Vaccine Eagerness: A New Framework to Analyse Preferences in Single Profile Discrete Choice Experiments. Application to HPV Vaccination Decisions among French Adolescents

"Few articles have focused on the adolescent perspective although adolescents' involvement in the vaccine decision-making process could reinforce their vaccine confidence..."

Human papillomavirus (HPV) vaccination uptake among adolescents remains low in many countries, including in France, where complete vaccine coverage (VC) at 16 years reached 32.7% among girls in 2020. The World Health Organization (WHO) recommends using the principles of social marketing to improve communication on vaccination promotion. To better understand which arguments could best motivate HPV vaccine acceptance among French adolescents, this group of researchers previously conducted a single-profile discrete choice experiment (DCE) to elicit preferences of adolescents around HPV vaccine communication, finding that only half of participants made variable choices (non-uniform respondents) from which preferences were elicited. In this paper, they provide a framework for eliciting preferences even among respondents who uniformly accepted (serial demanders) or refused (serial non-demanders) hypothetical vaccination scenarios. They also propose a new concept for DCEs on vaccination, "vaccine eagerness", which combines information of decision and choice certainty to provide additional (and more subtle) information on preferences.

As explained here, DCEs are grounded in economic theory. In standard DCE, respondents are asked to repeatedly choose their preferred option among several alternatives (including opt-out options) described by different characteristics or attributes. Econometric modelling of responses based on multinomial (conditional) logit models allows eliciting preferences and assigning weights of importance to the various characteristics of the alternatives being valued. In single-profile DCEs, participants are asked whether they would accept or refuse a presented hypothetical scenario. Indeed, for vaccination decisions, people are used to choosing whether to get vaccinated, rather than choosing the "best" vaccine among several alternatives. However, a disadvantage of single-profile DCEs is that there is often less information gathered from choices, which are binary by nature. "In single-profile DCEs, some respondents always make the same choice (accept or refuse) across scenarios, which leads to unidentified preferences for those respondents (so-called 'uniform respondents')."

Data were collected among 1,458 adolescent boys and girls from January 31 to March 13 2020 in five middle schools located in three French regions (Grand Est, Pays de la Loire, and Auvergne-Rhône-Alpes). The 15-minute questionnaire included questions on participants' characteristics and opinions on vaccination as well as the DCE tool. Four attributes were selected: (i) the disease against which the vaccine protects (three levels: respiratory disease, cancer in 20 years, genital warts), (ii) vaccine safety statements (four levels: no serious side effects (SSE), absence of scientifically confirmed SSE, no increase in risk of SSE in countries with high VC, presence of a rare SSE but greater benefit), (iii) potential for indirect protection (three levels: the vaccine protects only you, avoids transmission to other people, or helps making the disease disappear), and (iv) mention of VC among peers (four levels: not enough adolescents registered for vaccination, one-third did, most adolescents did, or 80% of teens were vaccinated in neighbouring countries).

Participants were asked to imagine the following hypothetical situation: a vaccination campaign, during which free vaccination would be offered by a physician during an individual consultation, will be organised at school in two weeks. Adolescents' parents have already been informed and asked for consent. Based on a variable set of information provided by the school nurse, participants need to decide whether they would register to get vaccinated during the campaign. They were then asked, after each choice task, to indicate their level of certainty about the choice, on a scale from zero (not at all certain) to ten (absolutely certain). The choice task was designed as a single-profile DCE format with an opt-out. In total, 36 scenarios were used and split in four randomly assigned blocks, leaving nine scenarios per participant.

Serial non-demanders (refusers) represented a small part of the sample (6%) and were less likely to be in favour of vaccination in general. They were also more likely to speak another language at home and be unaware of their parents' education level, which could potentially indicate a lower comprehension of the study or be a proxy for lower socioeconomic status.

The study found that attributes' impact on decision certainty were similar between serial demanders and non-uniform respondents (at least one acceptance and one refusal across scenarios: N = 711): mentioning a positive benefit-risk balance significantly decreased certainty to accept, while information on 80% coverage in other countries and potential for disease elimination increased it. Among serial non-demanders, significant attribute impacts were observed only after exclusion of low-quality responses (N = 31): a potential for disease elimination and 80% coverage in other countries significantly increased certainty of refusing vaccination.

The researchers developed a vaccine eagerness scale (ranging from -10 to 10) by combining information on the binary decision (accept vs. refuse the hypothetical vaccine) and the decision certainty level. They used random effects linear regressions to evaluate attributes' impact on vaccine eagerness. They found that combining decision and certainty into this vaccine eagerness indicator allowed analysing preferences in the full sample, including "hesitant" respondents. That is, it allowed describing sensitivities to particular communication contents even among serial (non) demanders, whose preferences remain unidentified in standard discrete choice models.

Thus, this study revealed a particular pattern of preferences: Communication on the potential to eliminate the disease through vaccination and on high VC in neighbouring countries significantly increased the certainty of refusal. One hypothesis is that the group with high vaccine hesitancy refuses any positive communication, in particular one appealing to social conformism or collective responsibility around vaccination. Statements motivating vaccine acceptance in most adolescents could have the opposite effect. More research is needed to test this hypothesis and see if it could be associated with mistrust towards institutions, which has been linked to vaccine hesitancy. The study's framework of vaccine eagerness could be useful to explore preferences specifically among highly hesitant persons by replicating in other studies using a larger sample size. Qualitative studies following DCEs could help better understand adolescents' motivations toward vaccination, especially for serial non-demanders, which may help tailor immunisation communication in specific subgroups.

In conclusion, this study "advocate[s] for the use of choice certainty scales specifically in single-profile DCEs, to better estimate the preferences of respondents in vaccine acceptance."