Medical: Whooping cough's resurrgence explained

Resurgent outbreaks of whooping cough in the United States and much of the developed world have drawn a flurry of research to understand why the vaccine-controlled disease, pertussis, is making a comeback.

Two reports published last month suggest that a shift toward using a vaccine made from inactivated pertussis toxin, or acellular, rather than a vaccine made from whole cells, may be at least partly responsible.

Another study done in New York and published online in early June by the journal Pediatrics points to an increase in the number of children exempted from vaccine requirements as a condition of school attendance for contributing to rising rates in some counties.

Last year, nearly 42,000 people were diagnosed with the highly contagious respiratory illness, the most since 1959, according to the federal Centers for Disease Control and Prevention. Eighteen people, most of them children, died from the disease, which is caused by a bacterium called Bordetella pertussis. The disease kills an estimated 300,000 people worldwide each year, most in developing countries.

The disease sickened hundreds of thousands of Americans each year until a vaccine was developed in the 1940s and became widely used over the next decade.

One study, done through the health maintenance group Kaiser Permanente, looked at more than 55,000 individuals born in 1999 or earlier who had gone at least five years since getting the recommended fifth pertussis booster shot. The group included 138 individuals with confirmed pertussis.

The study, conducted after a major outbreak in California in 2010-11, showed that teenagers who had gotten four doses of the accellular vaccine were almost six times more likely to develop whooping cough than those who had received four doses of the whole-cell vaccine. The research, also published in Pediatrics, showed that those who got a booster shot with acellular vaccine in their early teens did not overcome the protective advantage to those given whole-cell vaccine.

Other recent research has shown that the risk of becoming infected was as much as 20 times higher among children who got their last shot three or more years earlier.

A second study by scientists at the University of Michigan looked at the incidence of pertussis in Thailand, which mainly uses a whole-cell vaccine. They found no evidence that the disease had increased, and that vaccine coverage among children and adults is high enough that very few people become infected. The report was published online in May by the Proceedings of the National Academy of Sciences.

The study by researchers at SUNY Upstate Medical University in Syracuse, N.Y., looked at 12 years of religious exemptions from vaccine requirements needed for school attendance. They found that in counties with higher proportion of exemptions, pertussis rates were higher. Overall, the average incidence of pertussis among exempted children was 14 times greater than among vaccinated children: 302 per 100,000 children versus 22.

Thirteen of 62 counties in the study had exemption rates of 1 percent or higher among all enrolled children, compared with four counties in 2000.

Another concern is that a new strain of pertussis bacterium has emerged, although it probably is not the cause of the recent upsurge in whooping cough. It lacks the protein pertactin, which helps the bacterium attach to the airways' lining. The protein is an important component of vaccines.

The new strain, first identified in France, now makes up about 14 percent of cases reported there. The first cases in the U.S. were identified over the past two years in a Philadelphia hospital. 

Scientists aren't sure what impact the new strain has on immunity. The CDC says there are enough matches against other parts of the bacteria in the vaccine to keep it effective. But at least one small French study has found that vaccine still lowers the risk of severe disease from the new strain, but may not block infection as thoroughly as with the standard strain.

(Contact Scripps health and science writer Lee Bowman at BowmanL@shns.com.)