State health officials are encouraging Kentuckians, especially older children and teenagers, to get a booster vaccine to prevent pertussis, better known as whooping cough.State epidemiologist Dr. Kraig Humbaugh says there's been a spike in whooping cough cases in Kentucky since October, with the highest concentration in Bullitt, Franklin and Hardin Counties."Most experts think it is cyclical, and we're going through a cycle now where we're seeing more cases than usual. but we're probably diagnosing it a lot better with improved diagnostic testing,"Humbaugh said.Young children are vaccinated for whooping cough, but it's not unusual for the vaccine to wear off by the age of or so.Whooping cough is highly contagious and is especially dangerous for young children or people with weakened immune systems.