Overherd Immunity

Overherd Immunity

Caitlin Stewart discusses the importance of vaccination and the dangers of the anti-vaccination movement

Vaccination has been all over the news in recent years, as deadly, vaccine-preventable diseases are returning and infecting the population worldwide. One of these diseases, measles, was declared eradicated in the USA in the year 2000, but has since made a resurgence; with a 3-fold increase in cases between 2013 and 2014, according to the CDC, and an additional outbreak reported earlier this year at Disneyland in California. Measles, caused by Morbillivirus, is spread by coughing or sneezing, and is highly contagious. Infection causes a high fever and a distinctive rash that covers the whole body. It is considered as one of the most deadly childhood fever illnesses, with children being at high risk of complications; including ear infections, seizures and pneumonia. An estimated one in 1000 childhood cases of measles even results in death.

The USA isn’t alone in this phenomenon; an outbreak of pertussis, also known as whooping cough, occurred in the UK in 2012, with almost 10,000 cases recorded by Public Health England— that’s 10 times the average number of cases and the largest outbreak in the UK for 20 years! Cambridge was especially hit hard, experiencing 20 times the average number of cases. Pertussis is a highly contagious respiratory disease caused by the bacterium Bordetella pertussis. Infection results in uncontrollable, violent coughing, making it difficult for suffers to breathe. After fits of coughs, sufferers quickly take deep breaths that result in the characteristic ‘whooping’ sound the disease is named after. Although similar to measles, it is more serious and deadlier in children, especially in babies less than a year old. In the UK, 14 babies less than three months old died during the 2012 outbreak.

Both measles and pertussis are vaccine-preventable diseases. John F. Enders developed the first vaccine to measles in 1963, and an improved vaccine with reduced side effects was introduced five years later. The pertussis vaccine was introduced much earlier, in the 1940s. Vaccination against both has been credited for effectively eradicating these diseases in the USA and other developed countries. However, vaccines are not effective in 100 per cent of individuals as certain people, such as those with immunodeficiency or allergies to the vaccine contents, cannot be immunised. These people rely on herd immunity—if the majority of the population are immune the disease cannot effectively spread and infect those who are non-immune.

Interestingly, the increase in these diseases is not due to ineffectiveness of the vaccines or adaption by the pathogens, but by individuals choosing not to vaccinate themselves or their children. As a result, not enough people are being vaccinated against these diseases to maintain herd immunity. The anti-vaccination crowd, or ‘anti-vaxxers’ as they are known, typically believe vaccinating will cause more harm than good. They spread information about the risks vaccines and their side effects pose to children—information which is sometimes incorrect or even fraudulent. This belief was started largely by a paper, published in 1998 in the Lancet by Andrew Wakefield, claiming the measles vaccine results in autism spectrum disorders when given to children. After it was published, multiple papers published in the BMJ suggested that Wakefield had manipulated evidence and that the results were fraudulent. This resulted in the paper being retracted, but the damage had already been done and some still believe these findings. By 2006, cases of measles and mumps had increased, 13 and 37 times respectively, compared to their 1998 levels.

Vaccination does have side effects, although these are usually mild and can include fever, redness or soreness around the injection site. Serious side effects reported for the measles vaccine such as long-term seizures, coma or permanent brain damage are incredibly rare (<1 in a million doses); so rare that they cannot necessarily be attributed to the vaccine at all. These serious side effects are often portrayed as being common by anti-vaxxers and cited as reasons to avoid vaccination entirely. Another reason against vaccination is the belief that the current schedule includes too many vaccines at once or too close together, overloading a child’s immune system, resulting in brain damage and/or autism spectrum disorders. There is no scientific evidence supporting this. The unfortunate reality is that the measles vaccine is administered during a baby’s first year, a time when the emergence of autism spectrum symptoms, such as the regression of communication and other skills, quickly follows. This can lead some parents to mistake correlation for causation.

These parents fear harm coming to their children, and the availability of incorrect information about the risks of vaccines online has led them to believe the dangers of vaccination outweigh the benefits. It has been suggested that many anti-vaxxers have grown up without experiencing the symptoms and consequences of vaccine-preventable diseases, and do not realise the risks these diseases pose, or the reality of the danger they are putting their children in.

Many methods to change the minds of anti-vaxxers have been tried—most frequently the scientific evidence of their safety and absence of a link to autism is explained. Unfortunately this has been shown in previous studies to be ineffective and can even backfire—where some vaccine sceptics formed even stronger negative opinions about vaccination when this information was given to them and their views were challenged.

A more recent study, looking at changing the attitudes of anti-vaxxers was published by Horne et al. and showed encouraging results. However rather than trying to convince vaccine sceptics of no link between vaccination and autism, they tried to convince them of the negative effects of having no vaccination; i.e. that being unvaccinated leads to a high probability of catching a preventable disease and that the consequences of these illnesses are severe. 315 participants were first assessed for their attitudes towards vaccination, then randomly assigned one of three types of reading material (“Disease Risk”, “Autism Correction” and “Control” scientific literature), before finally being assessed again. Those assigned reading on Disease Risk received a paragraph from a mother describing her child contracting measles, a picture of children with measles, mumps, and rubella, and three short warnings about how important it is for people to vaccinate their children. Those reading about Autism Correction received materials from the CDC website summarising research that shows vaccines do not increase the risk of autism in children. Control scientific literature was on topics unrelated to vaccination. There was a slight increase in support of vaccination after participants read the Autism Correction literature compared to the control. However, this was not statistically significant. On the contrary, reading about Disease Risk increased support for vaccination in six times as many people as in the control group, a highly statistically significant result. Showing them the risks posed by the disease may have caused them to re-evaluate their attitudes towards vaccination to see that the benefits outweigh the risks. This is an important step forward in ensuring antivaxxers and the public are properly informed about the risks of remaining unvaccinated.

This type of shock tactic is similar to the antismoking campaigns showing diseased lung tissue and has been successful in other areas, suggesting it could be effective in changing anti-vaccination attitudes on a larger scale. To implement this, the authors suggest making this information more readily accessible on the internet and as easy to find as anti-vaccination information is currently, which can be found within a few clicks. Additionally, including this information in baby visits with parents would provide excellent opportunities to ensure higher vaccination rates in the future.

As somebody who fell victim to whooping cough during the last outbreak, desp ite being vaccinated, stemming the tide of these diseases from spreading and ensuring everyone is protected is a welcome prospect.

Image credit: NIAID