In the late 1990s, a new version of the whooping cough vaccine was introduced. The big benefit was that it had fewer side effects. But in the years since, evidence has been mounting that this newer vaccine loses its effectiveness -- and fast.
Now, another study sheds light on how well the "booster" dose works. California requires this booster -- called Tdap -- for all incoming 7th graders.
Researchers at the Kaiser Permanente Vaccine Study Center looked at the protection level of 175,000 adolescents in the Kaiser Northern California system vaccinated with Tdap. And just as with earlier studies, they saw that protection faded.
"What we found was the the Tdap vaccine offers moderate protection in the first year," said Dr. Nicola Klein, co-director of Kaiser's vaccine center, "but then that rapidly decreased over the next four years, so by the time we were at four years after vaccination, it was down to nine percent."
There have been two statewide whooping cough -- also called pertussis -- epidemics in recent years. The first was in 2010, the second in 2014, which was the first where all adolescents had received only the newer pertussis vaccine, the "acellular pertussis" vaccine: five doses by age 6, plus the booster before 7th grade. The 2014 epidemic was even more severe than the one in 2010.
"Unfortunately," said Klein, "routine vaccination of 11- and 12-year-olds does not appear to be preventing outbreaks."
The question becomes what to do about it -- especially since the authors predict the situation will only get worse. "Future pertussis epidemics," they write in the study, will "be larger as the cohort that has only received acellular pertussis vaccines ages."
At the very end of the study, published in the journal Pediatrics, the authors raise the question of a different strategy, one where the vaccine was "administered to adolescents in anticipation of a local pertussis outbreak, rather than on a routine basis."
It seems like a good idea, but it's one that would be a significant departure for the U.S., according to Dr. Arthur Reingold, head of epidemiology at the UC Berkeley School of Public Health.
"The United State doesn't have a lot of experience in doing mass campaigns in response to or in preparation of an outbreak," he said.
It might also seem reasonable to give doses more frequently. But Reingold, who also chairs the pertussis group for the nation's Advisory Committee on Immunization Practices, says first you need a lot of evidence that extra doses are both safe and effective.
"Give a dose at 12 [years], another one at 15 and another one at 18," Reingold questioned. "It's expense and it's complicated, and it's not clear what the effects would be. There's not a clear solution at this point."
A clear solution would be a new vaccine that is, again, safe and effective. Reingold says that's at least five years off.
In the meantime, no adolescents died or were hospitalized in the 2014 epidemic. Klein referred to pertussis as a "mild to moderate disease." Yet, the illness is often referred to as "the 100-day cough."
"It's not a deadly disease," Klein said, "but it is an impact on kids' lives." And parents, too, who may have to take time off work to care for their sick children.
It becomes a question of priorities in a limited-resources world. "At the public health level, the emphasis is on preventing deaths and hospitalizations," Reingold said. "How much money do we want to spend ... to prevent the 100-day cough?"
"It's a perfectly legitimate question, and I don't know the answer."