The most complete analysis yet of the new H1N1 swine flu virus shows it must have been circulating undetected for years, most likely in pigs, researchers said on May 22.

The report by Cox and an international team of researchers in the journal Science said the virus “might have been circulating undetected among swine herds somewhere in the world.”

The researchers confirmed the odd mixture of human, pig and bird genes in the new virus, which has infected more than 11,000 people in 42 countries, and killed 86. The World Health Organization is poised to declare a full pandemic of the virus, which so far causes mostly mild symptoms in people.

The researchers said it is likely other odd mixtures are infecting pigs but simply have not yet been seen. They sequenced the genetic codes of 70 different samples of the new virus from the United States and Mexico.

Meanwhile, Mexico City has lifted all restrictions imposed last month, following an outbreak of swine flu across Mexico. Mayor Marcelo Ebrard said no new infections had been reported for a week in the capital and there was no longer a need to wear a mask in public places.

Mexico’s flu death toll rose by three to 78 on Thursday, officials said. Some 4,000 people have been infected.

VietNamNet/VOV