Researchers have developed a mathematical model that predicts how infectious disease outbreaks incorporate fear -- both of disease and of vaccines.The model, named as "Triple Contagion" of disease and fears, helps better understand how pandemics can occur in multiple waves of infections, like those we are seeing with Covid-19.Human behaviours like social distancing (which suppresses spread) and vaccine refusal (which promotes it) have shaped the dynamics of epidemics for centuries."Fear of a contagious disease can shift how susceptible individuals behave; they may take action to protect themselves, but abandon those actions prematurely as fear decays," said lead author Joshua Epstein, Professor of epidemiology at NYU School of Global Public Health.For instance, the fear of catching a virus like SARS-CoV-2 can cause healthy people to self-isolate at home or wear masks, suppressing spread.But, because spread is reduced, the fear can evaporate -- leading people to stop isolating or wearing masks too early, when there are still many infected people circulating. This pours fuel -- in the form of susceptible people -- onto the embers, and a new wave explodes.
Likewise, fear of Covid-19 has motivated millions of people to get vaccinated. But as vaccines suppress spread and with it the fear of disease, people may fear the vaccine more than they do the infection and forgo vaccination, again producing disease resurgence.The "Triple Contagion" model couples these psychological dynamics to the disease dynamics, uncovering new behavioural mechanisms for pandemic persistence and successive waves of infection.Moreover, the model recognises that fear is not static: it can spread through a population as a result of misinformation or alarming updates, or fade with time or reassuring news.The model illustrates that the two fears evolve and interact in ways that shape social distancing behaviour, vaccine uptake, and the relaxation of these behaviours. These dynamics, in turn, can amplify or suppress disease transmission, which feeds back to affect behaviour, producing disease resurgence and multiple waves."Our 'Triple Contagion' model draws on the neuroscience of fear learning, extinction, and transmission to reveal new mechanisms for multiple pandemic waves of the sort we see in the current SARS-CoV-2 pandemic and novel ways to think about mitigating its spread," said Erez Hatna, clinical associate professor of epidemiology at NYU School of Global Public Health and a coauthor of the study.The study is published in the Journal of The Royal Society Interface.