Vaccine hesitancy and monetary incentives

The study addresses how to increase Covid-19 vaccination uptake to achieve herd immunity, given epidemiological estimates that 75–90% coverage may be required. Despite vaccine availability, hesitancy leaves a substantial gap from required thresholds, exacerbated by variant spread, heterogeneity in uptake, unvaccinated children, and less-than-perfect vaccine efficacy. The research asks whether specific informational messages (free cost, high efficacy, no side effects) and/or monetary incentives can effectively reduce hesitancy. The authors hypothesize that while salient information may modestly increase uptake, monetary incentives could further improve vaccination rates without backfiring via inferences of low quality or crowding out altruism. They conduct a randomized controlled survey experiment to measure baseline uptake, the marginal effects of information, and the willingness to accept (WTA) a vaccine among those initially refusing, to estimate how incentives could increase overall uptake and to identify segments among hesitants.

Prior work shows mixed success of strategies to increase vaccine uptake, including influenza vaccination campaigns with informational and monetary approaches (Betsch et al., 2015; Horne et al., 2015). For Covid-19, informational strategies can modestly increase acceptance but likely not enough to reach herd immunity (Moehring et al., 2021; Wilf-Miron et al., 2021). Economic theory suggests incentives should increase demand, yet concerns exist that payments could signal low quality or undermine prosocial motives (Loewenstein and Cryder, 2020; Largent and Miller, 2021; Lacetera and Macis, 2010; Nyhan and Reifler, 2015). Epidemiological analyses emphasize the need for high coverage and targeted approaches due to heterogeneity (Anderson et al., 2020; Piraveenan et al., 2021). Policy proposals have advocated payments (Delaney, 2020; Litan, 2020). This study contributes empirical evidence on how monetary incentives, alongside salient informational frames, influence Covid-19 vaccine acceptance, and quantifies WTP/WTA.

Design: Online randomized controlled experiment via Amazon Mechanical Turk conducted in the U.S. during the last week of November and first week of December 2020. Participants received $0.50 for a 4-minute study. Of 2500 contacted, 2461 completed the survey after excluding 39 (incomplete or failed attention check). Sampling targeted approximately 500 per arm to ensure at least 150 "No" responses per arm for within-condition analysis. Arms: Four conditions: (1) Control: "Will you get the coronavirus vaccine when it becomes available?" (2) Free: emphasizes zero cost ("If coronavirus vaccines were provided for free...") (3) High efficacy: emphasizes high efficacy (4) No side effects: emphasizes little to no side effects. Assignment was random. Outcomes and elicitation: Primary outcome was binary uptake (forced Yes/No). For Yes respondents, elicited willingness to pay (WTP). For No respondents (Hesitants), elicited willingness to accept (WTA) to vaccinate, with the option to select "None" (no amount would suffice). Monetary entries were constrained between $0 and $1000. Respondents also rated 12 validated statements on 7-point Likert scales: six vaccine hesitancy antecedents (Betsch et al., 2018), three CDC health behaviors (masking, distancing, hand hygiene), and three institutional trust items (OECD, 2019). To assess hypothetical bias, respondents rated certainty for their WTP/WTA (Champ et al., 2009; Ready et al., 2010). Statistical analysis: Compared uptake across arms with significance tests (p<0.05 threshold). Analyzed WTA distributions by condition (quartiles, means, and fraction selecting "no amount"), and constructed probability density functions for WTA by arm. Estimated classification models (logit) for uptake predictors and for segmentation of Hesitants into Reluctants (WTA ≤ $1000) vs Unwillings (no amount). Demographic covariates included age, income, political ideology; behavioral and attitudinal covariates included vaccine safety/effectiveness beliefs and institutional trust. Conducted k-means clustering (elbow and silhouette methods) on demographics and behaviors to explore natural segmentation among Hesitants. Computed cumulative uptake curves as a function of monetary incentive by summing initial Yes and Hesitants with WTA less than or equal to the incentive.

