Health officials worldwide prepared to step up their monitoring efforts in the wake of a warning from the World Health Organization that the swine flu virus was too widespread to make containment feasible, and that the virus could appear in any region.
"At this time, instituting travel bans would really not be very effective in preventing further spread of this virus," said Keiji Fukuda, WHO's assistant director-general for health security and environment, adding that the agency did not recommend that countries close borders or restrict travel.
The WHO raised its global alert level Monday to indicate the swine flu virus was capable of spreading through larger groups of people, but it stopped short of declaring the outbreak a pandemic.
U.S. officials said that the change in the WHO's alert level would not alter their preparations, as they were already operating as if a pandemic were possible.
U.S. Cases Rise To 50
The number of confirmed swine flu cases in the United States has increased from 20 to 50, according to numbers released by state health departments and the Centers for Disease Control and Prevention.
Twenty of the additional U.S. cases are from further testing related to a known outbreak in a prep school in the New York City borough of Queens. Some students at the school made a spring break trip to Mexico and may have become infected with the virus there.
Officials say the new cases from that school reflect more testing rather than an ongoing spread of that cluster. Mayor Michael Bloomberg says no other clusters have been found in New York City.
"Thankfully, so far, we've not seen severe disease in this country as has been reported in Mexico," says Dr. Richard Besser, acting director of the CDC.
He says the government will be releasing a new travel advisory recommending that nonessential travel to Mexico be avoided, out of an "abundance of caution."
At least 20 people in Mexico are known to have died from the H1N1 swine flu virus, with the suspected death toll now reaching more than 150.
President Obama said the emerging cases of swine flu in the U.S. are being closely monitored but that the situation is "not a cause for alarm."
Swine flu has also been found among patients in Canada and Spain, as well as Mexico and the five affected U.S. states: New York, Ohio, Kansas, Texas and California. Investigators are looking into reports as far apart as New Zealand and France, but so far, none of those cases has been confirmed.
In Luxembourg, European Union Health Commissioner Androulla Vassiliou urged Europeans to postpone nonessential travel to parts of the United States and Mexico affected by swine flu. Meanwhile, Hong Kong, Taiwan and Russia said they would quarantine visitors who show symptoms of the virus.
Besser of the CDC said he thought a recommendation to not travel to the U.S. was "premature."
What Will This Strain Bring?
It's possible the new strain — which has elements of viruses that infect birds and humans as well as pigs — will cause a devastating flu pandemic like that of 1918. Or it might touch off a much milder pandemic like one in 1968. But as this week begins, it seems less likely it will fade out quickly.
Experts say the new virus might not stay in its existing form. It might evolve over time into a more lethal threat, or, equally likely, it could become tamer.
On Sunday, U.S. officials declared a "public health emergency," which President Obama described as a "precautionary tool to ensure that we have the resources we need at our disposal to respond quickly and effectively."
The declaration of emergency allows the government to begin implementing long-prepared pandemic plans, including moving millions of doses of the anti-flu drugs Tamiflu and Relenza closer to affected states.
The CDC's acting chief says that 11 million courses of anti-viral drugs have been released from the nation's stockpile and are en route to affected states as well as other states around the country.
Besser also said the government plans to distribute yellow cards at points of entry so that people coming into the United States will know what to do if they become sick. And he urged people to wash their hands frequently.
Mysteries In Mexico
One of the mysteries that scientists are trying to understand is why cases outside of Mexico appear to be milder. Swine flu in Mexico seems to be a much more virulent disease, even though genetic tests suggest that swine flu viruses there are identical, or nearly so, to those isolated from U.S. patients.
Mexican President Felipe Calderon said Sunday that many of the 1,400 people reported ill since mid-April have recovered and that only about 400 remain hospitalized. Even with fewer people still ill, it's not known how many of the cases that required hospitalization might have been caused by swine flu. And there could have been many cases in Mexico that are mild and have gone unreported.
The central question in whether the world stands on the brink of a pandemic is this: How many milder cases of swine flu have there been in Mexico?
Without the full picture, flu experts can't answer two key questions: How dangerous the virus is, and how efficiently the virus can be transmitted from person to person.
Not knowing the percentage of people who die after getting the virus makes it impossible to determine whether this viral strain is more lethal than any other. It's been reported that more young adults have been affected in Mexico than in a normal flu outbreak, but so far, scientists haven't been able to determine for sure whether this pattern is truly different from other outbreaks.
It is possible that some other factor may be contributing to deaths among some Mexicans infected with swine flu — that is, those Mexicans whose cases have come to attention thus far because they are so sick. That other factor could be a concurrent infection with another virus or bacterium, or an environmental factor such as poor nutrition.
But one of the WHO's top flu experts is withholding judgment on the virulence of the new swine flu virus. "I think we are too early in our investigations to be able to address the lethality of the virus," says Keiji Fukuda.
Investigators from the WHO, the CDC and Canada began arriving in Mexico this past weekend to help local authorities sort out the mystery. They're looking closely at the characteristics of the identified cases and trying to see if their contacts have become ill, or perhaps have been infected without developing fever and classic flu symptoms such as aches, coughs, sneezing and, in many of the Mexican cases, diarrhea and vomiting.
Gearing Up For A Vaccine
While on-the-ground investigators and leading laboratories around the world are trying to figure out what sort of virus the H1N1 is, others are trying to assess humans' defenses against it.
The latest news isn't good. Tests at the CDC suggest that the flu shots many Americans and Europeans got this season probably don't offer protection against the new swine flu virus.
There had been hope that the current seasonal vaccine might do some good, since it contains an H1N1 virus component — meaning the new swine flu strain was at least in the same family of viruses. But the new H1N1 virus is apparently too different for the current vaccine to offer what scientists call "cross protection."
This has big implications: It means scientists and vaccine makers have to gear up as fast as they can to make a new vaccine that specifically protects against the new swine flu virus. But mass production of flu vaccine, which is grown inside chicken eggs, typically takes around six months.
"There are discussions that are ongoing about the decision to make a vaccine and whether that should be undertaken, but it's not an easy decision," says Besser of the CDC. "Those discussions are under way so that if we decide to manufacture a vaccine, we'd be ready to start that process."