want to know what can be done better and, ideally, how," Chan told the session.
