"I Was Right about Vaccination": Confirmation Bias and Health Literacy in Online Health Information Seeking

It has been shown that parents who exempt their children from vaccinations are more likely to have obtained information from the internet compared to parents who choose to vaccinate their children. Online health information (OHI) seeking is easy, but it may also facilitate confirmation bias by means of selective exposure to information that confirms one's existing beliefs and a biased evaluation of this information. This Dutch study tests whether biased selection and biased evaluation of OHI occur in the context of early childhood vaccination and whether people's health literacy (HL) level either prevents or facilitates these processes. The hope is that insights into the mechanisms underlying the relationship between OHI seeking, existing beliefs, and HL could serve as a basis for the development of effective communication strategies to prevent immunisation rates from decreasing any further than they already are in the Netherlands and some other countries.

In September 2017, vaccination beliefs of 480 parents of children aged 0-4 years of age were measured with an online survey, after which they were exposed to a list of 10 vaccine-related message headers, 5 of which were clearly negative and 5 of which were clearly positive. People were asked to select those headers that interested them most. They were also asked to evaluate 2 texts that discussed vaccination positively and negatively for credibility, usefulness, and convincingness.

HL was measured with the Dutch version of the Newest Vital Sign (NVS). This validated measure shows people the nutrition label of an ice cream container, followed by 6 open-ended questions that require interpretation and calculation skills related to the nutrition label. The researchers also assessed functional HL (e.g., "In reading instructions or leaflets regarding your child's vaccines, how often do you need a long time to read and understand the text?"), communicative HL (e.g., "Since your child was born, how often have you collected information on vaccination from various sources?", and critical HL ("Since your child has needed vaccines, how often have you considered the credibility of the information about vaccines?").

Over 80% of the participants sought vaccine-related OHI a few times per year or less. A quarter of the participants reported having never sought vaccine-related information online. A large majority (87%) of the participants had vaccinated their children according to the Dutch National Immunization Program.

It was found that:

• Vaccination beliefs significantly predicted the selection of messages: People holding positive beliefs towards vaccination selected more positive messages, whereas people holding negative beliefs selected more negative messages. Furthermore, confirmation bias in message selection was stronger among parents with higher HL compared to parents with lower HL. People with higher HL selected more messages that were in line with their own beliefs, and they did this regardless of whether they were in favour of or strongly against vaccination.
• This confirmation bias was found not only with respect to message selection but also with respect to message evaluation. People consistently perceived belief-confirming information as being more credible, useful, and convincing. The researchers also found a stronger confirmation bias with respect to message evaluation among people with higher HL, but this mainly applied to people who had positive vaccination beliefs. The HL differences in message evaluation on the positive side of the belief spectrum could be explained by the fact that the participants in the study were generally quite positive about early childhood vaccination.
• People with higher HL and negative beliefs towards early childhood vaccination perceive negative information as more convincing than positive information. This could possibly be explained by the discourse that is frequently used on anti-vaccination websites. Vaccine-critical websites generally promote alternative medicine, encourage parental autonomy and responsibility, and openly distrust medical expertise. These are complex themes, which especially health-literate parents could be attracted to.
• The findings of this study, using the performance-based NVS as the HL measure, are comparable to outcomes provided by the self-reported functional HL scale. Communicative and critical HL, in contrast, were positively associated with the preference of anti-vaccination messages. The formulation of the items for the communicative and critical HL scales possibly fits people who are skeptical towards vaccination better and therefore might not only measure HL but also vaccination hesitancy.

One implication of the findings is that under some conditions (i.e., deviating beliefs and OHI seeking), higher HL can be problematic. "If higher HL also means that people feel more certain and knowledgeable about certain health-related topics and therefore become less open to information that challenges their own ideas, this could impact health communication effectiveness. Future research is needed to investigate whether people with low and high HL differ in attitude strength towards certain health topics and whether one group is more open to attitude change than another."