WASHINGTON, June 11 (Xinhua) -- The U.S. is well prepared for theA/H1N1 flu pandemic as the World Health Organization (WHO) declared the alert against the disease, said the health secretary on Thursday.

    "What this declaration does do is remind the world that flu viruses like H1N1 need to be taken seriously," said Health Secretary Kathleen Sebelius in a statement.

    Despite a limited number of cases found in the U.S., "things could possibly be very different in the fall, especially if things change in the southern hemisphere, and we need to start preparing now in order to be ready for a possible H1N1 immunization campaign starting in late September," she said.

    The statement was made as WHO declared earlier the day that theH1N1 flu crisis has escalated into the world's first influenza pandemic in 40 years, with tens of thousands of infecting cases reported in 74 countries.

    As of Friday, the US Centers for Disease Control and Prevention(CDC) reported that a total of 27 people had died from the H1N1 flu in the United States, among 13,217 cases across all 50 states.

    Homeland Security Secretary Janet Napolitano also said in the joint statement that the U.S. government "responded to the H1N1 outbreak from the outset with the presumption that a pandemic was likely, so this decision comes as no surprise."

    "We acted aggressively to stay ahead of the virus as it spread across the country. Now our challenge is to prepare for a possible return in the fall."

    She also added that the government is "working with our scientists to test and prepare a possible vaccine," and " working with governments around the world to share what we know and learn from what is happening in their countries."