The U.S. Centers for Disease Control and Prevention is heightening its surveillance of a specific subtype of swine flu, according to a report from the agency. The report says that state public health labs across the country should notify the CDC immediately if they suspect someone is infected with the virus.
The CDC reports that since August, 12 people have been infected with the virus, called H3N2, in five states: Indiana, Iowa, Maine, Pennsylvania and West Virginia. Of the total, 11 were children, three were hospitalized and all have fully recovered.
The report said officials have observed two different scenarios of transmission: workers getting the virus from pigs and humans catching it from one another, which was the cause of two of the newest cases at a West Virginia day care.
"Nonhuman influenza virus infections rarely result in human-to-human transmission, but the implications of sustained ongoing transmission between humans is potentially severe," the report said.
Swine flu swept the globe beginning in 2009, when the H1N1 subtype of the virus was first detected in the United States. An estimated 43 million to 89 million Americans caught swine flu during the pandemic, and 8,870 to 18,300 people died from it, according to the CDC.
The government launched a major initiative to get the public vaccinated, giving $1.6 billion to pharmaceutical companies and vaccine makers to ensure there was enough vaccine to go around. But vaccine supplies exceeded demand as public interest dwindled.
The World Health Organization officially declared an end to the H1N1 pandemic in August 2010.
The flu vaccine available this year includes the H3N2 virus, along with the H1N1 virus. Officials are again encouraging people to get vaccinated. The CDC recommends that anyone age 6 months and older be vaccinated for the flu.
"We should remember that [seasonal] flu causes 36,000 deaths on average each year," Dr. William Schaffner, professor and chair of preventive medicine at Vanderbilt University in Nashville, Tenn., told ABC News. "It remains a serious illness."
ABC News' Courtney Hutchison and Lauren Cox contributed to this report.