Can the biomedical community eradicate a disease like measles?
This essay was written by Alastair McFarlane and was first published in the 2009 Mill Hill Essays.
Measles is a viral illness spread by droplet infection and it is most prevalent amongst children. Severe cases can induce the onset of pneumonia or encephalitis (inflammation of the brain), most commonly in infants, elderly people and those with a compromised immune system. Around one in a thousand people who contract the disease die from it and although this is only a small proportion, the high prevalence of the condition means that the absolute number of annual deaths worldwide (197,000 in 2007) is substantial while millions of people suffer serious complications. This dictates the need for a global approach to eradicate measles, as the high mortality can be affected by low vaccination coverage rates, preventing herd immunity in many regions. However, WHO report that measles deaths worldwide fell by 74% between 2000 and 2007.
In July 1996, the World Health Organisation (WHO) announced a strategy aimed at eradicating measles by 2010, a target that now seems overly ambitious. When smallpox was eradicated in 1978, epidemiologists became confident that diseases such as measles or polio could be managed with a similar approach. Unfortunately, several factors make these diseases harder to suppress.
For the worldwide elimination of a disease to take place, herd immunity must first be established. This entails interrupting the person-to-person transmission of a disease to protect the population at large. The level of coverage needed to achieve herd immunity is therefore largely determined by the contagiousness of the disease. Epidemiologists discovered that measles is an extremely contagious disease, with a high herd immunity threshold. The percentage of the population that need to be vaccinated to effectively stop the spread of the disease is 90-93%. Compared to a threshold of only 80-85% for smallpox, the contagiousness of measles requires very high vaccine coverage rates. In addition, the efficacy of the measles vaccine is not 100% but only around 95% (and may be less efficacious in developing countries). This means that even though vaccination rates are about 80% globally, only 76% of the population are actually immune. Compounding this, the measles virus possesses a number of characteristics that make it hard to eradicate: transmission can occur during the incubation period, before symptoms manifest themselves; therapeutically, there is no effective drug to counteract the measles virus. Fortunately, immunity to measles generally persists indefinitely after recovery from the disease and there is no animal or insect reservoir for measles: humans are the only host for the pathogen. Furthermore, unlike the influenza virus, the measles virus has a low antigenic variability, so the development of just one vaccine is sufficient.
In 1998, it was claimed that there was a link between the administration of the MMR (measles, mumps and rubella) vaccine and the onset of a pervasive development disorder, autism. The study, which comprised twelve subjects and included no control group, was later found to be scientifically flawed and unsubstantiated. The media, hungry for headlinegrabbing stories, published articles claiming the connection between MMR and autism was proven. The fall in public confidence which followed was exacerbated by various religious and cultural objections to the injection, some claiming it was ‘unnatural’. Vaccination levels plunged from above 90% to about 82%, and remain at 85% today. This is 10% below the level needed to induce herd immunity.
Whilst some UK newspapers and radio stations have carried on sensationalising stories linking the MMR vaccine with autism, there has also been a strong defence from the scientific community and journalists such as Ben Goldacre from the Guardian newspaper. Because of this, the likelihood that parents will refuse to vaccinate their babies may now be lower than ten years ago. Coupled with an attempt to deliver the vaccination in more practical ways, such as through aerosol sprays, the implementation of sustainable measles vaccination programmes with high coverage rates in the UK is becoming more realistic.
Neither scientific evidence nor money alone will eradicate global diseases like measles. In 1988, Bill Gates donated $100 million to fund an attempt to eradicate polio, which was stymied by the refusal of certain minorities to have the vaccination, perhaps because of a lack of trust in modern medicines. Polio is still endemic in Afghanistan, Egypt, India, Niger, Nigeria, and Pakistan. The continuing circulation of the polio virus in these six countries has global implications on attempts to eradicate the disease.
Despite the many epidemiological and cultural challenges, eradication of diseases like measles is still potentially possible. In July 1993 the last case of measles in Cuba was reported. Vaccine coverage in Cuba had risen to an impressive 98% through a policy of mandatory vaccination of twelve month old children (‘maintenance vaccination’), and a ‘follow-up’ service for children aged two to six years. Herd immunity was quickly established and the spread of the disease discontinued within a few years. This remarkable feat was attributed primarily to political will and public confidence in the vaccination, a stark contrast to the unparalleled distrust and resistance expressed in many European countries.
However, most epidemiologists feel that measles eradication by 2010 is an unrealistic aim in developing countries, where health officials lack the infrastructure needed to administer vaccinations. High levels of HIV infection in Africa compounds this problem, as immunosuppression makes people more susceptible to measles as well as inducing a higher fatality rate. Despite this, the Cuba case study illustrates that eradication is still feasible when governments endeavour to help the problem by developing firm policies.
The phenomenon of herd immunity shows that the elimination of an infectious disease in a population can be impeded by a minority of individuals. This raises ethical questions about whether parents have a duty to have their child immunised, not just for the child’s benefit but for the greater good. The 8% of the UK population who refuse their children the vaccination are making the problem worse for themselves and are a bad example to the developing world. Suboptimal vaccination coverage evokes doubts about eradication; ultimately, everyone must contribute if we are to remove the threat of measles.