Officials for the U.S. Centers for Disease Control (CDC) in Atlanta said they had confirmed more cases at a school in Queens, bringing the New York total to 28.
Cases have also been reported in Ohio, Kansas, Texas and California, but only one person has been hospitalized and all have recovered, according to the Associated Press.
With 40 cases of the flu strain now reported in the United States, President Barack Obama said there was reason for concern but not yet "a cause for alarm."
Direct contact with swine is not the source of, and U.S. pigs have not been infected with, the hybrid influenza, the National Pork Producers Council (NPPC) said.
"NPPC wants to assure domestic and global consumers about the safety of pork and urges pork producers to tighten their existing biosecurity protocols to protect their pigs from this virus, including restricting public access to barns," the industry group said on its website.
