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Officials at the Centers for Disease Control and Prevention (CDC) acknowledged on Friday that "concern has grown" since the first reports of a novel swine flu infecting patients in Texas and California emerged late March.

Dr. Richard Besser, acting director of the agency, said health officials are closely tracking the spread of the swine flu, after additional cases of flu and some deaths were reported in Mexico. Preliminary testing of flu viruses in patients in Mexico and the U.S. show that the strains are similar. Of the 14 samples of suspected swine flu from Mexico that the CDC has tested so far, half are positive for swine flu, a form of influenza that normally infects pigs and can be transmitted to people. (Read about the history of the flu vaccine)

So far, eight residents of San Diego and Imperial counties in California, and San Antonio, Texas, have tested positive for the new swine flu strain — an as yet unseen combination of swine flu, bird flu and human influenza viruses. There have been no deaths in the U.S. associated with the virus; so far the infections, which cause typical flu-like symptoms, are being controlled with antiviral medications, and only one patient has required hospitalization.

Besser says it's too early to raise alarms about a pandemic flu, but officials are watching the new virus closely and aggressively, since the geographic distance between the infected patients suggests that it can be transmitted easily from person to person (apparently none of the patients had come into contact with pigs). The CDC is working with the World Health Organization to keep track of any additional cases to determine whether and when a warning of a pandemic would be warranted. In preparation for such a scenario, the CDC has created a seed stock of a vaccine against the swine flu, which could be pushed into production should the number of cases jump significantly. The CDC did not specify what the threshold for vaccine production would be.

In the meantime, the government has not restricted travel to Mexico, California or Texas, but has issued an outbreak notice to inform travelers to those areas that cases of a contagious respiratory illness have been reported. In the affected regions, the CDC is recommending that doctors test samples from people complaining of flu-like symptoms to determine if they are infected with the new swine flu strain.

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• Alice Park