*Text courtesy Reuters.

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(Reuters) - Protection against whooping cough starts to weaken a few years after preschool children get their final shot, according to a U.S. study, meaning that some children may be at risk of developing the disease before they can get a booster shot.

The U.S. Centers for Disease Control and Prevention (CDC) recommends a booster shot at age 11 or 12. The usual practice is to give five doses of diphtheria, tetanus and pertussis (DTaP) shots, the last at age four to six.

"This evaluation reports steady increase in risk of pertussis in the years after completion of the 5-dose DTaP vaccines," wrote lead author Sara Tartof in the study, which appeared in Pediatrics.

Tartof, who is from Southern California Permanente Medical Group in Pasadena, and her team used immunization records and state-wide whooping cough data to track more than 400,000 children in the states of Minnesota and Oregon. All were born between 1998 and 2003 and received the recommended series of five shots.

Over the following years, 458 children from Minnesota came down with whooping cough. The rate of new cases rose from 16 per 100,000 in the first year after their most recent shot, to 138 per 100,000 in year six. In Oregon, there were 89 cases - six per 100,000 in the first year and 24 per 100,000 in the sixth.

"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 H Cody Meissner, a pediatrician from Tufts University School of Medicine who did not take part in the study.

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.

But researchers also said that switching back to the whole-cell version, at least for the first couple of shots, could improve protection as children get older. But even though the side effects for that shot were mild, that is unlikely to happen.

The CDC could also move up the booster shot to ages eight to 10, but it's harder to get children into the office at that point. The current booster is given at the same time as a number of other recommended adolescent vaccines.

"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 told Reuters Health. "Even though there is waning immunity ... getting the five doses on time is still the best protection you can give your kid." SOURCE: bit.ly/jsoh2P

(Reporting from New York by Genevra Pittman at Reuters Health; editing by Elaine Lies)