Protection against whooping cough declines during the five years after the fifth dose of vaccine is given to 4- to 6-year-old children, a new study suggests -- leading some experts to urging teens and adults to make sure they are up to date on all recommended booster shots.
An outbreak of whooping cough in California led experts to conclude that the current diphtheria, tetanus, acellular pertussis (DTaP) vaccine for children has not worked as well as the older version from nearly two decades ago, according to the study in Thursday's New England Journal of Medicine.
Whooping cough -- formally called "pertussis" -- is deadly for infants, with outbreaks occurring every few years despite vaccinations.
"We found that the effectiveness of the vaccine wanes 42 percent on average each year during the five years after the fifth dose," said Dr. Nicola Klein, the study's lead author and co-director of the Kaiser Permanente Vaccine Study Center in Oakland, Calif.
A new and longer-lasting vaccine has yet to become available.
Until then, "the current vaccine is safe and effective; it just doesn't last as long as we would like," Klein said. "And parents should know that some protection is better than no protection."
A tetanus booster shot is recommended starting around age 11 or 12 -- about five years after administration of the fifth dose. Health care workers, individuals age 65 or older, pregnant women and anyone with exposure to infants should get a booster shot, according to the U.S. Centers for Disease Control and Prevention.
As of Aug. 11, 46 states and Washington, D.C., reported increases in whooping cough compared with the same time period in 2011, according to CDC statistics. Provisional counts from the CDC's surveillance system showed that nearly 25,000 cases of whooping cough were reported through Aug. 24, with 13 deaths noted.
The majority of deaths continue to occur among infants younger than 3 months of age. The incidence rate among infants surpasses that of all other age groups. The second highest rates of disease are seen in children ages 7 through 10. Rates are also increased in 13- and 14-year-olds, according to the CDC.
In many states, five doses of the whooping cough vaccine are mandatory before a child enters school, Klein said. The first three doses in the series are typically given when an infant is 2 months, 4 months and 6 months old. The fourth dose is generally administered between the first and second years of life, usually at 15 months. And the fifth dose is recommended between ages 4 and 6, typically before kindergarten.
Despite the requirements, a large outbreak of whooping cough occurred in California in 2010 -- its peak incidence since 1958. The outbreak spurred Klein and colleagues to measure the degree to which protection wears off over time in school-aged children who had received the most recent version of the vaccine.
While this study focused on school-aged children in northern California, localized outbreaks of whooping cough have occurred elsewhere as well. In April, an epidemic was declared in Washington state. Other states with high rates of the disease this year included Wisconsin, Montana, Minnesota and Colorado, the CDC reported.
The disease tends to begin with cold-like symptoms and perhaps a mild cough or fever. A severe cough can take over one to two weeks after disease onset. Unlike the common cold, though, coughing fits can persist for weeks, until air has left the lungs and a loud "whooping" sound accompanies attempts to inhale.
An older form of the vaccine offered longer protection, but it was linked to more cases of fever and injection-site reactions. What's needed is a stronger vaccine.
On a positive note, "the findings have raised the attention of the manufacturers," Klein said of the study she led.
She hoped the results would encourage the development of more effective alternatives.
"Prevention of future outbreaks will be best achieved by developing new pertussis-containing vaccines that provide long-lasting immunity," the authors concluded in the study.
Aside from potential allergic reactions, new and frequent boosters should be evaluated for length of protection, cost-effectiveness and feasibility of introduction into the childhood care schedule, said Dr. William Schaffner, chairman of preventive medicine at Vanderbilt University Medical Center in Nashville, Tenn.
The Kaiser Permanente study "provides strong confirmatory evidence that the current vaccine's effectiveness declines over time," Schaffner said. "The magnitude of the decline is striking and will accelerate the deliberations concerning more-frequent booster doses."
Experts cautioned that the study does not suggest that parents stop vaccinating their children.
"The question it raises is whether we should give vaccine more frequently -- not ever or less frequently," said Dr. Gregory Poland, director of the Mayo Vaccine Research Group in Rochester, Minn.
Those who developed whooping cough after vaccination had mild or moderate illness, not the more-commonly-occurring severe disease that can strike in the absence of vaccination.
"Hence," said Poland, who studies vaccine response in adults and children, "even in this study, the vaccine offered benefit."