A woman leaves an immigration post wearing a mask as a preventive measure against infection from swine flu, in Tijuana airport.[Agencies]

The World Health Organization and the U.S. were following a playbook of precautions developed over the past five years to prepare for the next super-flu. The WHO on Saturday asked all countries to step up detection of this strain of A/H1N1 swine flu and will reconsider on Tuesday whether to raise the pandemic threat level, in turn triggering additional actions.

A potential pandemic virus is defined, among other things, as a novel strain that's not easily treated. This new strain can be treated with Tamiflu and Relenza, but not two older flu drugs. Also, the WHO wants to know if it's easily spread from one person to a second who then spreads it again — something U.S. officials suspect and are investigating.

"Right now we have cases occurring in a couple of different countries and in multiple locations, but we also know that in the modern world that cases can simply move around from single locations and not really become established," cautioned WHO flu chief Dr. Keiji Fukuda.

There is no vaccine against swine flu, but the CDC has taken the initial step necessary for producing one — creating a seed stock of the virus — should authorities decide that's necessary. Last winter's flu shot offers no cross-protection to the new virus, although it's possible that older people exposed to various Type A flu strains in the past may have some immunity, CDC officials said Sunday.

Worldwide, attention focused sharply on travelers.

"It was acquired in Mexico, brought home and spread," Nova Scotia's chief public health officer, Dr. Robert Strang, said of Canada's first four confirmed cases, in student travelers.

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