Swine flu is a respiratory disease in pigs that sporadically infects humans.
A relatively new strain of the swine flu virus that sickened more than 300 people last year is back again this summer, according to the Centers for Disease Control and Prevention.
The strain, called H3N2 variant virus, infected four people who visited a county agricultural fair in Indiana this month. None of the infected individuals were hospitalized, and none died.
Because this virus is typically transmitted from pigs to people, the CDC officials are urging people at high risk for flu complications — such as young children, the elderly and those with underlying health conditions — to avoid pigs at county fairs.
The first cases of people sick with the H3N2 variant virus were reported in 2011. In 2012, the virus caused 309 infections in 12 states, and 16 people were hospitalized. Because the 2013 strain is very similar to the 2012 strain, more cases of H3N2 variant virus could be detected this summer, the CDC said. Seasonal flu vaccines do not protect against H3N2 variant viruses.