By Alistair Bell and Noel Randewich
MEXICO CITY (Reuters) - A strain of flu never seen before has killed up to 60 people in Mexico and also appeared in the United States, where eight people were infected but recovered, health officials said on Friday.
Mexico's government said at least 20 people have died of the flu and it may also be responsible for 40 other deaths.
It shut down schools and canceled major public events in Mexico City to try to prevent more deaths in the sprawling, overcrowded capital. Authorities said they had enough antiviral medicine to treat about 1,000 suspected cases reported so far.
The World Health Organization said tests showed the virus from 12 of the Mexican patients was the same genetically as a new strain of swine flu, designated H1N1, seen in eight people in California and Texas.
"Our concern has grown as of yesterday," Dr. Richard Besser, acting director of the U.S. Centers for Disease Control and Prevention told reporters in a telephone briefing.
Global health officials were not ready to declare a pandemic -- a global epidemic of a new and deadly disease such as flu. "So far there has not been any change in the pandemic threat level," Besser said.
But the human-to-human spread of the new virus raised fears of a major outbreak. Mexico's government suspended classes for millions of children in Mexico City, where scared residents rushed out to buy face masks and kept their kids at home.
"We're frightened because they say it's not exactly flu, it's another kind of virus and we're not vaccinated," said Angeles Rivera, 34, a government worker who fetched her son from a public kindergarten that was closing.
Close analysis showed the disease is a mixture of swine, human and avian viruses, according to the CDC.
Humans can occasionally catch swine flu from pigs but rarely have they been known to pass it on to other people.
Mexico reported 1,004 suspected cases of the new virus, including four possible cases in Mexicali on the border with California.
Most of the dead were aged between 25 and 45, a health official said. It was a worrying sign as seasonal flu can be more deadly among the very young and the very old but a hallmark of pandemics is that they affect healthy young adults.
Health Minister Jose Angel Cordova said Mexico has enough antiviral drugs to combat the outbreak for the moment. "In the last 20 hours, fewer serious cases of this disease and fewer deaths have been reported," he told reporters.
The WHO said the virus appears to be susceptible to Roche AG's flu drug Tamiflu, also known as oseltamivir, but not to older flu drugs such as amantadine.
NO CONTAINMENT Continued...
According to Mexican health officials, an epidemic of swine flu has killed at least 68 people and infected a further one thousand inside the country.

Mexican health minister José Ángel Córdova said that the casualty rate appeared to be slowing down, and that there would be no plans to block off Mexican borders. “We’re dealing with a new flu virus that constitutes a respiratory epidemic that so far is controllable,” Córdova stated. He said that the disease had mutated from pigs and was transferred to humans at some point.

Museums and schools for seven million students near Mexico's capital were closed down in an effort to curb the epidemic, and the government has encouraged people with symptoms of the disease to take leave from work.

The outbreak has spread north to the United States, and US health authorities have reported that eight people were diagnosed with swine flu in Texas and California. However, these people have recovered.

“We are worried. We don’t know if this will lead to the next pandemic, but we will be monitoring it and taking it seriously,”  said Dr. Richard Besser, the acting head of the Centers for Disease Control and Prevention (CDC). Dr. Besser suggested "containment is not very likely" in a telephone briefing on Friday.

Tests conducted by the World Health Organisation (WHO) found that the virus from a dozen patients was genetically similar to a new outbreak of swine flu, designated as H1N1.