Reporting from Atlanta — The dramatic surge in whooping cough in California in 2010, in which 10 infants were killed by the bacterium and more people were sickened than in any year since 1947, has scientists looking for answers.
Researchers are focusing on a surprising trend: 7- to 10-year-olds are getting the disease at higher levels than doctors would have suspected.
Public health experts are concerned that children in that age group may not be protected by the final booster in a series of five shots that begin when they are 2 months old. The last booster is given between the ages of 4 and 6.
"The big question for the vaccine is, how long does it protect you for?" said Dr. Tom Clark, a medical epidemiologist and whooping cough expert with the U.S. Centers for Disease Control and Prevention.
"We haven't really studied this fifth dose," Clark said.
Earlier studies have shown the childhood vaccine, Dtap, to be effective for toddlers and children between 18 months and 5 years of age.
According to the California Department of Public Health, the age group hit hardest by whooping cough are infants younger than 6 months old, with 385 cases reported for every 100,000 babies.
But the high rate of the disease in babies is not surprising. At least three shots are needed before a baby gains a good degree of protection from whooping cough, also known as pertussis. There are five scheduled doses; one each at 2 months, 4 months, 6 months, between 12 and 18 months and the last when the child is between 4 to 6 years old.
By the time children are between 6 months and 6 years of age, the disease incidence falls to 50 cases for every 100,000. The mystery is why the rate of the disease goes up for 7- to 9-year-olds, whose rate is 58 cases per 100,000.
California health officials are working with the CDC to determine the effectiveness of the last shot, said Dr. John Talarico, chief of the immunization branch for the California Department of Public Health.
If scientists determine that the last Dtap dose is not working as long as expected, officials could decide instead to give the Tdap vaccine, currently given to 11- to 12-year-olds, earlier.
Beginning this summer, California will require the Tdap shot for entry to middle school and high school unless students have a medical or personal belief exemption.
The Tdap vaccine, which is given to adolescents and adults, was licensed in 2005. The shot has been credited with reducing whooping cough among adolescents and teenagers, who were hard hit during an outbreak that began the same year the vaccine was released.
Public health officials are also working to better understand why surges of whooping cough cases are reaching levels that haven't been seen in more than half a century.
Before the introduction of pertussis vaccinations in the 1940s and 1950s, as many as a quarter of a million Americans contracted whooping cough a year, and it was one of the significant killers of infants in the United States. For decades after the vaccines were introduced, whooping cough cases steadily declined, hitting a low of 1,010 cases across the nation in 1976.
Since then, however, the severity of outbreaks has worsened. Since 2003, more than 10,000 people nationwide have contracted pertussis each year.
California has faced dramatic increases in the last decade. Fewer than 700 people reported having whooping cough in 2000. But by 2005, more than 3,000 people contracted the disease. The total for 2010 was more than 7,800 cases in the Golden State.
"I think, to us, there's a bigger story in that we could use better vaccines. We could use vaccines that protect longer," Clark said over lunch at the CDC headquarters in Atlanta. "The vaccines we're using aren't so different than the ones that were developed in the 1940s. So if they don't protect you long enough, someone should be working on that. And there are no new vaccines for pertussis in the pipeline."
The good news, for now, is that the outbreak that began in California last spring seems to have crested, said Talarico, of the state's immunization program.
At the beginning of December, state records showed that disease reports had fallen to fewer than 200 new cases a week, down from peaks of more than 300 cases a week in the summer and autumn. Before the 2010 outbreak began, there were fewer than a dozen cases of pertussis a week.
Talarico said he hopes that the disease decline has been assisted by the state's massive inoculation effort. State health officials, using federal stimulus money from the American Recovery and Reinvestment Act, sent caches of vaccine to hospitals to inoculate pregnant women, fathers-to-be and their families, because people sickened with the pertussis bacteria can fatally transmit the illness to newborns.
Also, doctors throughout the state are increasingly offering pertussis vaccines routinely to their patients who seek care for unrelated reasons.
Half a year ago, some physicians did not have pertussis inoculations in their clinics, and some pharmacies, relying on outdated information, refused to provide vaccine to senior citizens despite state health officials' recommendation that the elderly be inoculated.
Some patients had to resort to seeking the vaccine from county and city public health departments.