The World Health Organization said Monday it still does not characterize a swine flu pandemic as inevitable, but raised its pandemic alert level to signal the increasing severity of the situation.
It also urged countries not to close borders or impose travel bans, saying such measures would be unlikely to stop spread of the virus and would cause economic harm.
But a number of countries had already taken actions, urging their citizens to avoid non-essential travel to places in the world where infections have been found. Canada, one of the affected countries, signalled it would warn Canadians against non-essential travel to Mexico, the country hardest hit by the swine flu outbreak.
It had been thought the WHO might raise the pandemic alert to Phase 5 from Phase 3, a position it has been at for years due to sporadic human infections with the avian influenza virus H5N1. But instead it took the more cautious approach of going to Phase 4.
The WHO's acting assistant director-general for health security and environment said experts on an emergency committee advising Director General Dr. Margaret Chan felt that while the world had taken significant steps towards the first pandemic since the Hong Kong flu in 1968, it's not there yet.
Further, Dr. Keiji Fukuda said, it may not get there with this unusual swine flu virus.
"A pandemic is not considered inevitable at this time," Fukuda said in a late evening press conference in Geneva.
The committee concluded there is definitely person-to-person spread of the virus, but felt it wanted more information about how capable the virus is at continuing to spread through generations of human hosts.
If the WHO saw a widespread community outbreak, he said, the committee would be reconvened and a decision likely made to go to Phase 5. Phase 6 is a pandemic.
"It's fair to say that the move from Phase 3 to Phase 4 signifies that we have taken a significant step closer but ... we are not there yet," said Fukuda, a leading influenza expert.
Influenza viruses are notoriously unpredictable. When asked about how the outbreak might evolve, Fukuda painted a picture containing a surprisingly wide range of options, given how quickly this outbreak has unfolded.
The world first learned of the existence of this unusual virus and that it had infected a human on April 17. Less than two weeks later infections have been found in Mexico, the United States, Canada, Spain and Scotland and experts expect it to be found elsewhere soon.
Mexico has linked 152 deaths to swine flu infection. The U.S. has diagnosed 50 cases, Canada six, Spain one and Scotland two. No country outside of Mexico has yet seen severe disease caused by the virus.
"It is possible that we could stay in Phase 4 for quite a long time. It is possible that as the situation evolves over the next few days, it could evolve to the point where it appears that we have moved into Phase 5. And it is also possible that the disease situation could be quieter and we could move backwards to Phase 3."
"I think right now it's a little bit difficult to give you likelihoods. But it does reflect the fluidity of the situation."
Fukuda said the WHO does not view closing borders or issuing travel bans as helpful. Studies have shown it would take a draconian cut in international air travel to even slightly slow spread, he said, but taking that approach would harm people in economic ways.
He said the decision to go Phase 4 rather than Phase 5 was based on the evidence available about the spread of the disease, not economic or political considerations. But he acknowledged the committee was mindful that moving up the scale would have "profound financial implications."
He said the WHO believes containment is not an option, given the range of disease spread - effectively dismissing a longstanding WHO plan to try to stop an emerging pandemic at source.
Instead countries should focus efforts on steps that would mitigate the impact of a pandemic, Fukuda said.
One such step is the development of pandemic vaccine to protect against these swine flu viruses. The WHO decided that for now it would not tell manufacturers to abandon production of next winter's flu shots in favour of making a swine flu vaccine. But he said serious discussions are underway to flesh out options for making vaccine.
A number of countries have already started that process and at least one vaccine maker, Baxter International of Deerfield, Ill., had asked for a sample virus to test, the first step towards potentially making a vaccine. But the company has not yet said whether it will proceed to production.
Canada is in discussions with vaccine manufacturers, including GlaxoSmithKline, which holds the country's pandemic vaccine control. GSK has a plant in St-Foy, Que.
And the United States on Sunday deployed part of its emergency pandemic stockpile, sending a quarter of the antiviral drugs in it out to the various states.
Dr. Richard Besser, acting director of the U.S. Centers for Disease Control, said given that the disease caused by the virus so far is mild - outside of Mexico, most cases haven't even needed antiviral drugs - the CDC wanted to start with a measured response, holding back supplies in case the situation escalates.
Pandemic planning has been underway in many countries for the past five years triggered by deep concern over the H5N1 avian virus. That virus causes severe disease in people, but so far has not acquired the ability to spread easily from person-to-person.
In the United States, the head of the Department of Homeland Security, Janet Napolitano, said that country is forging ahead with preparations as if swine flu is going to cause a pandemic.
"It matters less what we call this than what actions we take," Besser added. "And we are acting aggressively based on what we know today."
Speaking before the WHO news conference, Canada's chief public health officer said the WHO's decision doesn't change the situation on the ground in Canada.
"It does not change our preparations, it does not change our response," Dr. David Butler-Jones said in a press conference in Ottawa.
Health Minister Leona Aglukkaq said Canada would be issuing a travel advisory warning against non-essential travel to Mexico, but would not block entry of migrant seasonal workers from Mexico who come to Canada for agricultural work.
The European Union advised against non-essential travel to Mexico and parts of the United States. Canada and Spain were not mentioned in the EU communique.
The CDC's Besser called that move "premature" but said the U.S. would urge its citizens to avoid non-essential travel to Mexico.
"This is out of an abundance of caution as we learn more," he said.