The public health literature has long studied the drivers of vaccination choices. The coronavirus pandemic has made the reasons why people refuse vaccination a central question for policy-making (e.g. Blanchard-Rohner et al. 2021, Costa-i-Font 2021, Campos-Mercade et al. 2021 ). Economic reasoning explains the decision to get vaccinated as a rational choice between two perceived risks: contracting the disease and experiencing the side effects of the vaccine (e.g., Binder and Nuscheler 2017, Bohm et al. 2016, Chang et al. 2021) .
Empirical studies have shown that media coverage of vaccine safety controversies significantly encourages vaccine refusal, likely because it leads people to overestimate the incidence and severity of adverse events. For example, reports of the MMR-autism controversy have led to a decline in vaccination uptake, with negative spillovers to other vaccines (Carrieri et al. 2019, Chang 2017).
New study on the COVID vaccination campaign in Italy
But what happens when doubts about the safety of vaccines emanate from health authorities? In a new paper (Deiana et al. 2022), we exploit a quasi-experiment resulting from the suspension of the vaccine ChAdOx1-S – originally called AstraZeneca and later renamed Vaxzevria (VA) – to study the drivers of vaccine hesitancy. vaccination (defined as the refusal or delay of vaccination despite the availability of vaccination services).
In Italy, the National Medicines Agency (AIFA) halted inoculations of a VA batch on March 11, 2021, following reports of very rare thrombotic thrombocytopenia in vaccine recipients. On March 15, AIFA suspended all VA vaccinations for four days to review the potential magnitude and severity of adverse events while allowing Pfizer-Biontech (PB) injections to continue. We take advantage of the quasi-experimental framework, which stems from the decision of the health authorities, to explore the potential drivers of vaccine hesitancy. Exploiting a difference-in-differences design in which the VA vaccine is the treatment group and the PB vaccine serves as the control group, we begin by quantifying vaccine hesitancy in Italy during the first stage of the campaign. of vaccination, then we explore the potential factors of refusal of vaccination.
Figure 1 illustrates the total daily number of doses administered in Italy per 100,000 inhabitants. Solid red and blue lines depict trends for VA and PB injections, respectively. The green line represents the trend of daily online searches for the word “thrombosis”, which peaked during the suspension week. Inoculation rates for the two vaccines followed parallel patterns until the suspension of the first batch of AVs on March 11. During the week of the suspension, when centers did not administer the vaccine for four days, the VA vaccination rate decreased by approximately 46 daily doses per 100,000 population. The resulting value was approximately 60% lower than the level that would have resulted from the pre-event model.
Figure 1 Vaccines administered and searches for “thrombosis”
After the campaign resumed on March 19, the daily rate of VA inoculations decreased further. The authors observed a reduction of 63 daily doses per 100,000 inhabitants, implying a 55% lower vaccination rate. Two weeks later, the negative effect persists, resulting in a decrease of 86 daily doses per 100,000 inhabitants, corresponding to a lower level of 58%.
Balancing two perceived risks
To dig deeper into possible causes for the hesitation, we tested whether the response to AV suspension varied with outbreak severity and Google searched for “thrombosis” in Italian regions. The maps below show the geographic distribution of search intensity for the term “thrombosis” from March 11 to 18, 2021 (left) and the cumulative number of COVID-19 cases per 100,000 people since the start of the pandemic until March 11, 2021.
Figure 2 shows the geographic distribution of the variables we use in the heterogeneity analysis to understand drivers of vaccine hesitancy after VA suspension.
After authorities resumed VA rollout, the gap in inoculation rates between the two vaccines narrowed with local COVID cases and widened in regions paying greater attention to related adverse events to vaccines.
Overall, the results suggest that the slowdown in the vaccination campaign was demand-driven. The four-day suspension led to a statistically significant reduction in AV injections that persisted weeks after authorities reinstated the vaccination campaign and reassured the public about the safety of the vaccine. The widening gap between the two vaccines in regions showing greater interest in “thrombosis” suggests that the suspension of AV may have drawn public attention to vaccine-related adverse events linked to AV injections. , perhaps inflating their risk. This evidence is consistent with the hesitancy literature which identifies concerns about the incidence and severity of adverse events as one of the main drivers of the decision to delay or refuse vaccination. On the other hand, the narrowing of the gap between the two vaccines in regions with higher incidence of COVID suggests that the severity of the outbreak has mitigated the impact of vaccine candidate safety concerns. .
A daily study of events shows that there are no anticipatory effects related to reporting on blood disorders among Vaxzevria recipients before the suspension. This evidence suggests that the release of alarming news about adverse vaccine events did not elicit any statistically significant reactions in candidate recipients. Instead, it appears people began to revise their assessment of the two risks to inform vaccination decisions when doubts about the safety of the VA vaccine emerged from health authorities.
picture 3 Main outcome event study
In summary, the suspension of the ChAdOx1-S vaccine may have unintentionally altered the balance between the perceived risks of contracting the disease and the adverse effects of the vaccine in many people, leading to increased reluctance to be vaccinated. Overall, our results reaffirm the importance of providing accurate institutional information about the relative incidence of alternative risks that individuals weigh in their vaccination decisions.
Authors’ note: The opinions expressed are the sole responsibility of their authors and can in no way be considered as the expression of an official position of the European Commission.
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