This, of course, means that any child not reacting during the tiny 10-day window (and both DPT and MMR typically cause reactions after a latent stage) or not taken to the doctor would not have been recorded as having a reaction.

The US and a number of European countries are beginning to substitute the acellular pertussis vaccine for the old whole-cell variety as they regard it as safer, producing fewer of the more serious reactions linked with the old vaccine: very high fever, hypotonia/hyporesponsive events, persistent high-pitched crying, and convulsions or seizures (N Engl J Med, 1995; 333: 1045-50).

The entire rationale for the acellular vaccine is that it is safer than the whole-cell variety. However, the US Nationwide Multicenter Acellular Pertussis trial, which compared over 2000 babies given either acellular or whole-cell vaccine, found that the number of serious adverse reactions linked to the jab, but not proven to be caused by it - sudden infant death, nearly death, seizures, developmental delay, hospitalisation, encephalitis and vomiting - was similar in both groups.

However, those side-effects acknowledged as due to the vaccine, such as high fever, occurred less often and with less severity among the acellular groups. Other studies from Italy and Sweden concur (JAMA, 1995; 274: 446-7).

One question concerns how many components of the whole pertussis cell make for a safe, effective vaccine. The Swedish Ad Hoc Group for the Study of Pertussis Vaccines found that some of the most serious side-effects, like hypotonia/hyporesponsiveness, high fever and seizures, were most common among those given vaccines with the least number of components and those given the whole-cell vaccine. The safest vaccines were those containing three and five components.

Although deaths were comparable between various component vaccines and the whole-cell variety, basically, the more components present in the vaccine, the more reactions there were, with 39 adverse events in the three-component group vs 60 in the whole-cell group.

Five times as many babies had a high fever and three times as many had convulsions with the whole-cell jab as with the five-component acellular vaccine.

Many more cases of hypotonia/hyporesponsiveness occurred among the children receiving all varieties of acellular vaccine than the researchers expected (nearly one in every thousand babies).

As for effectiveness, most studies show that the acellular vaccine is about 85 per cent effective, but this may not last for very long (Presc Int, 2000; 9: 220-3; J Pediatr, 1998; 132: 983-8).

The bottom line is, though the acellular vaccine is far from ‘safe’ or highly effective, it is probably safer than the whole-cell vaccine and just as effective (see box, p 1).

Babies and the whole-cell vaccine The use of the acellular vaccine in older children begs the obvious question: why are we still using the whole-cell vaccine in babies?

In 1996, acellular pertussis vaccines were licensed in the US for routine use in 15-month-olds (MMWR CDC Surveill Summ, 2002: 61). By 1997, the US Advisory Committee on Immunization Practices (ACIP) recommended switching to the acellular pertussis vaccine for infants.

According to the DoH, the reason for sticking with the whole-cell vaccine has to do with efficacy. A review, written by the PHLS’s Elizabeth Miller, says, 'With the exception of the five-component vaccine, acellular vaccines are less efficacious than a good whole-cell vaccine' (Biologicals, 1999; 27: 79-86).