7.5 Perceived risk of vaccination
An important consideration in vaccination programmes is the perceived risk associated with the vaccine. As the incidence of a disease falls (possibly as a result of vaccination), the risk of contracting it also falls. Of course, the risk to individuals from the vaccine remains the same – but what changes is the relative importance of the disease compared with the perceived importance of any adverse effects of the vaccine. There is less incentive to be vaccinated or to have a child vaccinated in a population with a low incidence of the disease, and concerns about possible side-effects of the vaccine can further reduce its uptake.
What consequence might falling vaccine uptake have for the incidence of a vaccine-preventable disease in a population?
As vaccination coverage falls, the proportion who are susceptible to the infection rises and the level of herd immunity to the pathogen declines. If a source of infection enters the population in conditions in which transmission to susceptible hosts is possible, an outbreak of the disease will occur. If the proportion of immunes in the population has fallen below the critical immunisation threshold (i.e. the proportion of the population who must be immunised for the infection to be eliminated), then – in the absence of a return to high vaccine coverage – the infection will become endemic in the community again.
The perception of risk is a subtle and complex phenomenon. Many studies have shown that people tend to overestimate the chance of suffering a rare adverse event (such as being struck by lightning) and underestimate ‘everyday’ sources of risk (such as traffic accidents), which are actually much more likely to happen. Public perception of the risks associated with vaccinations seems to follow this pattern, in that people in countries where a vaccine-preventable disease has fallen to a very low level tend to overestimate the residual risk associated with the vaccine. An additional consideration is that most vaccine recipients are babies or very young children who cannot ‘consent’ to the procedure. Parents may be more reluctant to have children vaccinated than a rational assessment of the risks and benefits would dictate, because they will feel responsible for any adverse outcomes of a procedure they deliberately chose to accept. Two examples of vaccination programmes illustrate these ideas.
In 1999, the new conjugate ‘MenC’ vaccine against Group C Neisseria meningitidis, one of several causes of meningitis, was introduced in the UK in the first mass vaccination programme with this vaccine in the world. An earlier subunit vaccine against Groups A and C was available, but had never been used in a routine vaccination schedule. No vaccine currently protects against Group B, which is twice as prevalent as Group C, but causes a less severe illness. Before 1999, there were about 1500 cases of Group C meningitis in the UK annually, with a 10 per cent case fatality rate and an incidence of serious permanent disabilities in about 15 per cent of those who survived. The septicaemia can cause such rapid and extensive tissue damage in the extremities that amputation of the hands, forearms and lower limbs may be unavoidable.
The vaccine was offered initially to young children, then to everyone of school age, and later to young adults (e.g. college students), these being the highest risk groups. The programme was strongly promoted by the government, health professionals and independent bodies such as the Meningitis Trust. Even though meningitis C is a rare condition – around 5 cases annually per 100,000 children aged under 5 years – parents saw it as such a serious disease that there was a high uptake of the new vaccine. The perceived threat from the disease was considered to be so great that almost all parents accepted government assurances that any risk associated with the new vaccine was low (as indeed turned out to be the case). Following the introduction of the MenC vaccine, the incidence of Group C meningitis fell by about 75 per cent.
Estimate the annual number of lives saved by the introduction of the MenC vaccine in the UK.
Around 110, based on the figures given above. With 150 fatalities annually, a 75 per cent reduction in the disease incidence implies that mortality is reduced accordingly.
Contrast the lack of public anxiety about vaccine safety in the MenC programme, with the continuing controversy about vaccination against Bordetella pertussis, the causative agent of whooping cough. A pertussis vaccine has been available in the UK since the mid-1950s, accelerating the downward trend in disease incidence. However, in the mid-1970s, concern was expressed about possible neurological consequences of the vaccine. Uptake fell very rapidily from 81 per cent to 30 per cent, followed soon after by a sharp increase in the incidence of whooping cough (Figure 12).
As the perceived threat of the disease rose, parental confidence in the vaccine was re-established and vaccination rates increased again, bringing the incidence of whooping cough back down to almost zero in the early 1990s.
By comparison with bacterial meningitis, whooping cough is highly contagious. The estimated value of R0 for England and Wales in the 1970s, assuming a totally susceptible population, is 16–18. With a transmission risk of 50–90 per cent (i.e. effective transmission between an infective case and a susceptible individual occurs in 50–90 per cent of contacts), most children in an unimmunised population will become infected. Thus, as vaccination rates fell in the 1970s, epidemics very quickly occurred and parents became increasingly concerned about its effects. Historically in the UK, whooping cough was a major cause of infant death, and it still carries a risk of neurological damage or detached retina due to the coughing in a minority of cases. However, as vaccination levels have risen, parental experience of the effects of whooping cough have faded, and concerns about possible adverse reactions to the vaccine have resurfaced.
The cyclical resurgence of whooping cough associated with the decline in vaccination levels highlights the societal dimension of attempts to eradicate an infectious disease. In 2002, concerns were expressed in the UK and in the USA that the combined measles, mumps and rubella vaccine (MMR) might be associated with the development of autism or gastrointestinal abnormalities. Vaccination rates have fallen in those countries, and not just in relation to MMR. Some parents have lobbied for separate vaccines against measles, mumps and rubella to replace the triple vaccine in the belief that this approach would be safer, despite government assertions that it will result in less protection. By the time you are reading this, the debate will have moved on.