habits (Carins and Rundle-Thiele 2014) and reducing social stigma in different areas (for a complete review, see Shawky et al. 2019).<sup>6</sup> However, the effectiveness of such campaigns has been, to our knowledge, entirely restricted to assessing on-platform engagement metrics or short-term “Brand Lift” surveys,<sup>7</sup> which may not be informative about medium-term behavioral change and public health outcomes.

We contribute to the existing literature by measuring the effects of a social media campaign on malaria-related behaviors and incidence, using both self-reported surveys and objective health facility data for up to five months post-advertisement exposure. Our study provides new and substantial evidence that social media can be an important channel in fighting malaria and similar diseases through the promotion of preventative behaviors and response actions. In this respect, our research complements evaluations of traditional interventions conducted in the field to combat malaria, including initiatives to subsidize and distribute antimalarials, rapid tests, and bed nets. (Cohen et al. 2015; Tarozzi et al. 2014). We show that digital ads that remind individuals to use bed nets and timely seek treatment in case of symptoms can be a cost-effective way of reducing malaria incidence. Moreover, in line with previous studies (Lambrecht and Tucker 2019; Sapiezynski et al. 2022), our analysis of the mechanisms sheds light on the importance of accounting for algorithmic targeting when delivering ads with public health objectives, as populations that are more at risk of the disease are also less likely to engage with the ads.

In this respect, our study demonstrates the importance of data in targeting users and tracking them to measure ad effectiveness, contributing to the ongoing debate on privacy and the use of thirdparty data in digital marketing (Acquisti et al. 2016; Tucker et al. 2018). The constraints imposed by privacy regulations, such as the GDPR, CCPA, and recent changes by Apple and Google, pose significant challenges (Goldfarb and Tucker 2011 _b_ ; Johnson et al. 2020; Wernerfelt et al. 2022).<sup>8</sup> However, they also underscore the need for innovative approaches that respect consumer privacy while leveraging data for effective targeting and measurement (Godinho de Matos and Adjerid 2022; Goldfarb and Tucker 2011 _a_ ; Rafieian and Yoganarasimhan 2021). By integrating privacy considerations into our experimental methodology, our approach highlights how first-party data, directly collected from users on social media, can inform targeting and help reach the most valuable segments of the population—in our context, those most at risk of malaria.

More broadly, our study relates to the work emphasizing the importance of marketing techniques to address social issues (Chandy et al. 2021; Kotler and Levy 1969; Kotler and Zaltman 1971), with

> 6Traditional media (television/radio/print) campaigns have also been shown effective for public health outcomes such as the prevention of childhood obesity and COVID-19 (Croker et al. 2012; Ghosh Dastidar et al. 2023. For a complete review of the effects of mass media on health outcomes, see Orozco-Olvera and Malhotra (2023).

> 7Brand Lift Studies are commonly used to measure ad effects on outcomes such as ad recall, brand sentiment, and purchase intent. Companies such as Meta and Google provide Brand Lift experiments to advertisers, typically executed via a brief three-question survey administered to respondents one to two days following exposure to the advertisement.

> 8Regulatory measures such as the General Data Protection Regulation (GDPR) in Europe and the California Consumer Privacy Act (CCPA) in the United States have been enacted to protect consumer privacy and limit the extent to which personal data can be used without explicit consent. Concurrently, tech giants like Apple have restricted user tracking through device IDs on mobile devices, and Google has announced plans to deprecate thirdparty cookies in its Chrome browser.

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