(CBS) Twelve people have become infected with swine flu since August, the CDC said Friday. The people infected with this type of the virus, called influenza A H3N2, lived in Indiana, Iowa, Maine, Pennsylvania, and West Virginia. Eleven of the cases occurred in children.
The cases have caused the CDC to ask all state public health laboratories to notify the CDC immediately if any swine flu cases occur so investigators can limit human to human transmission of the disease.
The CDC's report - which covers cases from August 17 through December 23, 2011 - is published in the Dec. 23 issue of Morbidity and Mortality Weekly Report.
The latest three cases occurred in two children who attended the same day care in West Virginia and a man from Indiana who worked with swine. All 12 patients have recovered fully.
"Nonhuman influenza virus infections rarely result in human-to-human transmission, but the implications of sustained ongoing transmission between humans is potentially severe," the CDC wrote.
In 2009 a swine flu pandemic of the H1N1 virus sickened more than one million people and killed another 477 in the U.S.
The CDC says this strain is different enough from current human flu viruses so the vaccine is "not expected to provide significant protection." But the CDC still urges everyone to get vaccinated.
The CDC says people can "Take 3" actions to prevent the flu.
Everyone 6 months and older should "take time to get a flu vaccine," "take everyday preventive actions to stop the spread of germs" - such as covering your nose and mouth when you cough or sneeze and washing your hands - and "take flu antiviral drugs if your doctor prescribes them." Pregnant women, young children, and people 65 and older are more likely to be given flu medication early when symptoms first appear.