Barriers of Influenza Vaccination Intention and Behavior - A Systematic Review of Influenza Vaccine Hesitancy, 2005 - 2016

Despite influenza's severity and the availability of safe vaccines, low influenza vaccine uptake rates within specific risk groups remain a challenge throughout the globe and contribute to the burden of disease. This review aims to: (i) identify relevant studies and extract individual barriers of seasonal and pandemic influenza vaccination for risk groups and the general public; and (ii) map knowledge gaps in understanding influenza vaccine hesitancy - on both the macro- and micro-levels - to derive directions for further research and inform interventions in this area.

There are several levels at which one can analyse vaccine hesitancy.

• On the meso-level, the World Health Organization (WHO)'s Strategic Advisory Group of Experts (SAGE) on Immunization working group proposed in their model that individual/social influences, contextual influences, and vaccine and vaccination-specific issues play a role. For instance, influenza vaccines, as compared to other standard vaccines, have some special characteristics that should be considered when looking at influenza vaccine hesitancy. Namely, vaccine effectiveness varies annually and is frequently low. Vaccination is required annually; in most countries, it is recommended for specific risk groups only, and there are influenza-specific myths (e.g., the flu shot can cause the flu).
• On the micro-level, the model can be further specified and characterised by psychological profiles that refer to psychological theories of health decision-making and behaviour, such as the Theory of Planned Behavior (TPB). These models provide psychological insights that contribute to our understanding of why some individuals get vaccinated while others refuse to do so. On the TPB, the intention is a function of an individual's attitude (negative or positive evaluation of behaviour and outcome), perceived behavioural control (PBC; perceived ability to perform a behaviour), and the subjective norm (perceived social pressure of significant others). TPB can thus predict various health behaviours, such as vaccinating.

This review uses an extended version of TPB as a comprehensive theoretical framework for identifying and clustering barriers to influenza vaccination. As this review aims to integrate all potentially relevant barriers rather than psychological processes alone, it also includes physical, contextual, and sociodemographic aspects in the conceptual framework, which is displayed above (and as Figure 1 in the article).

Thirteen databases covering the areas of medicine, bioscience, psychology, sociology, and public health were searched for peer-reviewed articles published between the years 2005 and 2016. Following the PRISMA approach, 470 articles were selected and analysed for significant barriers to influenza vaccine uptake or intention. The largest proportion of research focused on seasonal influenza (331/470; pandemic influenza 156/470; avian influenza 5/470; unspecified 3/470). Health care personnel (HCP) and the general public were the most studied populations, while parental decisions for children at high risk were underrepresented. Actual vaccine behaviour was the main outcome variable in the majority of the studies (377/470), while the intention to vaccinate against influenza was assessed in 100 of the 470 studies (unspecified 4/470).

Over time, the number of published studies about influenza vaccine hesitancy increased. There are peaks in the numbers of published articles after the avian influenza outbreak 2005, particularly in the Americas and European regions, and after the influenza pandemic 2008/2009 in the Americas, European, and Western Pacific regions. The number of publications from authors from the African, South East Asian, and Eastern Mediterranean regions remained low throughout the entire period.

On a micro-level, with regard to the original model of the TPB, a negative attitude towards vaccines and attitudinal beliefs such as a decreased perceived effectiveness of the vaccine and a lack of trust in health authorities were the most frequently reported barriers. Perceiving that influenza vaccination was not the norm in the relevant peer group was frequently reported as a barrier. Relatively few research articles identified low perceived behavioural control as a significant barrier to influenza vaccine uptake. This might be due to the fact that perceived behavioural control is correlated with actual access, and research articles from regions with low reliability of vaccine supply, such as Africa, were underrepresented. Taking extensions of the TPB (described in the article) into account, frequently reported additional barriers of influenza vaccine uptake were utility evaluations such as worries about the safety of the vaccine, low perceived severity of the disease, and a low perceived risk of the disease. Furthermore, the lacking of recommendations from medical personnel, low frequency of interaction with health services, and missing vaccination habits were frequently reported as barriers. To frame it differently, the major and most consistent enablers for vaccine uptake on the micro-level were a positive attitude towards influenza vaccines, high perceived utility of vaccination, cues to action, and previous influenza vaccinations.

On a macro-level, the researchers view the literature through the lens of the 4C model of vaccine hesitancy (complacency, convenience, confidence, and calculation). The available evidence suggests that confidence, as well as complacency, are major reasons for influenza vaccine hesitancy. Complacency was mostly expressed by low worry, low perceived risk, and severity of the disease. The lack of confidence was expressed by doubts about the safety and effectiveness of the vaccine, as well as a lack of trust in health authorities and, among other knowledge gaps, the belief that the vaccine can cause the flu. Convenience and calculation played only a minor role.

According to the researchers, the 4C model provides a heuristic for selecting and designing appropriate strategies to address and overcome vaccine hesitancy. Combining the results from this systematic review with conceptual frameworks like the 4C model could be instructive. For example, if one aims to increase influenza vaccine uptake within a hospital setting, the results of this review suggest that addressing confidence (by debunking misconceptions and increasing awareness of an ethical and professional obligation to vaccinate) is a promising lever for interventions. It is also shown that structural interventions such as mandatory vaccinations that are suitable to overcome complacency should be treated with care, as negative attitudes towards vaccination are a major barrier and could lead to reactance following structural interventions.

Thinking beyond the present study, the researchers argue that future research needs to address underrepresented regions and populations so that these findings can inform design of appropriate interventions in all WHO regions and for all risk groups of influenza. They also stress that it is not only important to improve who and which region is studied but also what is studied. Though their review may lead to the impression that psychological variables are frequently analysed, the actual measurements are rarely based on psychological theories, and the items used to measure the constructs vary substantially across studies.

Thus, the researchers conclude by suggesting that a more theory-based approach to measuring vaccine hesitancy - not only regarding influenza vaccines - could improve the knowledge base to design effective evidence-informed interventions. This approach to overcoming vaccine hesitancy, they assert, could enhance the effectiveness of vaccine advocacy and reduce the burden of vaccine-preventable diseases.