Infoveillance is the application of infodemiology with a primary aim of surveillance, which refers to surveilling and analyzing the unstructured information available on the internet in order to inform public health and public policy [15]. In this review, we focused on the surveillance of information available on social media in order to inform public health. Specifically, articles in this category analyzed social media data to (1) surveil potential public health risks including the onset of individual illnesses and disease outbreaks in a population (n=70) and (2) monitor public responses to health issues (n=25).

First, social media data have been analyzed to predict the future illness onset for individual users. For example, the language used in users’ posts such as life stress and anxiety can predict future occurrence of mental illness and distinguish different types of mental illness [16]. In addition, special characters used in social media posts such as emojis can also help identify illness syndromes such as asthma and difficulty breathing syndrome [17]. The use of social media to detect individual illness is especially beneficial for people who are at risk but less motivated to seek diagnosis and treatment; social media surveillance can supplement the traditional doctor-patient interaction method for illness detection [17] and can be combined with social media–based interventions to proactively provide health advice to those who are less likely to seek help [18].

Second, social media data can be analyzed to predict infectious disease outbreaks among a population such as in the COVID-19 outbreak [19]. The studies showed that social media data can provide an accurate prediction of disease outbreak case count [19]. In addition to outbreak prediction, demographic and geographic data obtained from social media can inform medical research and practice of the characteristics of people who are at risk of being infected [20]. Consistent with a recent systematic review of social media–based surveillance systems for health care [21], this study found that Twitter was the most used social media platform for individual illness and disease outbreak surveillance.

Third, user-generated social media posts can provide insights about the public’s cognitive and behavioral responses to health-related issues. Studies have analyzed social media posts to understand public concerns and sentiments about ongoing disease outbreaks such as Zika [22] and COVID-19 [23], investigated public attitudes toward and actual implementation of recommended disease prevention actions [20,24], monitored public discussion about controversial health topics such as vaccination [25] and e-cigarettes [26], and gauged public support of health-related policies [27]. Assessing social media data to understand public responses to health issues not only yields results comparable to national survey data but also provides insights for health-related theoretical models [25]. More importantly, traditional public surveys may take weeks and are expensive, while social media provides real-time and inexpensive data to evaluate the effectiveness of public health communication, which allows communication practitioners to adapt timely communication strategies to public needs [23].

With the ability to disseminate information quickly and widely, social media can be used by health institutions to post health information and share information with the public. Health institutions can inform the public about common health topics such as healthy living, immunization, and smoking [7]; communicate the risk of disease outbreaks [28]; quickly provide instructions about prevention behaviors to a broad audience during disease outbreaks [29]; and share the latest news and inform the public of the government’s handling of disease outbreaks [28,30]. Research efforts have been devoted to exploring strategies for generating health content on social media and increasing retransmission [31,32].

Social media has several advantages over other media outlets when used for disseminating health information. First, social media is seen as the fastest among available channels to share alerts and updates about disease outbreaks [28]. Second, social media allows leveraging various media forms to engage the public. For example, by integrating hyperlinks, social media posts can direct the public to other online resources for additional health information [28,33]. Health institutions can also share podcast audios and YouTube videos on various social media platforms to deliver health information [34,35]. Third, social media can provide a communication platform for stakeholders during disease outbreaks. Government social media accounts may become official information sources that provide content about disease outbreaks for local agencies and journalists in a timely manner [28,33].

With the increased volume of misinformation during the COVID-19 pandemic, many studies focused on the concern of rapid misinformation transmission on social media and proposed approaches to combat misinformation through social media. For example, researchers suggested health institutions increase efforts to monitor and enforce fact-checking of dubious information on social media [36,37], promote strategic and timely refutation to misinformation on social media [38], share personal experiences on social media to refute rumors [39], and use search optimization strategies within the social media platforms to redirect users who ask health-related questions to reliable information sources [40]. In addition to combating misinformation through social media, health institutions may also encourage health professionals to establish interpersonal relationships with patients and invite patients to conversations about misinformation that they may encounter online [41]. Although the effectiveness of the approaches proposed by articles in this review have not been formally tested, there have been many studies testing the effectiveness of different message design features in correcting misinformation. For example, Gesser-Edelsburg and colleagues [42] found that a theory-based correction message posted on Facebook by health authorities was more effective in correcting vaccination misinformation than a common correction message. Practitioners may design and post misinformation-correcting messages on social media based on the strategies suggested by these message design studies.

