# **1. Introduction and Literature Review**

COVID-19 upended daily life and exacerbated global inequalities, resulting in about 6.88 million reported deaths globally by March 2023 (Johns Hopkins University, 2023) and 14.83 million excess deaths (i.e., the number of deaths during a crisis that exceeded the expected number under normal conditions, including those directly caused by the crisis and those resulting from indirect impacts like healthcare disruptions) (Msemburi et al., 2023). The development and regulatory approval of COVID-19 vaccines changed the trajectory of the pandemic. However, limited access to and acceptance of vaccination resulted in global vaccine coverage inequalities that continued to persist throughout the pandemic. Although 72.3% of the world’s population had received at least one dose of the COVID-19 vaccine as of March 2023 when many countries stopped regularly reporting vaccination data, there remained stark gaps in vaccination rates across regions (Holder, 2023). For example, 37% of the population received at least one dose of vaccination in Africa, 58% in the Middle East, 70% in Europe, 81% in the U.S. and Canada, and 82% in Asia-Pacific and Latin America (Holder, 2023). Among LMICs, surveys found that Europe and Central Asia showed the highest vaccine hesitancy (58%), followed by the Middle East and North Africa (47%), East Asia and Pacific (26%), Sub-Saharan Africa (15%), and Latin America and the Caribbean (8%) (Dayton Eberwein et al., 2023). In particular in LMICs, COVID-19 vaccination can reduce transmission and mortality, prevent healthcare system overload, support economic recovery, and address global vaccine inequity (Ali et al., 2022; WHO, 2021).

The journey to vaccination is a complex process involving structural factors, individual decisionmaking, and last-mile actions. The success of vaccination programs is determined by promoting vaccination intentions and translating these intentions into vaccine uptake. Issues of vaccine confidence (i.e., lack of trust in the vaccine or provider), complacency (i.e., perceived lack of need for a vaccine), and low convenience (i.e., limited access) can contribute to vaccine hesitancy<sup>3</sup> (Larson et al., 2014) and ultimately discourage vaccine uptake.

<mark>Communications campaigns offer one approach to strengthening vaccine confidence and promote intentions to get vaccinated. A</mark> lthough public health messages generally have small to moderate effects on attitudes and behaviors (e.g., Noar, 2006; Wakefield et al., 2010), these messages can have societal impacts as they reach large populations repeatedly (Funder & Ozer, 2019; Noar, 2006). A recen <mark>t summary of research found that media campaigns, both through traditional (e.g., television, radio, and printed materials) and new social media, can significantly impact health knowledge, attitudes, and behaviors</mark> (Orozco-Olvera & Malhotra, 2023). These approaches might be particularly effective when coupled with other behavioral and policy interventions such as vaccine certificates or automated vaccination appointment systems (e.g., van Bavel et al., 2020; Ruggeri et al., 2024).

As a key antecedent of vaccine uptake, it is important to understand the type of communication that fosters vaccine confidence and intentions to get vaccinated. Given the limited empirical evidence to guide such communication strategies as the pandemic was ongoing (Lim et al., in press), <mark>we launched a global research program of 28 online survey experiments leveraging social media across 23 mostly low-</mark>

> 3 Vaccine hesitancy refers to a “delay in acceptance or refusal of vaccines despite availability of vaccination services” on a continuum between full acceptance and refusal (WHO, 2014).

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