The World Health Organization (WHO) has raised the influenza pandemic alert level to phase 4, just two steps shy of a global pandemic alert, according to Keiji Fukuda, the WHO's assistant director-general for health security and environment. And while the WHO does not recommend countries close borders or restrict travel, they are moving forward with plans to produce a swine flu vaccine as soon as possible, he told reporters in a teleconference this evening.

According to the WHO Web site, a phase 4 alert describes a viral outbreak in which human-to-human transmission is efficient and sustained enough to cause community-level outbreaks. Level 3 describes a situation in which human-to-human transmission is limited and disease cases are sporadic and occur in small clusters. (Phase 6 designates a global pandemic.)  According to the WHO, there are at least 73 confirmed cases of human swine flu -- 40 in the U.S., 26 in Mexico (including seven deaths), six in Canada, and one in Spain. But Mexico has over 1000 suspected cases, according to Time Magazine, and 149 suspected deaths, according to Reuters.

"A pandemic is not considered inevitable at this time," Fukuda said, but he emphasized the WHO intends to send a strong signal to nations that they need to strengthen their preparations for a pandemic. "The situation is fluid," he said, noting that the level could drop to 3 or rise to 5 in the next several days, or remain at level 4 "for quite along time," depending on how efficiently the virus spreads and causes disease.

In this age of global travel, there is no country that is immune to swine flu, but it's already too late to stem the outbreak by closing borders, Fukuda said. "The virus is too widespread to make containment a feasible consideration," he said, but added that people with symptoms of swine flu or any infectious disease should refrain from travel.

The WHO is taking all possible steps to facilitate the production of a swine flu vaccine, Fukuda said, but he estimates it will take four to six months to develop one. (There is currently no effective vaccine against the virus.)

At this point, little is known about the swine flu's incubation period -- the length of time between infection and the appearance of symptoms -- or who is most at risk for serious illness. Fukuda says scientists are also working to figure out why people are dying in Mexico but not in the U.S. (David Dobbs, who frequently writes for Scientific American, tries to answer that question in this piece for Slate.)

See our in-depth report for more on the swine flu outbreak.