Public health officials claim that there are fewer serious neurological side-effects with the new vaccine, but few studies have compared its safety with the older pertussis vaccines, except for injection site reactions or fever (Pediatrics, 1995; 96: 557-66; Vaccine; 1998; 16: 320-2).

In one animal study, the production of proinflammatory cytokine interleukin-beta, associated with neurodegenerative conditions, was significantly increased in the brain of mice vaccinated with whole-cell pertussis vaccine. Acellular vaccine did not have this effect. The researchers concluded that active bacterial toxins are responsible for the neurological disturbances with the whole-cell vaccine. However, animal studies are not always applicable to humans (Infect Immun, 2001; 69: 4217-23).

A comparison of adverse reactions reported to the US Vaccine Adverse Events Reporting System (VAERS), a national system of surveillance, when the whole-cell vaccine was in exclusive use in 1995 and when acellular vaccines were introduced in 1998 shows 2071 reports of death or injury in 1995 compared with only 491 reports for the first half of 1998. Non-fatal reactions requiring hospitalisation, life-threatening illness or permanent disability declined by about a third, although the number of deaths reported (about 80 per year) were similar (Pediatrics, 2000: 106: E51).

In one study of children getting a preschool DaPT booster, fever higher than 38 degrees C occurred in 3.8 per cent of children and vomiting in more than one in 50. Thirty days after vaccination, there were 247 adverse events in 580 children, but only 63 were thought to be vaccine-related, including haematomas, headache, stomachache and sleep disturbance.

The study found that local reactions are more pronounced in children aged four to six compared with younger children and also after a fifth dose, possibly because the body reaches a level of tolerance, causing a bigger reaction (Pediatr Infect Dis J, 2001; 20: 981-8).