(CNN) -- A new strain of swine flu in humans continues to spread, health officials said Thursday, with more than 100 cases reported.
The Centers for Disease Control and Prevention says 145 cases of the influenza A (H3N2) variant have been found in four states since mid-July: 113 in Indiana, 30 in Ohio, one in Illinois and one in Hawaii.
The CDC says it expects the case count to increase. Two people were hospitalized, but both have been released, officials said.
The agency's numbers did not include seven new cases reported in Indiana. Dr. Gregory Larkin, Indiana state health commissioner, said the number of cases has risen to 120 as of Thursday.
"Surprisingly, the greatest, overwhelming percentage (of cases) is in people 16 years and younger," Larkin said. "As our investigation continues, we're seeing transmission from ill or infected swine, or hogs, to their handlers, which in most of these cases are kids."
Dr. Joseph Bresee, an epidemiologist in the CDC's Influenza Division, said that there has been a predominance of cases in children and young adults and that all of the cases have been associated with close or indirect exposure to swine, often at state or county fairs. The other pig-to-human transmissions occurred in farmers or veterinarians.
"This time of the year is the time when you have fairs around the country ... thousands of them," Bresee said. "That accounts for the increased transmission more than anything else."
The CDC says people exposed to pigs should take precautions to protect themselves from this new strain of flu, namely:
• Wash your hands with soap and water before and after touching pigs.
• Don't drink or eat near pigs, and don't take food into animal areas.
• Avoiding contact with animals such as pigs may be the best protection if you are among those likely to suffer severe symptoms if you get the flu: people with lung disease or diabetes, for instance.
According to the CDC, H3N2 flu viruses are common among pigs. H3N2 viruses are a subgroup of influenza A viruses, and they are known to adapt in humans.
What makes this new version of the H3N2 flu virus different is that it has picked up a gene from the novel H1N1 flu virus that became a pandemic three years ago. This can happen when a person or an animal is exposed to two different viruses at the same time.
Somewhere along the line, H3N2 and H1N1 viruses were present in a mammal at the same time, and the "matrix-gene" (or m-gene) from the H1N1 pandemic virus was picked up by the H3N2 swine flu, creating a new or variant version of H3N2.
It is this m-gene that has experts on the lookout, because the presence of the m-gene can make it more easily transmissible to humans.
Health officials point out that this flu is not a foodborne illness. Instead, it spreads like any other flu: someone sneezes or coughs, spreading the virus to other mammals (humans included) and onto surfaces.
Dr. Lisa Ferguson, a veterinarian for the National Animal Health Policy Programs at the U.S. Department of Agriculture, said this variant of swine flu was first detected in 2010.
Bresee said the first human cases were reported in July 2011.
Even though the regular seasonal flu vaccine contains a strain of the A-flu virus group, it will not prevent you from getting sick if you come into contact with the new flu strain.
Bresee said last week that preliminary steps have been taken to develop an H3N2 vaccine, part of the overall pandemic preparedness planning of the CDC and other health agencies.
When a new flu virus pops up, "we immediately begin to think about the process of making a vaccine," Bresee said.
The incremental process involves finding a good vaccine candidate, reassessing and testing the virus, developing seed vaccines and ensuring their safety. The goal is to have a vaccine quickly available in case a pandemic occurs, as with H1N1 in 2009.
CNN's Miriam Falco contributed to this report.