Protection against whooping cough starts to weaken a few years after preschool children get their final diphtheria, tetanus and pertussis (DTaP) shot, a new study confirms.
The Centers for Disease Control and Prevention (CDC) recommends a booster shot at age 11 or 12. But researchers found that slightly younger kids may be at risk of developing whooping cough before then, as their original immunity declines.
"What has become apparent is there's a fairly dramatic and startling increase in pertussis in children in the seven- to 10-year-old age group," said Dr H Cody Meissner, a paediatrician from Tufts University School of Medicine in Boston who didn't participate in the new research.
Meissner and other researchers think that trend results from a change in the 1990s to a new type of pertussis vaccine, called an acellular vaccine, which comes with fewer side effects than the original whole-cell version.
Although the acellular vaccine works well in the short-term, experts said it might end up providing less-complete or shorter-lasting protection.
What the study found
Researchers led by Sara Tartof from Southern California Permanente Medical Group in Pasadena used immunisation records and state-wide whooping cough data to track more than 400 000 Minnesota and Oregon children.
All of those kids were born between 1998 and 2003 and received the recommended series of five DTaP shots, the last of which is given at ages four to six.
Over the following six years, 458 kids from Minnesota came down with whooping cough. The rate of new cases rose from 16 per 100 000 children in the first year after their most recent DTaP shot to 138 per 100 000 in year six.
In Oregon, there were 89 cases - six per 100,000 kids in the first year and 24 per 100 000 in the sixth.
"Ideally, we need a better vaccine," said Dr Eugene Shapiro, a paediatrician and infectious diseases researcher from the Yale School of Medicine in New Haven, Connecticut, who wasn't involved in the new study.
Researchers said switching back to the whole-cell pertussis vaccine, at least for the first shot or two, could improve protection as kids get older.
But even though the side effects tied to that vaccine were typically mild - such as swelling and fever - that's unlikely to happen, they said.
The CDC could also move up the booster shot to ages eight to 10, but it's harder to get kids into the office at that point, according to Shapiro. The current booster, called Tdap, is given at the same time as a number of other recommended adolescent vaccines, such as one of the human papillomavirus (HPV) doses.
Five doses on time
Researchers agreed that for now, the best thing for parents to do is follow the CDC's vaccine schedule, which is online here: "An important thing to remember is the kids who do receive all five doses on time generally have milder (whooping cough) than those who are under-vaccinated or unvaccinated," Tartof said.
"Even though there is waning immunity getting the five doses on time is still the best protection you can give your kid."
Making sure pre-teens get their booster shot and expecting that parents are vaccinated will help cut down on transmission of whooping cough as well, said Tartof.
"It is so important that people do not interpret this as the vaccine doesn't work, and then not get vaccinated at all," Meissner said, "because then we would really have a problem."