GENEVA (Reuters) -- New H1N1 flu cases across Europe and a second U.S. death have kept health officials on alert despite signs Mexico's epidemic had passed its peak.
Sweden joined the list of affected countries, saying a woman in her 50s who had visited Chicago had tested positive.
"She is healthy now. She feels absolutely fine," said a health-authority spokesman in the capital Stockholm.
In Geneva, the World Health Organization (WHO) said earlier there had been 1,516 officially reported cases in 22 countries, a cautious tally based on its own laboratory tests.
It confirmed more infections in Britain, Spain, Italy, and Germany. If the UN agency detects a sustained spread within Europe -- as there has been in North America -- this could trigger the official declaration of a pandemic.
The bulk of cases remain in North America, where a Texan woman died this week, officials said -- only the second death outside Mexico, where the WHO has confirmed 29 fatalities.
The 30-year-old had chronic health problems. A Mexican toddler also died in Texas last week of the virus known as swine flu -- actually a mix of pig, human, and bird flu elements.
The WHO has certified 822 infections in Mexico and 403 in the United States whose authorities said they suspected another 700 "probable" cases.
"Those numbers will go up, we anticipate, and unfortunately there are likely to be more hospitalizations and more deaths," said U.S. Health and Human Services Secretary Kathleen Sebelius.
Mexican authorities said the outbreak seemed to be slowing in the country hit hardest by the virus.
Pandemic Alert Remains
For authorities worldwide, the question remained how far the virus would spread and how serious would it be. The WHO remained at alert level 5, meaning a pandemic was imminent.
"If it spreads around the world you will see hundreds of millions of people get infected," said the WHO's Dr. Keiji Fukuda.
If it continues to spread outside the Americas, the WHO would probably move to Phase 6, a full pandemic alert.
This would be no indication of how mild or severe the outbreak would be, but prompt countries to activate pandemic plans, distribute antiviral drugs and antibiotics, and perhaps advise on other precautions like limiting large gatherings.
The WHO's Emergency Committee was not formally set to meet on May 6 but a spokesman said its members were talking all the time to see if an alert change was necessary.
Fukuda said it was important to watch the Southern Hemisphere, where the winter flu season is beginning.
Other pandemics have started with a mild new virus in spring that came back to cause severe disease later in the year.
An aircraft carrying 98 Chinese stranded in Mexico by the flu outbreak arrived in Shanghai on May 6 and all appeared healthy but will have to spend a week in quarantine.
A day earlier, a plane picked up dozens of Mexicans who China had kept in quarantine despite having no symptoms, sparking a row with Mexico.