Because of the increase in cases among infants, not children, five-year-olds are being given a booster in an attempt to reestablish countrywide herd immunity rather than protect them per se. In effect, the immune system of your child is being exploited to prevent this ubiquitous disease from spreading among infants.

Whooping cough returns In the Netherlands, despite universal vaccine coverage since the 1940s, whooping cough cases are increasing (CDC, Emerging Infectious Diseases [Suppl] June 2001). Indeed, the rate of infection may be as high as 1-4 per cent among the population (Infect Control Hosp Epidemiol, 1999; 20: 120-3).

Many other countries with vaccination coverage of over 90 per cent, such as Canada and Australia, have recently had epidemics of whooping cough (Can Commun Dis Rep, 1995; 15: 45-8; Commun Dis Intell, 1997; 21: 145-8).

In the US, the Centers for Disease Control and Prevention (CDC) recently announced that, in the last three years, the number of cases of whooping cough has more than trebled to 7000 cases per year since the 1980s, when only 2000 cases were reported a year.

As with England, this increase - 29 per cent - has mainly occurred among children under a year old. This is despite the fact that at least two-thirds of all children receive some form of the whooping cough jab, and the vaccine is touted as being 88 per cent effective among children 7-18 months old (MMWR, 2002; 51: 73-6).

The reemergence of whooping cough in the US is hardly a new trend. After the vaccine was launched in the 1940s, cases of pertussis declined to an historic low in 1976. But, since the early 1980s, the incidence of whooping cough has increased cyclically, peaking every three to four years independently of vaccination (MMWR, 2002; 51: 73-6).

During a nationwide epidemic of whooping cough in 1993, a group of researchers at Cincinnati Children’s Hospital, examining 352 children who had come down with the disease, discovered that 82 per cent had completed their full complement of DPT vaccines (N Engl J Med, 1994; 331: 16-21).

One immediate cause is that the vaccine simply wears off after a few years. In one Italian study of 38 children, aged five to six years, who’d routinely received three doses of pertussis vaccine, very few children had significant antibodies to all three pertussis antigens five years after vaccination (Infect Immun, 2001; 69: 4516-20).

The mutated gene If Bordetella pertussis has made a worldwide comeback, for the most part it has done so in a slightly different guise. Epidemiologists believe that, among highly vaccinated populations, B. pertussis has mutated, changing its DNA ‘fingerprint’ and the genetic coding of its outer coat of surface proteins (Infect Immun, 1998; 66: 670-5; Microbiology, 1996; 142: 3479-85).

In the Netherlands, scientists have observed significant changes in the structure of the wild circulating bacteria when comparing blood samples of victims of whooping cough with those who were vaccinated. The main differences involved not only the outer membrane protein pertactin, but also the pertussis toxin itself. A similar genetic ‘drift’ has been observed in Poland, Finland and the US (Vaccine, 2001; 20: 299-303; Emerg Infect Dis, June 2001).

'It seems like the bacterium is changing part of its coat, thereby disguising itself' from the immune system, said Dr Audrey King of the National Institute of Public Health and the Environment in Bilthoven, the Netherlands, at the annual meeting of the American Society for Microbiology in June 2001.