Despite the proven efficacy of vaccines in reducing mortality and morbidity from vaccine-preventable diseases, vaccination rates have been declining for years in many areas of the world even before the coronavirus pandemic [1]. This has led to the resurgence of some diseases that were largely controlled or eradicated. Refusal to be vaccinated has been linked, in some countries, to outbreaks of pertussis, measles, and other vaccine-preventable diseases [2, 3]. Likewise, in a review carried out with data from 33 countries, it was observed that the Covid-19 vaccination acceptance rates worldwide were very different. In some countries, such as the United States, Russia, Poland, Italy and France, the acceptance rates were less than 60%, and in countries such as Kuwait or Jordan they were less than 30% [4]. In Spain, until May 2022, the percentage of people who have received the full schedule of the Covid-19 vaccine is 92.6% [5]. But vaccine hesitancy is an emerging public health problem in Spain, which is slowing down the process of eliminating measles and other diseases [6,7,8].
Although the decision to be vaccinated or not is individual, it is influenced by the historical, political and sociocultural context of the reference country where the vaccination is carried out [9]. The media, as well as social networks, have great influence on a significant part of the population. In a recent study carried out in Spain, the coverage that the printed media carried out on vaccines between 2012 and 2017. It was observed that the tone of most articles changed from negative in 2012 to positive and neutral until 2017. It was also found that the fewer articles with a negative tone, the higher the vaccination rates [10]. Another study also carried out in Spain analysed the influence of anti-vaccine groups through social networks. The objective was to study the ideology of these groups using discourse analysis, in order to prepare responses based on scientific evidence. The results showed that the speeches of these anti-vaccine groups refer to aspects related to the safety and effectiveness of vaccines. They also strongly emphasize the importance of people’s values and beliefs, and that each person should freely choose whether or not to vaccinate. Likewise, it was observed that the argument that stands out the most is the distrust in health personnel and in official sources of information, which are governments and pharmaceutical companies [11]. In a more recent study, the Covid-19 anti-vaccination messages published on Twitter during December 2020 were analysed to find the key elements in their communication strategy. The results again raised arguments about the safety of vaccines, including viewing the vaccine as a means of manipulating the human genetic code [12]. Therefore, it is important to understand the attitudes that lead a person to refuse vaccination in order to develop more effective public health campaigns.
Regarding the instruments used to assess attitudes towards vaccination, there are several scales designed to assess the attitudes of parents towards vaccinating their children. For example, the Attitudes and Behaviours Regarding Vaccination Decisions [13], the Parent Attitudes about Childhood Vaccines Survey [14] and the Vaccine Hesitancy Scale [15]. Recently, this scale has been modified to also assess the attitude of parents towards specific vaccines, such as influenza and human papillomavirus [16], and for the assessment of attitudes towards vaccination against coronavirus in people with acquired immunodeficiency virus (HIV) [17]. There are also other scales designed to assess attitudes towards specific vaccines in adults, such us the HIV Vaccine Attitudes Scale [18], a variation of the Vaccination Hesitancy Scale adapted to assess adult attitudes toward influenza and coronavirus vaccination in the United States and China [19], and more recently, the COVID-Vaccination Attitude Scale [20].
The only existing instrument used to assess attitudes toward vaccines, without focusing on a specific population or vaccine, is the Vaccination Attitudes Examination (VAX) scale [21]. Although attitudes towards vaccination may differ depending on the type of vaccine, recent studies have shown that the reasons are often similar, including being against vaccine, including coronavirus vaccines, lack of confidence, concerns about their safety, doubts about the origin of vaccines, and vaccines safety due to its rapid development [22, 23]. Therefore, a single measure may be the most efficient way to identify people with vaccine concerns.
VAX adaptations have been carried out in several countries. In the United Kingdom the psychometric properties of the original version of the scale have been studied, with good results [24]. It has also been adapted in Turkey [25] and Romania [26], and its psychometric properties have been studied in a Spanish sample [27], but of these three countries, only the Turkish version performs a back-translation of the items, as advised by the International Test Commission [28]. In all cases, construct validity was studied using Exploratory Factor Analysis (EFA) and/or Confirmatory Factor Analysis (CFA), obtaining the best solution with four related factors. In some countries, convergent validity has also been studied with measures of susceptibility to possible adverse effects of the medication, with current health or with medical mistrust. The predictive validity of the VAX has been studied in the United States and the United Kingdom asking the participants about their intention to vaccinate themselves or their children, if they had been vaccinated against the flu the previous year, or if they would be vaccinated next year.
Regarding the study carried out in Spain, in addition to not carrying out the back-translation, the analyses of the study lead to uncertain conclusions. In addition to performing an EFA that offers a one-factor solution, the authors perform a CFA to study whether the one-factor structure or the four-related factor structure is better. In addition, Cronbach’s alpha coefficient is offered, instead of the recommended McDonald’s omega coefficient [29] for rating scales, or the equivalent Composite Reliability index [30, 31]. These are the most appropriate reliability indicators when factor loadings of a CFA are used. Furthermore, after confirming that the four-factor model fits better, the authors carry out a convergent and concurrent validity study. For concurrent validity, measures similar to those of previous studies are used, and for concurrent validity, the intention to be vaccinated against COVID-19 when the vaccine is available is used. However, in both cases, Pearson correlations are calculated with the total VAX score, instead of using the total score in each of the four factors, which, given the results of the construct validity study, is inadequate.
On the other hand, measurement invariance has not been studied in any country, but it is important to check the existence of measurement invariance to be able to make comparisons between groups. Furthermore, in all convergent and concurrent validity analyses, correlations between total scores or regressions have been used. For all these reasons, the main objective of this study has been to adapt the Vaccination Examination Attitudes (VAX) scale in a Spanish sample, larger than those used in all previous studies, and study its psychometric properties (not only construct validity) and measurement invariance using structural equation modelling, which offers less measurement error than the calculation of Pearson correlations with total observed scores.