It s the law of unintended consequences. In the 1990s, the U.S. switched from a whole-cell vaccine for pertussis (whooping cough) in favor of the acellular Tdap vaccine, in which the a represents acellular which also protects against diphtheria and tetanus. But now, new research is showing that protection offered by the acellular vaccine (a vaccine comprised of fragments of bacteria cells, rather than entire cells) fades over time, even for those who get all five recommended doses.
Lara Misegades, PhD, of the Centers for Disease Control, and colleagues examined data from the 682 children who developed pertussis during the California whooping cough outbreak, along with 2,016 children in a control group. They found that the vaccine was 98 percent effective in preventing pertussis in children 12 months from the time they d received their fifth dose but at 60 months or longer, its effectiveness was reduced to 71 percent.
There s a growing consensus that the effectiveness of the current pertussis vaccine wanes with time and that teenagers, and maybe adults too, should get a booster shot, ACSH s Dr. Gilbert Ross says. The CDC recommends children get one booster shot at 11 or 12, but maybe they need one more as teenagers.
So if acellular vaccines don t work as well as the whole cell vaccines, why did we ever switch from them in the first place? Dr. Ross says the whole cell pertussis vaccine caused a dramatic decline in whooping cough cases after it was introduced in the 1940s, but it also caused some adverse effects such as swelling, local pain, fever, and behavioral problems of short duration. One study shows 6 percent of children vomited after receiving the shots, and in rare cases infants went into convulsions or seizures. However, the tradeoff of using the newer acellular vaccine, while reducing adverse reactions to almost zero, has proven to be the lack in staying power for protecting against whooping cough, which can be lethal in newborns and infants.