Health officials from around the world are attending WHO's annual meeting in Geneva this week to discuss the outbreak that has infected 9,000 people in over 40 countries, killing 76 of them.
Flu experts have told WHO that vaccine manufacturers will not be ready to produce a swine flu vaccine until mid-July at the earliest, WHO reported Tuesday. Previously, WHO officials had thought production could start in late May.
Experts also found no evidence that regular flu vaccines offer any protection against swine flu.
Meanwhile, dozens of countries urged the WHO to change its criteria for declaring a pandemic, saying the agency must consider how deadly a virus is - not just how far it spreads across the globe.
"This flu will continue to spread across the United States," acting director of the U.S. Centers for Disease Control and Prevention Richard Besser told CBS' The Early Show.
Fearing a swine flu pandemic declaration could spark mass panic and economic devastation, Britain, Japan, China and others asked the global body on Monday to tread carefully before raising its alert. Some cited the costly and potentially risky consequences, such as switching from seasonal to pandemic vaccine, even though the virus so far appears to be mild.
Although no formal changes were made Monday, WHO said it would listen to its members' requests.
"It's certainly something we will look at very closely," said Dr. Keiji Fukuda, WHO's flu chief.
The alert for swine flu is now at phase 5, which means the virus is spreading unchecked inside at least two countries in a single region. Under the existing rules, phase 6 indicates outbreaks in at least two different regions of the world and that a pandemic is under way.
"We need to give you and your team more flexibility as to whether we move to phase 6," Britain's Health Secretary Alan Johnson told WHO Director-General Dr. Margaret Chan, a public health veteran who has made combating the outbreak her top priority since the new virus appeared in North America last month.
Key developments on swine flu outbreaks, according to U.S. Centers for Disease Control and Prevention, World Health Organization and government officials as of May 19, 2009: