Lay Understanding of Vaccine Efficacy.

Although perceived vaccine efficacy is strongly associated with vaccination behavior, little is known about how well laypeople understand what vaccine efficacy rates mean. Three experiments (N=3402) explored errors in how laypeople process numerical information about vaccine efficacy rates and examined the effects of three tutorial interventions designed to bring lay perceptions in line with normative efficacy calculations. Findings indicate that responses to questions about efficacy rate tend to align with normatively correct calculations, whereas responses to questions about the rate of infections indicate an incorrect calculation whereby the post-vaccination risk of infection is assumed to be the efficacy rate subtracted from 100%. Both quantitative and conceptual tutorial interventions helped participants calculate efficacy rates that were more aligned with the normatively correct values and infection rates that were less aligned with the incorrect values, particularly when participants had higher numeracy levels or responded with numeric inputs rather than choices between options.