Social media has been widely integrated into health interventions and can perform multiple functions. First, social media was used to offer health resources including delivering health information to audiences [43], motivating participation in health-related events [44], and directing campaign audiences and intervention participants to other health resources [45,46]. Second, social media was used to support interaction with audience including engaging campaign audiences in 2-way communication with the institutions and health professionals [47,48] and providing platforms for peer support group discussion [49]. Third, social media was used to motivate health behavior change; intervention participants used social media to document and share their own progress of health behavior change such as uploading a picture of an everyday healthy meal [50] and sharing virtual awards of weekly achievement in physical activities [51], setting group challenges toward health goals [52], and engaging in health behavior competition with peers [53]. Last, at the preparing stage of an intervention, social media was also used to reach broader and more varied populations, including hard-to-reach populations [54,55].

Research effort has also focused on improving the design and evaluation of social media–based health interventions. Studies have examined approaches to evaluating campaign outcomes [56], proposed theoretical frameworks for effective health interventions design [57], explored social media user segments to inform intervention development [58], and examined user acceptance and suggestions of receiving social media–based interventions [59]. In general, empirical evidence shows that integrating social media in health interventions is accepted by target populations [59] and effective in increasing health knowledge, reducing risky behaviors, and adopting health behaviors [60]. Researchers have also discussed challenges and raises concerns regarding social media–based health intervention; for example, researchers must rigorously evaluate social media–based interventions throughout the stages of development and implementation, involve populations experiencing health disparities in the studies, and address privacy concerns [61].

Since social media can reach varied stakeholders and a wide audience in society, nongovernmental health organizations can use social media to mobilize social resources. Studies found that these organizations leveraged social media to advocate for change in public policies related to health issues such as HIV/AIDS [62] and mental health [63], raise funds for individual medical care and health-related research [64,65], and raise awareness and promote actions to address health problems such as medical equipment shortages [66]. Although a formal test of the effectiveness of using social media for social mobilization is lacking, the successful cases described in the articles show that social media provides a platform for nongovernmental health organizations to effectively mobilize resources and advocate for collective actions to achieve their goals [62].

Social media can facilitate health-related research in two ways: provide additional data to learn about patients’ disease experience and recruit research participants. First, by analyzing patients’ conversations on social media, researchers can gauge patients’ understanding of the disease and their coping strategies [67], identify their concerns about the disease [68], understand their barriers to health behavior change [69], identify symptoms related to the disease [70], and assess patients’ experience after recovery [71]. Because patient self-reported illness experience is not routinely reported to and recorded by physicians, analyzing patient discussions on social media may enhance health researchers’ and professionals’ understanding of patient experiences beyond what can be learned from traditional sources of health data [72].

Second, social media can help researchers recruit participants for health-related research. Studies found that social media performs as well as or even better than traditional recruiting methods in terms of cost and number of participants enrolled [73]. Social media is also useful in recruiting hard-to-reach populations such as immigrants [74]. Social media recruitment can generate a representative sample; a study comparing a social media–recruited sample with the underlying population found that the sample was representative in 8 of the 13 characteristics studied including demographics and health-related factors [75].

Health professionals and researchers may use social media for their own professional development such as learning, collaboration, and career advancement. Social media can be used to collaborate on research projects and practices [76], access and share trending research findings and medical knowledge [77,78], broaden their exposure to funders and publishers [79], conduct a job search [80], follow medical conferences remotely [81], market their team and services, and discuss interesting or difficult cases with colleagues [4]. Studies found that different social media platforms serve different roles in the professional development of health professionals and researchers; Pinterest was mostly used for health care–quality education [82], Twitter for gathering news and information on conferences, and LinkedIn for career advancement [81]. Nearly 85% of health professionals agreed that social media can be an effective tool for educational purposes [82], and 71% of health professionals, researchers, and businesspeople in the urology discipline agreed that social media is useful for career development [83].

Health professionals use social media for doctor-patient communication including responding to questions posted by patients [84], offering online consultation [85], and proactively providing advice and health information to social media followers [86]. In addition, social media can be used to support offline health services. Health professionals can use social media to inform patients of the results of their examination [4], encourage compliance with medication [87], receive feedback from patients about their health services [4], and collect information from patients after discharge to inform future practices [88]. Encouraging interactions between health practitioners and patients on social media may benefit both patients’ well-being and patient-physician relationships [89]. In addition, social media can also be used to provide medical services such as appointments, medical inquiries, personal information management, and medical charge payment, which can increase the accessibility of medical care and improve patient experiences [90].

