The World Health Organization (WHO) named vaccine hesitancy one of the top 10 threats to global health in 2019. “Vaccine hesitancy – the reluctance or refusal to vaccinate despite the availability of vaccines – threatens to reverse progress made in tackling vaccine-preventable diseases,” the public health arm of the United Nations wrote.1

Although the vast majority of parents vaccinate their children according to the immunization schedule recommended by the Centers for Disease Control and Prevention, the percentage of children who have not undergone any of the 14 recommended vaccinations has more than quadrupled from 0.3% to 1.3% since 2001.2

WHO points to a “3Cs” model to explain why people refuse to vaccinate. The 3 Cs stand for complacency, convenience, and confidence. Complacency occurs when the perceived risks of disease are low and vaccination is deemed unnecessary. Convenience is measured by the extent to which availability, affordability, willingness to pay, health literacy, and other potential obstacles prevent vaccination. Confidence refers to distrust in the safety and efficacy of the vaccines, the system that delivers them, and the motivations of policymakers who decide which vaccines are necessary.3

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In a study published in Proceedings of the Royal Society B, Dartmouth College researchers explained that hysteresis may describe persistent vaccine skepticism.4 Hysteresis describes how a system is dependent on its history and how the impacts of a force can persist well after the initial force has been altered or eliminated. This phenomenon can be seen in many physical systems but also in society. In the context of vaccines, it prevents an increase in vaccination levels even after negative objections have been debunked. As a result, society is more vulnerable to outbreaks of disease.

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“Once people question the safety or effectiveness of a vaccine, it can be very difficult to get them to move beyond those negative associations,” noted Feng Fu, an assistant professor of mathematics at Dartmouth College who led the research. “Hysteresis is a powerful force that is difficult to break at a societal level.”5

“This study shows why it is so hard to reverse low or declining vaccine levels,” added Xingru Chen, a graduate student at Dartmouth College who co-authored the article. “The sheer force of factual, logical arguments around public health issues is just not enough to overcome hysteresis and human behavior.”5

The researchers hope that by identifying hysteresis effect in vaccination, public health officials can create campaigns to increase voluntary vaccination rates, specifically by promoting it as an altruistic and socially desirable behavior.


  1. Ten threats to global health in 2019. World Health Organization. Accessed January 18, 2019.
  2. Hill HA, Elam-Evans LD, Yankey D, et al. Vaccination coverage among children aged 19-35 months – United States, 2017. Morb Mortal Wkly Rep. 2018;67(40):1123-1128.
  3. Report of the SAGE working group on vaccine hesitancy. World Health Organization. November 12, 2014. Accessed January 18, 2019.
  4. Chen X, Fu F. Imperfect vaccine and hysteresis. Proc Biol Sci. 2019;286(1894):20182406.
  5. Research explains public resistance to vaccination [press release]. Hanover, NH; Dartmouth College; January 9, 2019.