When the nation experienced a spike in whooping cough cases last summer, doctors speculated that something was wrong with a popular children’s vaccine.

Now, an increase in whooping cough cases among Minnesota grade-schoolers has become national evidence that the pertussis vaccine loses effectiveness before children receive booster doses at ages 11 or 12.

The Centers for Disease Control and Prevention (CDC) studied the number of whooping cough infections in Minnesota and Oregon and found a sharp increase among vaccinated children ages 7 to 10, starting in 2006. There was no comparable increase in cases among other age groups, according to a report published Monday in the journal Pediatrics.

The age-specific nature of the increase is the key evidence that the pertussis vaccine loses strength over time, said Sara Tartof, a lead author of the report and a former CDC epidemic intelligence officer.

Today’s 7- to 10-year-olds were the first to receive the “acellular” version of the vaccine, which was recommended by federal health officials in 1997 because it appeared safer than the older “whole cell” version. The kids were, in essence, test cases for the new vaccine’s long-term effectiveness.

“If it was just due to increased recognition or diagnosis [of whooping cough], you would see the increase across all age groups,” she said. “But this was happening in this specific age group at this specific time.”

The report noted, however, that other factors might be to blame, including an increase in diagnoses.

Whooping cough is known for causing coughing fits, followed by a low whooping noise when children try to inhale. Infections often are harshest in infants, and can lead to pneumonia and other complications. The coughing can be so severe that some children suffer fractured ribs. Infections were linked to the deaths of 18 children in the United States last year.

Five doses, then a booster

Children are supposed to receive five doses of “DTaP” vaccine between the ages of two months and six years. It protects against diphtheria, tetanus and pertussis. A booster dose usually follows around age 11 or 12.

The new report does not suggest any change in that schedule. If anything, studies have found that vaccination results in milder cases of whooping cough, said Tartof, whos study was among three that recently examined the issue of waning pertussis vaccine immunity. “We do know that kids that are vaccinated with all five doses do have milder disease than those who are undervaccinated or unvaccinated.”

The study focused on whooping cough only in vaccinated children in Minnesota and Oregon, which suggests that any increase in the infection rate is not from fewer children getting their shots.

“Uptake of DTaP is actually pretty high,” Tartof said. “We don’t think that the unvaccinated population plays a really large role in this increase in cases we’ve been seeing nationally.”

The rate of pertussis infections in Minnesota increased from nine per 100,000 children ages 7 to 10 in 2006 to 133 cases per 100,000 children in 2009, the study found. The rate didn’t spike as much in Oregon, but Tartof said whooping cough tends to cycle through regions every three to five years. Minnesota could already have been poised for a bad year in 2009 that got worse because of the waning immunity of vaccine in grade-school children.

Minnesota and Oregon were studied because their state health departments had robust data, even if it had its limitations: Doctors frequently did not report the lots and brands of vaccines received by children who later came down with whooping cough. That prevented a determination of whether one type of vaccine was more likely to lose effectiveness than another.

One solution could be giving the booster dose at an earlier age, but Tartof said that would create problems. The booster dose now is timed with the typical adolescent health screening when children also receive shots for meningitis and human papillomavirus.

“It’s already difficult to get kids that age into the office,” said Tartof, who now conducts research for Kaiser Permanente in California. “So we’re trying to make it easier for parents. We’re hoping to consolidate a lot of the preadolescent vaccines to one time to increase uptake.”