How Communicating Vaccine Benefits and Harms in Fact Boxes Affects Risk Perceptions: Two Randomized Trials

"Communicating a vaccine's benefits - that is, its power to effectively prevent disease and severe conditions - via fact boxes can increase the perceived risk of omitting vaccination and decrease the perceived risk of vaccination."

When making medical decisions, such as for or against vaccination, individuals should ideally know the relevant benefits and harms of the behavioural options before making a decision. A promising communication tool is the fact box, which is a compact table summarising the benefits and harms of a health-related intervention and comparing the intervention to a control group. Fact boxes enable an easier comparative assessment of two behavioural options than text-based summaries. However, the impact of fact boxes on people's risk perceptions (e.g., in light of the role of mental shortcuts) deserves more attention. This paper describes two experiments that investigated how different profiles of vaccine benefits and harms influence risk perceptions and the intention to vaccinate.

In both experiments, participants received one out of eight fact boxes about a vaccine against the fictitious disease "dysomeria", caused by the "DS-virus". The fact box displayed probabilities of benefits and harms given a placebo vaccination versus the dysomeria vaccination. The upper-left table area informs participants about the risk of infection and secondary diseases, given exposure to the pathogen while unvaccinated against the disease. The upper-right table area illustrates the vaccine benefits by displaying the reduced probability of contracting the disease. The probabilities of adverse events in the case of a placebo vaccination (lower-left table area) serve as a reference point to evaluate the harms in the case of the dysomeria vaccination (lower-right table area).

In Experiment 1, 430 participants were included in a 4 (benefits: no information, low, high, or perfect vaccine effectiveness) × 2 (harms: low or high probability of vaccine adverse events) between-subjects design. In Experiment 2, 541 participants were included in a 2 (benefits) × 2 (harms) × 2 (comprehension test conducted before or after assessment of risk perceptions to assess whether the participants could find, compare, and perform calculations using the numbers presented in a fact box) between-subjects design. Measures included: perceived risk of vaccination, intention to vaccinate, comprehension, and, in Experiment 2 only, perceived risk of non-vaccination.

The two experiments show that individuals incorporate the presented vaccine fact box content into their risk perception and vaccination intention. Greater benefits decreased the perceived risk of vaccination and increased the intention to vaccinate. More benefits increased the perceived risk of non-vaccination. More harms increased the perceived risk of vaccination and, in Experiment 2 only, decreased the intention to vaccinate. Furthermore, the intention to vaccinate was lower when there was no information on the vaccine benefits than when there was information that the vaccine was 100% effective.

The result that greater benefits decreased the perceived risk of vaccination is an indicator that participants use a mental shortcut, as described by the inverse benefit-risk relationship. This relationship is apparent when a person associates the high effectiveness of a vaccine with lower perceived risks, even though vaccine benefits and risks are distinct and independent events. According to the affect heuristic, providing information about benefits should change the overall affective evaluation and, thereby, the perceived risks. Furthermore, this inverse relationship and the absence of the negativity bias (where negative events exert a greater influence) found in the experiments highlight the importance of communicating the vaccine benefits.

The results for omitting the benefits information and for the risk of non-vaccination further underline the significance of communicating vaccine benefits. When the vaccine benefits were not presented, the perceived risk of vaccination was similar to a vaccine with low effectiveness. Also, the intention to vaccinate was lower in the case of no benefit information compared to the condition of a perfectly effective vaccine. In addition, higher benefits also led to a higher perceived risk of non-vaccination in Experiment 2.

The researchers note that evidence-based information can be rather abstract and lack relevance for people's everyday life decisions and experiences. Further research could test whether combining the fact box with storytelling generates more interest in the information, can enhance comprehension, and help to elaborate key messages without introducing unintended effects.

Importantly, most vaccines have indirect effects in addition to the direct effects of preventing disease (e.g., they thus have a social benefit, too). Presenting this particular benefit can be challenging in the current versions of the fact boxes. Demonstrating the positive benefit for others and the individual may be a useful and effective further development of fact boxes - and depict a more comprehensive picture of the benefits of vaccination.

The researchers conclude: " Taken together, communicating vaccine benefits could lead to more accurate risk perception and greater intention to vaccinate for most vaccines....Readers could benefit from measures that cue deeper processing of the fact box content and can facilitate the comprehension of how the vaccine omission or uptake affects a person without relying on numbers."