However, increasing use of social media by health professionals and researchers is accompanied by concerns and challenges. Many studies raised concerns about risks to patients’ privacy and confidentiality [80] and health professionals lacking knowledge about social media [4]. Studies also discussed challenges such as protecting content generators’ intellectual property [91] and managing negative comments from patients [4]. Most of these articles called for developing a guideline or a regulation regarding ethical, legal, and technological issues [4,80] and providing training for health professionals and researchers on the proper use of social media [91,92].

Seeking and sharing health information is the most common use by the public. The public uses social media to follow and share news about trending health issues such as flu [93] and COVID-19 [1], find information on daily health behaviors such as fitness and healthy food choices [94-96], search for health advice for their own health concern such as pregnancy-related information [97], request a crowd diagnosis or second opinion after seeing a health care professional about their illness [98], access health care law [70], follow social media accounts of health organizations and professionals [99,100], and learn about physicians and hospitals to inform their choices [101]. Social media has become a primary information source for the public with varying information needs [1,102].

However, many studies raised concerns of social media information quality and its impact on individual health outcomes [103]. Indeed, empirical studies provided grounds for the concern. Sallam et al [104] found that people who relied on social media for health information had a lower COVID-19 knowledge score compared with those who relied on doctors and scientific journals for information. In addition, people who sought information on social media did not confirm its accuracy with health professionals, and health professionals disagreed with the information found on social media on 36.7% of occasions when people checked the information accuracy with health professionals [105]. This evidence calls for efforts to reduce misinformation on social media and improve the ability of the public to find reliable information sources on social media.

Social support includes informational support, emotional support, esteem support, network support, and tangible support [106]. Except for tangible support, other types of social support were widely found in interactions in the online health community.

People with certain health concerns can receive and provide informational support content. They may read the experience of others diagnosed with the same illness, share details of their own medication and diagnosis [107,108], share and access medical knowledge such as treatment options and self-care activities [109], learn about health laws [110], and solicit recommendations of doctors and hospitals [99]. They may also proactively request health information and advice [111] and validate health information with others [112].

Emotional and esteem support, expressions that convey love and caring (emotional support) and respect and confidence in ability (esteem support), were considered as one category [106]. People with health concerns may express their disease feelings to mitigate their stress and appeal for sympathy [113], offer hope and a sense of empowerment to each other [114], and encourage peers’ engagement in medical care [115].

Network support content refers to expressions that communicate belonging to a group of people who have the same concerns or interest [106]: individual users use social media to build community with people who have similar health concerns or health goals [116], foster a group identity [116], enhance a sense of belonging [117], achieve shared health goals together [118], get connected with peer patients [119], and get involved in social events such as swimming lessons with peers [120]. Network support also empowered people with health concerns to influence health awareness at the societal level such as raising awareness of queer and trans issues [121].

People with health concerns may benefit from participating in online communities in several ways. First, they do not have to reveal personal identity and thus are free from the risk of being stigmatized when participating in online communities [107]. Second, online communities as an information channel supplement offline communication channels in terms of content and time. Peers may provide more information on treatment options not raised by health professionals because of time constraints [109]. Also, people can get answers and emotional support from online communities 24/7, while other social networks may not be able to provide such timely support [122]. Third, people have a sense of control over the help-seeking process when participating in online communities, which may fulfill patients’ need of autonomy and reduce treatment avoidance [123]. Last, engaging in online communities may benefit health outcomes; studies found that perceived social support predicted subjective well-being [124].

Social media enables users to track and share their health statuses or activities and view those of others in the community. The public can document and share their illness experiences such as a breast cancer journey [125] or experience with chronic disease [126], achievements in health behavior change [127], and fitness activities [128] such as cycling and walking [129]. By self-tracking and sharing their physical activities, users can build networks with other users and discuss their activity performances [129]. Writing about and sharing their illness experiences may help patients cope with illness [125] and achieve health goals [126]. Other users may also benefit from such sharing as they can easily find peers who have similar experience to get advice on lifestyle changes and discuss treatment options [130].