Conversations with Feng Fu: Untangling the causes of low vaccination compliance

Anyone paying attention to the news these days knows that serious problems exist when it comes to members of the public getting vaccinated. Unfortunately, the problem goes much deeper than the misinformed and misguided anti-vaccine hucksters crying “Fire!” all over the Internet. Dartmouth University’s Feng Fu explains how something called a hysteresis loop can influence public vaccination levels.

SCIENTIFIC INQUIRER: What prompted you to undertake this study?

FENG FU: Addressing vaccine compliance problems is of particular relevance and significance to public health. Especially, measles, mumps, and whooping cough, which are are thought no longer a major threat to our society, make a surprising comeback in recent years. In some regions, such as France, measles even becomes an endemic disease.

Despite the resurgence of vaccine-preventable diseases and public awareness of vaccine importance, why is it so challenging to boost population vaccination coverage to desired levels especially in the wake of declining vaccine uptake? We are intrigued by this puzzling phenomenon and set out to address this problem by combining evolutionary game theory with epidemiology. We had a great time studying this problem, especially the moment we discovered the hysteresis effect, which I believe, is the first time to discover such effect in the public health domain.

SI: What are the advantages of the approach you took in the study?

FF: We use game theory models combined with infectious diseases to shed insight on the vaccine compliance problem. Vaccine compliance can occur in a variety of contexts. One of the most relevant factors is the fear of vaccine adverse effects, such as the infamous MMR-autism controversy. In this work, we looked at this problem from the perspective how vaccine effectiveness and perceived vaccine cost/risk can affect decision-makings of vaccination. We discovered the hysteresis effect can appear as an unforeseen roadblock for the recovery of vaccination rates.

SI: Hysteresis loops are more commonly associated with magnetism. Can you discuss it in terms of public health?

FF: Hysteresis means that “A hysteresis loop causes the impact of a force to be observed even after the force itself has been eliminated.” The occurrence of hysteresis is traditionally associated with magnetic properties of materials, and also has been found in biological and socio-economical systems. It’s why physical objects resist returning to their original state after being acted on by an outside force. It’s why unemployment rates can sometimes remain high in a recovering economy.

In this paper, we showed that bifurcation and hysteresis can occur in the important context of public health and vaccination behavior in particular. The hysteresis loop can be caused by questions related to the risk and effectiveness of vaccines. Negative experiences or perceptions related to vaccination impact the trend of uptake over time, and cause such “vaccination trajectory” to get stuck in the hysteresis loop. In other words, the existence of hysteresis loop makes the population sensitive to changes in factors that drives vaccination behavior such as cost and effectiveness. The presence of hysteresis also makes it hard to recover to previous states, as different path is followed. Thus, our results have practical implications for increasing vaccine compliance, particularly through means of overcoming the hysteresis effect.

SI: Describe the model you used for your study.

FF: Vaccines may be imperfect in the sense that (i) there can exist unwanted, adverse side effects of various degrees, albeit being minor most of the time, causing exaggerated perceived risk or cost of vaccination, and (ii) vaccination only can confer partial protection against the disease. Simply because vaccine is not perfect, rumors or myths about vaccine can fuel the fear of vaccination and thereby lead to vaccine refusals. In this work, we studied how social imitation dynamics of vaccination can be impacted by the presence of imperfect vaccine, which only confers partial protection against the disease. Besides weighing the perceived cost of vaccination with the risk of infection, the effectiveness of vaccination is also an important factor driving vaccination decisions. We used game theory models combined with infectious diseases to shed insight into vaccine compliance problem.

SI: What did you discover?

FF: In this work, we show that social imitation dynamics of vaccination can exhibit hysteresis, namely, the dependence of population vaccination rate on its past trajectory. The occurrence of hysteresis loop hinders the recovery of vaccine uptake, in spite of decrease in the perceived cost of vaccination or improvement of vaccine efficacy. Our work offers a novel mechanistic explanation for vaccine compliance problems. Such hysteresis effect can be a roadblock for efforts to recover vaccination rates. We think our theoretical results offer a novel dynamical systems-based insight into understanding human health behavior.

SI: What is the bistability in vaccination dynamics?

FF: There can exist multiple stable equilibrium vaccination levels if a vaccine is imperfect and cannot provide sufficient protection against contracting the disease. In other words, the population can settle either at a high vaccination level or possibly at a low vaccination level, depending on the initial state. The existence of bistability causes the hysteresis effect where the population can get stuck at low vaccination levels.

SI: In one of your figures, you have a Region R0>1/(1-E) and say that “In spite of elevated vaccine take up, the effective vaccination coverage actually reduces and thus results in larger epidemic sizes.” Can you discuss this?

FF: Sure, this phenomenon is counterintuitive and we can explain it using the phrase: “the number is traded for efficiency”. In this scenario, as the risk of infection for unvaccinated is substantially higher than that for vaccinated even in the presence of comprised vaccination effectiveness, choosing to vaccinate still leads to a better prospect of payoff than not to, thereby causing the population to be fully vaccinated in equilibrium. It is worth noting that, in spite of such elevated vaccine take-up, the effective vaccination coverage actually reduces and thus results in larger epidemic sizes.

SI: What was the specific role of hysteresis your study discovered?

FF: One important insight arising from our present study is that in order to improve vaccine compliance, we will have to overcome hysteresis effect to reduce the resistance and reluctance to vaccination. For example, vaccination campaign should promote vaccination as an altruistic behavior that is desired for societal benefit (vaccination not only protects oneself but also others — including family, friends, and strangers — through the notion “herd immunity”).

SI: What are the implications of your findings in terms of public health and its applications?

FF: The identification of the hysteresis loop is a powerful finding that will help public health officials better manage vaccination campaigns. For example, the modelling result that shows that vaccines need to reach a certain level of efficacy to increase uptake should tell officials to focus on two things: (1) develop sufficiently efficacious vaccine and, importantly, (2) make people aware of this efficacy once it has been achieved.

Moreover, fundamentally, vaccine compliance is a kind of social dilemma where mechanisms for promoting human cooperation, such as reputation effect and rewarding, can be leveraged to overcome such hysteresis. Therefore, aside from the public health prevention perspective, vaccination campaignS should promote vaccination as an altruistic behavior that is desired for societal benefit (vaccination not only protects oneself but also others — including family, friends, and strangers — through the notion “herd immunity”).

SI: What is next for you in terms of research?

FF: We are currently working on exploring effective ways of harnessing the social contagion of vaccine knowledge and positive attitudes towards vaccination in order to overcome this previously unforeseen hysteresis effect.

For more information about Feng Fu and his research, visit the Fu Lab Page.

IMAGE SOURCE: Creative Commons

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