• Lottie Buttle

CMI Explains: Vaccines and Behavioural Economics

As the duration of the Covid-19 pandemic turns into years, there is now a light at the end of the tunnel: vaccines. The Covid-19 vaccine has been generating widespread interest recently. This comes as no surprise. The vaccine has been mass-produced in a remarkably rapid turnaround of only 9 months, compared with the previous record holder of 4 years for the mumps vaccine. The fast construction of an effective vaccine has undoubtedly attracted widespread praise but is not without its sceptics.


Vaccine efficacy


Vaccine efficacy is often used to describe how ‘good’ a vaccine is at preventing infection. However, the use of this terminology is often confusing. A vaccine’s efficacy is determined by comparing the proportion of people who get Covid-19 within a group of people who have had the vaccine and one where they have been given a placebo injection. Consequently, a vaccine that has up to 95% efficacy, as in the case of the Pfizer-BioNTech vaccine (as published by the CDC), means that those with the vaccine have a 95% lower chance of getting the virus than those without. The probability of getting the virus without a vaccine was 1 in 1000 when the trials were done, so 99.95% of the vaccinated population would be virus free.


However, the Johnson & Johnson single-shot vaccine only has a 66% efficacy. Does this mean it carries a greater risk of getting the virus? No, crucially, vaccine efficacy rates are calculated during specific clinical trials, and thus they cannot be fairly compared. This is due to the clinical trials being unique to the conditions within which the study was done. The Johnson & Johnson clinical trials were conducted at much more critical points in the pandemic than the Pfizer ones, so viral exposure was higher, which would have lowered its efficacy.


Although there are cases of people having had the vaccine and still getting sick from covid, these are considered ‘breakthrough’ cases, as they are very rare, and all vaccines confer a significantly lower chance of being hospitalised or dying from the virus.


Increasing vaccine uptake

Despite widespread scientific advice to take vaccines when they are offered, many individuals are vaccine hesitant. It has been theorised that we would need around 70% of the population to be immune to Covid-19 (either through vaccines or exposure) to achieve herd immunity. In England, vaccine uptake is anticipated to reach 94%, which is significantly higher than in many other countries. For example, current estimates expect 60% of the French population to be vaccine-hesitant and has been linked to a lack of trust of public health officials.


In places where vaccine hesitancy is high, behavioural economic strategies are being employed to increase uptake. An example of this is using incentives. On the 3rd of May, the Governor of New Jersey tweeted the launch of their ‘Shot and a Beer’ program where those over 21 who get vaccinated in the state can receive a free beer. The state of Ohio has attracted attention in the last few days after announcing that vaccinated adults in the state could enter a $1 million lottery where five would win. Ohio since has reported a 6% increase in vaccine uptake. Thus, demonstrating how the capacity uncertain rewards have to change behaviour. Regardless of the incentive at play, these campaigns are successfully raising awareness of the need to get vaccinated.


Other behavioural economic techniques include public health awareness campaigns and social influence. An example of a valuable public health campaign comes from Boston Children’s Hospital, which has created an array of informative videos highlighting the differences between the Covid-19 vaccines available and key information on the vaccination process in children. By increasing factual awareness of vaccines, people are less likely to be guided by informational social influence and be deterred from getting the vaccine due to a lack of knowledge. The education of children could further reduce vaccine hesitancy in the future.


Social influence methods to increase take up of the vaccine have used both celebrities, as the NHS is doing, and on people’s circle of family and friends, as with giving people online and in person stickers once they have been vaccinated. To influence behaviour, this relies on the halo effect, where the overall perception of a person impacts how they continue to be perceived by an individual.. For example, if you like an actor, you may also believe they are good at choosing whether or not to be vaccinated and trust their opinion.

Vaccine side effects

Recently, vaccine side effects have been gathering global attention due to some reported cases of blood clots related to having the AstraZeneca vaccine. The base rate of blood clots in the UK is reported to be around 3000 per month. The case rate of blood clots in the AstraZeneca vaccinated population UK is reported to stand at around 1 in 250,000. Initially, the rate of blood clots associated with the vaccine was drastically dramatised by describing it using simple probability rather than using Bayes theorem, which takes the prior probability of blood clots into account. This commonly occurs due to a cognitive bias known as the law of small numbers outlined by behavioural economists Daniel Kahneman and Amos Tversky. The law of small numbers describes a fallacy of over generalising from small samples. We tend to treat conclusions from small samples as if they are from large samples, so we generalise erroneously. For comparison purposes, 1 in 1000 people are predicted to be affected by blood clots from air travel and an estimated 30% of people who test positive from Covid-19 experience thrombocytopenia (a deficiency of blood platelets linked to blood clotting).


Although the risk of blood clots from the vaccine is markedly less than in the general population, the UK government has decided to stop offering the AstraZeneca vaccine to under 30-year-olds as this age group has very such health risks associated with catching Covid-19 so the costs of offering it outweighs the benefits.


Ultimately, all the vaccines on offer statistically reduce the chance of hospitalisation and death from Covid-19. As the risk of side effects is so low there is no rational reason to not have the vaccine. Therefore, employing behavioural science techniques to successfully reduce vaccine hesitancy is the best course of action.