• Baseline and informational effects: Control uptake ~70%. The Efficacy and No Side Effects messages increased uptake by about 4.5–5.0 percentage points to approximately 74.5–75.0% (p<0.05). The Free message did not increase uptake (observed 68.3%).
• Monetary incentives: Monetary incentives substantially increased acceptance beyond informational effects. A $500 incentive increased acceptance to roughly 76–81% depending on condition (e.g., Control 80.2%; Efficacy 78.9%; No Side Effects 80.9%; Free 76.1%). A $1000 incentive raised uptake to as high as 86.9% (No Side Effects condition), with Control reaching 86.2%. Averaged across conditions, $1000 yielded approximately 85.6% acceptance.
• Segmentation among Hesitants: Of 690 No respondents, 355 (51.45%) selected that no amount of money would incentivize vaccination (Unwillings). The remainder (Reluctants) reported positive WTA up to $1000. Unwillings were more likely to believe Covid-19 is not severe, have lower trust in public health institutions, and be older than Reluctants. Income negatively predicted willingness to accept payment among Hesitants. Everyday hassles ("stresses prevent vaccination") positively predicted being Reluctant, suggesting convenience interventions could help.
• Free condition effects: The Free message increased the fraction selecting "no amount" (16.6%) versus Control (13.8%, p<0.10) and shifted some Hesitants to higher WTA (pdf bump near $500), consistent with possible inferences of lower quality or safety when highlighted as free.
• Predictors of uptake: Political conservatism strongly predicted refusal across conditions. Age positively predicted uptake in Control and Free but not in Efficacy or No Side Effects arms. Trust in public health and belief that vaccines are safe/effective predicted acceptance in some models.
• Clustering: K-means favored two clusters among Hesitants in all arms, with one cluster enriched for Unwillings (more "no amount" selections) and the other reflecting Reluctants (lower WTAs). Indirect clustering supported the direct segmentation into two groups with differing responsiveness to incentives.
• Alternative "soft" approach: Requiring vaccination for access to large events garnered limited agreement among Hesitants, suggesting modest impact relative to monetary incentives.
• Overall: Combining effective informational frames with monetary incentives up to $1000 can increase projected uptake to approximately 86–87%, potentially sufficient to approach herd immunity thresholds in the U.S.

The study demonstrates that while salient information about high efficacy and minimal side effects modestly increases Covid-19 vaccine acceptance, monetary incentives produce substantially larger gains in uptake without clear evidence of backfiring. The results directly address the research question by quantifying how much incentives can move Hesitants and by identifying two distinct subgroups: Reluctants who respond to incentives and Unwillings who largely do not. This segmentation implies that blanket informational or incentive campaigns will have diminishing returns; targeted strategies focusing on Reluctants and on reducing convenience barriers (e.g., on-site vaccination) are likely to be more cost-effective. The finding that highlighting "free" may inadvertently increase skepticism underscores the importance of message framing. Political ideology and institutional trust remain key determinants, indicating that non-monetary strategies to build trust could complement incentives. Overall, incentive levels up to $1000 could raise acceptance to around 86–87%, improving prospects for reaching herd immunity when combined with other measures.

This work is among the first to elicit both WTP and WTA for Covid-19 vaccination and to quantify the impact of combined informational and monetary strategies. The authors identify a meaningful split among Hesitants—Reluctants versus Unwillings—and show that targeting Reluctants with incentives up to $1000, along with credible efficacy/safety messaging and convenience enhancements, could raise uptake to levels consistent with herd immunity thresholds. Policymakers should prioritize targeted, data-driven incentive programs (e.g., focusing on high-hesitancy geographies) and trust-building communications. Future research should examine real-world behavioral responses to implemented incentive programs, assess longer-term durability of acceptance, explore optimal incentive sizes and forms (cash vs. tax credits), and evaluate combined "carrot" and permissible "stick" policies in diverse settings.

• Hypothetical valuations: WTP/WTA were self-reported and hypothetical, though certainty checks did not indicate systematic bias by certainty level.
• Sample and generalizability: MTurk sample, while geographically and socio-demographically diverse, differs marginally from the U.S. population (wealthier; underrepresentation of Hispanic and Black respondents), potentially affecting generalizability.
• Incentive range: Monetary options capped at $1000; true WTAs may exceed this ceiling for some individuals.
• Temporal and contextual factors: Data collected in late 2020; evolving pandemic conditions, vaccine perceptions, and policies may change responsiveness.
• Focus on "carrot" strategies: The study did not experimentally test punitive or mandate-based approaches; a "soft" passport-like prompt showed limited stated impact.
• Messaging scope: Only three informational frames were tested; other messages (e.g., endorsements, social norms) were not evaluated within this design.