The Tdap booster is meant to protect against pertussis, or whooping cough, tetanus and diphtheria. The study found it protected about 69% of adolescents against whooping cough in the first year after vaccination, but protection dropped to 57% of adolescents in the second year, then 25% and 9% in the third and fourth years after vaccination, respectively.

Researchers looked at diagnosed pertussis infections in about 280,000 children from 2009, when the children were 10 years old, until 2015. Nearly everyone received Tdap by the time they were 11 or 12 because California mandated the booster for seventh graders starting in 2011.

"It provides moderate protection during the first year but years two and three after vaccination, there is not that much protection left," said Dr. Nicola P. Klein, co-director of the Kaiser Permanente Vaccine Study Center in Northern California, and lead author of the study, which was published on Friday in the journal Pediatrics

Researchers hoped that Tdap vaccine would fare better than the five doses of DTaP, which are recommended for all children between 2 months and 4 to 6 years of age. Studies have shown that immunity fades starting one year after the last DTaP dose, and so Tdap is given later in adolescence to fill in the gap.

"The idea of giving the booster to 11- and 12-year-olds -- the recommendation, and in California, the requirement -- is to prevent infections and outbreaks, and that is unfortunately not what we found," Klein said.

Rethinking vaccine strategies

The researchers found that the pertussis cases in the study, whether in vaccinated or unvaccinated children, were mild or moderate. "But it's still a pretty substantial cough and kids can be out of school. It impacts their life even though there were no hospitalizations or death," Klein said. In addition, these infections help pertussis spread to babies and young children who can develop more serious infections, she added.

"We need to reopen the discussion about how we can most effectively target vaccination to make the best use of it," Klein said.

One possibility could be to give children Tdap only if a pertussis outbreak is on the horizon, which would require local health departments and medical centers to do regular surveillance, she added. Another possibility could be to administer Tdap to adolescents of all ages every three or four years, as outbreaks generally occur at this frequency.

By now, it is not that surprising that protection against pertussis from Tdap is short lived, said Dr. James D. Cherry, distinguished research professor in the David Geffen School of Medicine at the University of California, Los Angeles. Before the current study, which Cherry called a "me-too study," researchers found that immunity plummeted sharply in children in Washington state and Wisconsin a year or two after the booster.

The bigger concern than improving immunity among adolescents, who generally have mild disease from pertussis, is how to protect babies -- especially those younger than 3 months old -- in whom pertussis can be deadly, Cherry said.

"The most important thing is to vaccinate pregnant women [because protective antibodies get passed through the placenta] and start immunizing at 6 weeks and not 8 weeks," Cherry said.

With these strategies, "we can prevent all deaths with our current vaccines," he added. Although the Tdap vaccine is already recommended for pregnant women , many women in the United States do not receive it, Cherry said.

Can we make a better pertussis vaccine?

The current study offers some hints for making a better pertussis vaccine. During the study period, from 2009 to 2015, there were two important pertussis outbreaks in California, one in 2010 and another in 2014 . In both outbreaks, 10- and 11-year-old children bore the brunt of whooping cough cases, because by that age most of the immunity from their fifth dose of DTaP at age 4 to 6 would have worn off, Klein said. She and her colleagues conducted a study showing the risk of pertussis increased by additional 42% for each year after DTaP immunization.

However, in the 2014 outbreak, many older children, especially those between 14 and 16 years of age, were also diagnosed with pertussis. Teenagers would have been protected during earlier outbreaks because they received their childhood immunizations in the 1990s when a different form of pertussis vaccine, called whole cell pertussis, was still given, Klein said.

Cherry and other researchers around the world are trying to modify the vaccine so that it causes fewer side effects and still provides long-lasting immunity. The idea would be for it to replace acellular pertussis in childhood and adolescent immunizations, Cherry said. However, the versions being developed are a "long ways away" from being ready to use, he added.

The idea of a new-and-improved whole cell pertussis vaccine "would be fantastic and if that approach ultimately pans out, that maybe is the way forward," Klein said.