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Whooping cough levels among children in the United States are at its highest in decades, as protection against whooping cough starts to weaken a few years after preschool children get their final shot, a new study said Monday.

Whooping cough, also called pertussis, is a highly contagious bacterial disease that attacks the respiratory system. This study, published in the Pediatrics journal, tracked the immunization records of 224,378 children in Minnesota and 179,011 in Oregon and compared them to cases of whooping cough. The study showed that protection against whooping cough wanes significantly between the fifth dose of the vaccine -- given sometime during the 4- to 6-year-old age range -- and the adolescent booster vaccine given at 11 or 12.

Researchers speculate that immunity from the vaccine diminishes each year after the fifth dose is given. This means that some children may be at risk of developing the disease before they can get a booster shot.

"This study provides fairly strong evidence that the trends we're seeing are real -- and a couple of other studies with similar findings have recently come out," said study lead author Sara Tartof, who was at the CDC at the time of the study and is now a researcher at Kaiser Permanente in Los Angeles.

Nonetheless, "the vaccine is still a great tool," she said. "Kids who are fully vaccinated who get whooping cough have a much milder disease than those who aren't vaccinated. Vaccines are still the best protection against disease, and the incidence of whooping cough is low."

The children evaluated for the study were all born from Jan. 1, 1998, to Dec. 31, 2005 and had all five doses of the pertussis vaccine combined with diphtheria and tetanus toxoids (DTaP).

The researchers acknowldeged that there were limitations to the study, included a limited length of follow-up time and variability in data between the 2 states. They also were not able to assess the possible effects of vaccine formulations because of a large amount of missing lot and manufacturer data.

"Continuing to monitor disease burden and vaccine effectiveness in fully vaccinated children in the coming years will be important to assess ongoing risk as additional cohorts vaccinated solely with acellular pertussis vaccines are introduced," the researchers conclude.