There is still some dispute over what elements ought to be included in an acellular vaccine, largely because scientists don't really understand which components in the pertussis vaccine are required to stimulate an immune response.
Is it pertussis toxin (PT) itself, the outer membrane pertactin (PRN), filamentous haemagglutinin (FHA; a protein that causes red blood cells to clump), or one or both types of fimbrial antigens (FIM) that are needed to kickstart the immune system?
In Sweden, the Ad Hoc Group for the Study of Pertussis Vaccines, comparing two-, three- and five-component products with the whole-cell variety, found that the most effective was the five-component jab (Lancet, 1997; 350: 1569-77).
The UK government maintains that you need all five components to have a truly effective vaccine. Nevertheless, there is evidence that the number of components is a red herring and that a single-component vaccine using pertussis toxin only is just as effective as the multicomponent vaccines in preventing standard pertussis (with about an 80 per cent chance of working).
As one study has suggested, the apparently greater efficacy of multicomponent acellular pertussis vaccines compared with a single-component one may be a laboratory 'artifact' as a result of the strict diagnostic criteria used by the World Health Organization.
So, how effective a vaccine appears to be depends on your definition of whooping cough - simple pertussis or infections that could include other bacteria (Pediatrics, 2001; 108: E115).