WASHINGTON, May 20 (Xinhua) -- The number of confirmed and probable human A/H1N1 flu cases has risen to 5,710 in 48 U.S. states, with eight deaths, the U.S. Centers for Disease Control and Prevention (CDC) reported on Wednesday.

    The number of confirmed and probable cases on Tuesday was 5,469,with six deaths in 48 states.

    The CDC said on its website that it's uncertain at this time how severe this novel H1N1 outbreak will be in terms of illness and death compared with other influenza viruses. Because this is anew virus, most people will not have immunity to it, and illness may be more severe and widespread as a result. In addition, currently there is no vaccine to protect against this novel H1N1 virus.

    The CDC anticipates that there will be more cases, more hospitalizations and more deaths associated with this new virus in the coming days and weeks.

    U.S. Health and Human Services Secretary Kathleen Sebelius said on Monday at a World Health Assembly plenary session in Geneva that the U.S. government is "cautiously optimistic" that this virus may be less severe than was first feared, based on initial reports from the close neighbor, Mexico.

    She said the U.S. government is continuing to act aggressively and appropriately to help mitigate the consequences of this outbreak and protect public health.

Special Report:  World Tackles A/H1N1 Flu