The World Health Organization says 42 countries are reporting there
have been more than 11,000 cases of the swine influenza A-H1N1 virus.
In its latest figures, the Geneva-based WHO says 86 deaths have been reported from the virus, with most of the fatalities in Mexico, the epicenter of the outbreak.
In other developments, U.S. researchers say new test results show that people in their 60s and older show signs of greater immunity to the virus.
The Atlanta-based Centers for Disease Control and Prevention issued the assessment Thursday as the number of U.S. swine flu cases climbed past 5,700. The CDC says more than 60 percent of U.S. infections have occurred in patients younger than 25.
The CDC says one possible explanation is that older adults were either infected with or vaccinated against a much older strain that more closely resembles the H1N1 virus. But the researchers say it is still not clear how safe older people are from the new infection.
Health officials say the current batch of seasonal flu vaccine provides virtually no protection against the new H1N1 strain. Seasonal flu is usually more severe in very young children, older people and those with certain health conditions.
The U.S. has the greatest number of infections, followed by Mexico with nearly 3,900. But Mexico has the most deaths, 75, while the U.S. has had 10 fatalities.
In Mexico City, the swine flu alert level Thursday was dropped to green, the lowest level. Officials say there have been no new infections in a week. The flu's impact is expected to cost the Mexican economy $2.2 billion.