"Even if this outbreak is a small one we can't anticipate we wont have follow up outbreaks," said U.S. Homeland Security Secretary Janet Napolitano.
In Mexico, more than a thousand people have been infected. The Mexican government has advised people to stay homebound, and the government has indicated those infected could be isolated. In the country's deserted capital, public events were cancelled for the next week or so. Sales of masks have soared as people try to prevent themselves from the potentially deadly disease.
"If we move the pandemic threat, want to make sure on pretty good solid ground. Such a move would be a big signal to the world. We already have several countries involved (with positive cases), but we know in current global situation that cases can occur in many places without ever taking hold. We have decided to wait and get more information," said Dr Keiji Fukuda, assistant director-general ad. interim for Health Security and Environment at WHO.
WHO officials said so far, the virus has only been confirmed in Mexico and the United States, and that the transmissability is limited and the outbreak is small. WHO officials said are working on a vaccine and testing and identifying this particular never-seen-before virus.
The severity of the flu is still unknown. Some U.S. doctors say they do not trust information from Mexico, but WHO officials said they do not believe that Mexico created any delays in reporting the outbreak.
"We only have isolated reports from various places. You have to have more than this" to declare a pandemic, said Peter Katona, an associate professor at the University of California, Los Angeles (UCLA) Medical Center. "This virus may burn itself out tomorrow. We don't know that yet."
Several countries have reportedly announced measures to test American visitors and imposed travel warnings for Mexico. The United States has not yet imposed a travel advisory to and from Mexico, with officials only suggesting to Americans to reevaluate their plans if visiting the country.
American Airlines, US Airways and Continental are waiving fees for flight changes to Mexico. They are not cancelling flights, and American Airlines said it's only received a handful of calls seeking changes in travel plans.
Katherine Andrus, an attorney for the Air Transport Associated, told ABC News that the organization is taking this seriously and employing prudent measures, but at this point it would be an extreme measure to restrict any flights.
Dr. William Schaffner, chairman of Preventive Medicine at Vanderbilt University Medical Center in Nashville, Tenn., said doctors like him have been advised by the CDC and state health department to set up a system that would test patients with flulike symptoms and help define how widespread this outbreak is. He said the severity of the virus is hard to gauge because of the wide discrepancy in how it has affected Mexicans and Americans, and because it is occuring in places that are warm, which is very unusual.