Field studies provide evidence of free-riding on influenza herd immunity. For example,
“self-other” optimistic bias about influenza risk was demonstrated in a sample of New Jersey adults where three out of four individuals estimated their risk of infection below average
[27]. Using game theory, it was previously shown that, in the case where individuals know only whether an epidemic takes place every influenza season, free-riding on herd immunity may cause severe drops in the vaccination coverage, leading to major epidemics
[17],
[18]. Furthermore, these major epidemics could be suppressed by a public health program offering free vaccination for a number of years to every individual, every time the individual decided to get vaccinated
[17]. However, other means of epidemic control may be investigated. In a society where vaccines are rarely in short supply, the success of voluntary vaccination programs depends very much on the perceived benefit of vaccination. Mass media is an important factor in shaping social trends and its reach has never been broader. In this work we investigated the potential impact of epidemiological newscasts on the dynamics of voluntary vaccination coverage against seasonal influenza infections.
Our first result addresses the possibility of eliminating influenza epidemics by increasing the vaccination coverage. Recently, the seasonal influenza vaccination coverage has steadily increased in the developed world. The average in five European countries reached 25.9% in 2007–2008
[25], while in the US the coverage reached 41.1% in 2009–2010
[26]. Still, these figures fall short of the critical vaccination coverage, estimated to be in the range of 50–70%; see
[17] and references therein. Will the vaccination trend continue such that the coverage exceeds the critical level year after year, eliminating influenza epidemics? The answer is most likely no. If the coverage approached the critical value, epidemic severity would decline and individuals would no longer perceive the same benefit in vaccination, instead attempting to free-ride on herd immunity. Hence, epidemics would persist. Furthermore, our study suggests that mass media would not be able to change this course of events, no matter what epidemiological information were released to the public to change its perceived benefit of vaccination (Proposition 1).
Our second result shows that newscasts providing the individuals with more epidemiological information (e.g., incidence, coverage, or both) may lead to a stable coverage dynamics, suppressing severe epidemics caused by free-riding of herd immunity (Proposition 2). Hence, mass media could effectively be used for public health just as incentives providing free vaccination
[17]. The practical implications of this result may be profound as newscasts require no compliance from participants and less logistics than implementation of vaccination incentives.
Our third and last result states that the stabilized level of the vaccination coverage follows the trend of the perceived vaccination benefits (Proposition 3). Still, as they learn more about seasonal influenza epidemiology, individuals have two major choices in changing their vaccination strategies. First, they may assign more utility to vaccination, increasing the overall vaccination coverage. Second, they may take even more risk to free-ride on herd immunity, causing a decrease in the vaccination coverage. Which of these two choices would likely be realized is a difficult question. Using reasonable assumptions, our example models and numerics show that the vaccination coverage in the general population could become better or worse when disclosing both vaccination coverage and disease incidence than when disclosing either piece of information separately.
A significant body of literature in cognitive psychology addresses perception biases of clinical risk (see Refs.
[27]–
[29] and references therein); however, applications to influenza are limited. Data to characterize the public perception of the benefits of vaccination and its potential trends when epidemiological information is broadcast are currently insufficient. A comprehensive study
[25] of seasonal influenza vaccination in the general population was conducted in five European countries over seven influenza seasons, identifying reasons invoked pro and contra vaccination. The chief reasons invoked for getting vaccinated were advice from a family doctor (58.6%) and the perception of influenza as a serious illness (51.9%). The two major reasons for not getting vaccinated were the feeling of not being likely to catch influenza (39.5%) and never having considered the option of being vaccinated (35.8%). More field investigations are needed to explicate these reasons in terms of epidemic variables (e.g., prevalence, coverage, etc.) and build realistic models for the score function
. Still, to provide a broad understanding of possible modeling outcomes, here we investigated several examples of how mass media may influence individuals' perceptions of the benefits of vaccination, by reporting infection incidence and vaccination coverage. We note that game theory was particularly suited for our study as it explicitly models risk taking, adaptability and decision making.
In conclusion, the impact of mass-media on social behavior and, implicitly, vaccination coverage is complex. Our game theoretic approach allows for illuminating some of the underlying mechanisms, proposing new perspectives for public health.
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