Effects of Side-Effect Risk Framing Strategies on COVID-19 Vaccine Intentions in the United States and the United Kingdom: A Randomized Controlled Trial

Vaccination rates have stagnated in the United States and the United Kingdom leading to the continuing spread of COVID-19. Fear and concern over vaccine side-effects is one of the main drivers of hesitancy. Drawing from behavioral science and health communication theory, we conducted a randomized controlled trial among 8998 adults to determine whether the way COVID-19 vaccine side-effects are framed and presented to individuals can influence their willingness to take a vaccine. We presented participants information on a hypothetical future COVID-19 vaccine -- including information on its side-effect rate -- and then examined the effect of three side-effect framing strategies on individuals stated willingness to take this vaccine: adding a qualitative risk label next to the numerical risk, adding comparison risks, and for those presented with comparisons, framing the comparison in relative rather than absolute terms. Based on a pre-registered and published analysis plan, we found that adding a simple descriptive risk label ("very low risk") next to the numerical side-effect increased participants willingness to take the COVID-19 vaccine by 3.0 percentage points (p = 0.003). Providing a comparison to motor vehicle mortality increased COVID-19 vaccine willingness by 2.4 percentage points (p = 0.051). These effects were independent and additive: participants that received both a qualitative risk label and comparison to motor-vehicle mortality were 6.1 percentage points (p < 0.001) more likely to report willingness to take a vaccine compared to those who did not receive a label or comparison. Taken together, our results reveal that despite increasingly strong vaccination hesitancy and exposure to large amounts of vaccine misinformation, low-cost side-effect framing strategies can meaningfully affect vaccination intentions at a population level.