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Wisconsin leads the country in what could be the worst national outbreak of whooping cough in 50 years. Experts are warning that the current regimen of immunizations may not be enough to protect children from the serious and sometimes fatal disease.

New research confirms the whooping cough vaccine is failing at a higher rate than expected, and scientists are considering adding a seventh dose to the national immunization schedule published by the federal Centers for Disease Control and Prevention. Two recent studies have found the majority of people getting sick are up to date with their immunizations.

Wisconsin has the nation's highest rate of reported whooping cough cases in 2012. Through July 5, according to the CDC, Wisconsin's incidence of the disease was 50.7 per 100,000 persons, nearly 10 times the national average. Washington had the second highest rate, with 39.2 cases per 100,000 residents.

Through July 31, Wisconsin had reported 3,496 cases, including 2,498 confirmed and 998 probable cases. Washington state reported 3,484 cases through Aug. 11.

Washington has declared an epidemic. Wisconsin has not, but officials insist they are taking appropriate steps, including encouraging vaccinations and booster shots and making sure infants are protected from unvaccinated people.

"We're calling it a widespread outbreak," said Beth Kaplan, spokesperson for the state Department of Health Services, adding that using the word "epidemic" does not trigger any specific actions. "Our focus is on awareness and prevention."

Whooping cough, also known as pertussis, is a highly contagious respiratory illness that may mimic a cold for the first 10 days. It then can produce a violent and persistent cough with a unique "whooping" sound.

For adults, pertussis may only be a nuisance, like a bad cold. But to infants it can be deadly because they can't cough up what collects in their lungs, and infections can spread.

The state of Wisconsin reported that one infant with pertussis died in February.

Vaccine sometimes fails

More than 21,000 cases of whooping cough were reported nationwide through Aug. 4, according to the CDC. If that trend continues, 2012 would be the largest outbreak in more than 50 years.

Dr. Patrick Remington, a professor with the UW School of Medicine and Public Health, said the current pertussis vaccine is "one of the least effective vaccines out there," meaning the percentage of people who should be vaccinated to protect against an outbreak is correspondingly higher.

"You can see outbreaks propagate even with a high level of vaccination," Remington said.

Last year in Wisconsin, the state Department of Health found that 70 percent of persons 18 or younger "were up to date for age for pertussis immunizations at illness onset." So far in 2012, the state said, 56 percent of 162 cases of pertussis involving children age 1 or younger, the most vulnerable group, were up to date on their immunizations.

New studies published in prominent medical journals have called into question the 85 percent efficacy quoted in the vaccine's package inserts. For example, one study by scientists at Kaiser Permanente found vaccine effectiveness in Marin County, Calif., in 2010 to be between 24 and 79 percent for children ages 2 to 18.

Time for a new approach?

Public health officials and scientists agree the existing vaccines are still the best tool available to prevent and curb outbreaks. They say children who aren't immunized against the disease are eight times more likely to contract it than fully immunized children.

But some officials, prompted by the numbers of fully immunized people caught up in the outbreaks, have begun to question how much protection the vaccines provide, and for how long.

Of particular concern was the number of cases in California in 2010 occurring in fully vaccinated children 7 through 10 years of age. For cases in which immunization history was known, more than 80 percent of the children in that age group diagnosed with pertussis had been fully vaccinated, according to an analysis of state data.

The Marin County study also found vaccinated kids between ages 8 and 12 to be especially vulnerable at 24 percent effectiveness. Researchers suggested more shots more often might be necessary.

"We're seeing waning immunity over time, so it would suggest that we may need more boosters in order to keep elevating the immune system until we come up with a vaccine that has longer protection," said Dr. Mark Sawyer, a pediatric specialist at Rady Children's Hospital in San Diego and a member of the committee that advises the CDC on its immunization schedule.

Remington said the number of vaccinations is "up to the scientists" but acknowledged there's room for improvement: "It would be good to have a more effective, safer vaccine for whooping cough."

This article was reported primarily by Kevin Crowe of the nonprofit Investigative Newsource and Joanne Faryon of KPBS, both in San Diego. The Wisconsin Center for Investigative Journalism's Bill Lueders reported from Madison.