MONTPELIER, Vt. (AP) — Scientists and public health officials are studying a Vermont whooping cough epidemic to determine if the current vaccine is less effective against a new strain sweeping the country.

The Centers for Disease Control and Prevention and the Vermont Health Department are examining the records of 854 people, ages 4 to 19, who likely contracted whooping cough from late 2011 to early 2013.

They are also examining a control group of people from the same age group who did not get sick. They want to compare their vaccination history against those who did contract whooping cough.

For several years, scientists have noted a dramatic resurgence of the disease. In 2012, pertussis cases were up in 48 states. Outbreaks were particularly bad in Colorado, Minnesota, Washington, Wisconsin and Vermont.