3.2.3.1 Type of reports included
I included all published articles reporting original data which provided sufficient information to calculate bivariate effect sizes (e.g. frequencies, means and standard errors). I excluded studies reporting only multivariate associations, as these analyses may understate the true relation between vaccination behaviour and the assessed measures. However, attempts were made to contact the authors of such studies to request the additional information needed (Honkanen P, Keistinen, et al., 1996; Nexøe J, Kragstrup J, et al., 1999; Tsutsui Y, Benzion U, et al., 2012). I also excluded reports which aggregated “I do not know” and yes or no responses because this strategy can bias results, and studies assessing the acceptability of new routes of administration (e.g. dermal), since novel products fall outside the remit of this thesis. If multiple published reports used the same sample, I included only the one which provided the most detailed information on the assessed measures.
3.2.3.2 Type of participants included
I included relevant articles reporting on adult influenza vaccination uptake. Yet, for the same reasons described in Chapter 2, I excluded studies which focused or included pregnant women or healthcare professionals.
52 3.2.3.3 Type of outcome measures included
The outcome measure was vaccination uptake. Although actual vaccination (e.g. extracted from medical records) is recognised as the most accurate measure of vaccination, for practical reasons, the vast majority of studies in this area use self-reported vaccination as the outcome measure. Since actual and self-reported influenza vaccination are strongly correlated (Irving, Donahue et al., 2009), the latter was acceptable as a reliable outcome measure.
3.2.3.4 Type of survey measures included
3.2.3.4.1 Influenza risk perception
I included three distinct sub-dimensions of risk: perceived likelihood of getting influenza, perceived susceptibility to influenza and perceived severity of influenza, as defined by Brewer et al. (Brewer, Chapman et al., 2007). Perceived likelihood is the perceived probability of harm conditioned on not taking action to prevent it (i.e. not getting the influenza vaccine). It is illustrated by the question “If you do not get the influenza vaccine, what is the likelihood that you will get the influenza this year?” Although it is often used as a synonym of likelihood, perceived susceptibility, is hereby understood as perceived constitutional vulnerability to harm. It is represented by the question “if you got influenza, would you feel sicker than people your age?” The third construct, perceived severity, refers to the extent of harm a disease could cause. This construct can be captured with the question “If you were to get influenza, how severe would it be?”
Influenza risk measures were acceptable if they captured the individual’s own perceived risk (e.g. “Influenza could make me severely ill”) rather than general risk (e.g. “Influenza is a serious disease”), as the latter may be interpreted as knowledge and misrepresent actual perceived risk. Likelihood measures were required to condition the perception on not having received the influenza vaccine (e.g. “Without the influenza vaccine, I would get influenza this winter”). Susceptibility measures were included only if they specifically addressed the individual’s intrinsic vulnerability (e.g. “If I got influenza, I would feel sicker than other people my age”).
3.2.3.4.2 Vaccine risk perception: perceived side-effects
The perceived harm of influenza vaccines is commonly addressed in the literature as side-effects and regarded as one of the most important barriers to vaccination. However, there is important variance in the measures used to assess perceived side-effects. Some measures capture general attitudes toward side-effects (e.g. “Are you concerned about the side-effects from the influenza vaccine?”),
53 whilst others focus on specific types of side-effect or perceived safety issues (e.g. “People can get influenza from the influenza vaccine”). I believe it is important to unpack this construct to uncover which aspects generate the most anxiety.
Due to the characteristics of the available evidence, the inclusion criteria for side-effects measures were less strict than for influenza risk perception measures (e.g. they did not have to concern the individual). Included measures were categorised as follows: perceived general side-effects (e.g. “I am concerned about influenza vaccine’s side-effects”), perceived post-vaccination illness (e.g. “The influenza vaccine makes you ill”), perceived likelihood of side-effects (e.g. “It is common to experience side-effects after having the influenza vaccine”), previous side-effects (“I have
experienced side-effects after having the influenza vaccine), perceived severity of side-effects (e.g. “Side-effects from the influenza vaccine can be serious”), perceived vaccine safety (e.g. “Some of the contents from the influenza vaccine could be harmful” or “The influenza vaccine gives you
influenza”) and perceived pain (e.g. “The influenza vaccine can be painful”).
3.2.3.4.3 Perceived vaccine effectiveness
Perceived vaccine effectiveness is understood as the perceived ability of a vaccine to prevent one or more diseases. Since the effectiveness of the influenza vaccine is partial and varies depending on individuals characteristics such as age and health, and how well the influenza virus strains match the vaccine’s in any given year, this construct is particularly relevant when assessing the perceived utility of the vaccine. As with side-effects, the criteria applied to perceived effectiveness measures were more relaxed. For example, the terms efficacy (how well a treatment works in clinical trials) and effectiveness (how well a treatment works in practice) were often used interchangeably. Thus, I included studies which addressed both perceived vaccine effectiveness (e.g. “The influenza vaccine protects people from catching influenza”) and efficacy (e.g. “The influenza vaccine is very
efficacious”), regardless of the specificity of the measure. Measures which assessed the vaccine’s ability to ameliorate influenza symptoms were excluded, as this is a different construct.
3.2.3.4.4 Physician recommendation
Influenza vaccination is usually prompted by the recommendation of a physician. The existing evidence suggests that a physician recommendation is the single most important predictor of influenza vaccination uptake. Most studies assessing physician recommendation use straightforward measures (e.g. “My doctor recommended the influenza vaccine”). However, some studies use ambiguous statements such as “I discussed the influenza vaccine with my doctor”. I excluded such
54 measures because discussing the pros and cons of the influenza vaccine with a doctor does not necessarily result in it being recommended.