The whooping cough vaccine given to babies and toddlers loses much of its effectiveness after just three years. Doctors believe that could help explain a recent series of outbreaks in the U.S. among children who were fully vaccinated, a study suggests.
The study is small and preliminary, and its authors said the results need to be confirmed through more research. Nevertheless, the findings are likely to stir debate over whether children should get a booster shot earlier than now recommended.
“I was disturbed to find maybe we had a little more confidence in the vaccine than it might deserve,” said the lead researcher, Dr. David Witt, chief of infectious disease at the Kaiser Permanente Medical Center in San Rafael, Calif. Witt presented his findings Monday at the American Society for Microbiology conference in Chicago.
The study was done in California. The state had a huge spike in whooping cough cases last year, during which more than 9,100 people fell ill and 10 babies died. California schools have turned away thousands of middle and high school students this fall who haven't gotten their booster shot.
An empty bottle of Tetanus, Diphthera and Pertussis, (whooping cough) vaccine is seen at Inderkum High School in Sacramento. Calif. A law passed last year by the state Legislature requires all middle and high school students to receive the vaccination by the start of the 2011- 2012 school year. A 30-day extension had been granted, but many school districts began hitting that deadline last week. A free clinic was set up at Inderkum High for students to get the shot.
Government health officials recommend that children get vaccinated against whooping cough in five doses, with the first shot at age 2 months and the final one between 4 and 6 years. Then youngsters are supposed to get a booster shot around 11 or 12. That means a gap of five to eight years.
Witt's study looked at roughly 15,000 children in Marin County, Calif., including 132 who got whooping cough last year. He found that youngsters who had gone three years or more since the last of their five original shots were as much as 20 times more likely to become infected than children who had been more recently vaccinated. The largest number of cases was in children 8 to 12 years old.
Marin County has a reputation for anti-vaccine sentiment, and Witt said that when he started the study he expected to see the illness concentrated in unvaccinated people. But more than 80 percent of the children who developed whooping cough in Witt's study were fully vaccinated.
Whooping cough, or pertussis, is a highly contagious bacterial disease that in rare cases can be fatal. It leads to severe coughing that causes children to make a distinctive whooping sound as they gasp for breath.
California health officials told doctors last year that they could give the booster to kids as young as 7 in an effort to stifle the outbreak. Federal health officials said that they are still studying the issue and that it is too soon to make that a standard practice. At the Centers for Disease Control and Prevention officials acknowledged that the vaccine's protection declines, but they said the agency's own studies show the drop-off is not as pronounced as Witt's research found. The CDC has estimated that the risk of the disease can increase fourfold several years after vaccination, not 10 to 20 times.
One reason may be differences in how a case of whooping cough is defined: Witt counted positive test results, while the CDC also requires more than a week of symptoms. CDC officials stressed that the vaccination is still much better than nothing — it reduces how sick a child becomes. Also, the nation no longer sees thousands of whooping cough deaths each year, as it did before there was a vaccine.
The shots “are still our best protection against pertussis, and they still protect well against fatal disease,” said Dr. Tom Clark, who leads the CDC's epidemiology team focused on vaccine-preventable diseases. A preliminary study conducted by the CDC last year found the five-dose vaccination for children was about 70 percent effective five years after the last shot. Witt's research suggests the effectiveness may drop much lower than that, perhaps below 50 percent after just three years. Witt also found that shots work great in the short term. Rates of whooping cough dropped dramatically after kids were age 11 and 12, when many get the booster shot.
Versions of the vaccine are made by two companies — Sanofi Pasteur and GlaxoSmithKline. The companies have acknowledged that the immunity conferred by the vaccine wanes over time, but they declined to comment on Witt's study.
Nurse Susan Peel draws whooping cough vaccination before giving an injection to a student at Inderkum High School in Sacramento. Calif. A law passed last year by the state Legislature requires all middle and high school students to receive the vaccination by the start of the 2011- 2012 school year. A 30-day extension had been granted, but many school districts began hitting that deadline last week. A free clinic was set up at Inderkum High for students to get the shot.
“It's a little too soon to say much” about the longer-term effectiveness of that booster, said Lara Misegades, a CDC epidemiologist.
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