The number of children who have died from the H1N1 virus is “increasing substantially,” health officials said Friday, and children with disabilities and underlying medical conditions appear to be hardest hit.

Already 76 children have died from H1N1, or swine flu, this year and the flu season is just beginning. Flu season traditionally lasts until May.

The number of deaths is especially striking given that fewer than 90 children died in each of the last three years from seasonal flu.

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Most of the children who have died from the H1N1 flu strain had disabilities or underlying health conditions, officials at the Centers for Disease Control and Prevention said Friday.

“The majority of children that we have information on had an underlying disease,” according to Anne Schuchat, head of the CDC’s National Center for Immunization and Respiratory Diseases. “Among children, muscular dystrophy and cerebral palsy are quite prominent.”

These findings are consistent with a CDC report released in September, which indicated that two-thirds of children who died from H1N1 during the spring and summer had a chronic illness or developmental disability.

Officials say they are hopeful that the threat of the virus will begin to subside as the swine flu vaccine becomes more readily available, but they do expect more deaths in the coming weeks.

The first doses of the vaccine were administered this week, but only a nasal spray version was available. The nasal spray contains a weakened live version of the flu virus. An injection version of the vaccine, which does not include the live virus, is expected next week.

Individuals with developmental disabilities including cerebral palsy, intellectual disability and developmental delay are considered to be at high risk for H1N1, especially if they have respiratory issues as well. Those who are at high risk are first in line for the vaccine, CDC officials say, but are advised to wait for the injection version.