Table 1.

Characteristics of included articles (n=12).

Authors	General characteristics	Population	Intervention/type of social media	Comparison/control group	Outcome/measurement	Risk of biasa
Daley et al [63]b	Study design: RCTc Vaccine type: vaccines in general Colorado, United States	Member of KPCOd Recruited during pregnancy Age: 31.6 (SD 4.4) years Only females N=1093	A study website with vaccine information and social media components (VSM arm).	A website with vaccine information only Usual care	Vaccine acceptance Change in parental vaccine attitudes over time by baseline degree of vaccine hesitancy.	Good quality
Glanz et al [64]b	Study design: RCT Vaccine type: vaccines in general Colorado, United States	Member of KPCO Recruited during pregnancy Age: 31.6 (SD 4.3) years Only females N=1093	Multidirectional communication model: (1) website developers created and presented content to users; (2) users created content and interacted with website developers; and (3) users interacted with each other and shared information.	A website with vaccine information only Usual care	Days unvaccinated From birth to age 200 days	Good quality
O’Leary et al [65]b	Study design: RCT Vaccine type: influenza and Tdape Colorado, United States	Women in the third trimester of pregnancy integrated into KPCO Age: 32 (SD 4.5) years Only female N=1093	A website with vaccine information and interactive social media components. Included a blog and a discussion forum and an “Ask a question” portal.	A website with vaccine information only Usual care	Uptake of vaccines compared with baseline Receipt of influenza and Tdap vaccines among pregnant women.	Good quality
Liao et al [69]	Study design: RCT Vaccine type: childhood SIVf China, Hong Kong	Mothers of child(ren) aged 6-72 months Age: N/Ag Only female N=365	WhatsApp weekly vaccination reminders WhatsApp discussion group	No intervention	Uptake of vaccines compared with baseline SIV uptake in children	Good quality
Zhang et al [70]	Study design: online survey experiment Vaccine type: influenza, HPVh, MMRi, Tdap, Zika United States	Adults recruited from Dynataj Age: 41.13 (SD 13.42) years 50.2% female N=1198	Mock Twitter page and fact-checking labels: the treatment groups added a simple fact-checking label below the misinformation message, which consisted of a red warning sign, a falsification message, and a source logo.	A tweet consisting of a picture of a bottle of a specific vaccine and a misinformation claim only.	Vaccine acceptance Vaccine attitudes	Good quality
Ugarte et al [68]	Study design: RCT Vaccine type: COVID-19 United States	Adults recruited from online advertisements Age: 39.02 (SD 10.90) years 78.7% female N=108	Online support community of peers trained in behavior change science Facebook groups	Online community without peer leaders	Vaccine acceptance Vaccine uptake	Good quality
Abdel-Qader et al [71]	Study design: RCT Vaccine type: COVID-19 Jordan	Adult population who were reluctant or resistant to the COVID-19 vaccine Age: 18-64 years 56.1% female N=320	Pharmacists-physicians collaborative coaching intervention was delivered to active group participants over 2 months through Facebook live sessions.	The control group did not receive intervention	Uptake of vaccines compared with baseline The proportion of hesitancy and resistance to a COVID-19 vaccine The proportion of patients vaccinated	Fair quality
Brandt et al [72]	Study design: a controlled, quasi-experimental mixed methods study Vaccine type: HPV United States	College students: two undergraduate classes at a public university in the southeast region of the United States Age: 21.6 (SD 2.2) years Female n=47, male n=11 N=58	Facebook private group posts Weekly emails	Behavioral weight gain prevention intervention (Healthy Weight) Classes were randomized to receive either an HPV vaccination awareness intervention or a behavioral weight gain prevention intervention (Healthy Weight; control). Each group served as the control for the other group, allowing for simultaneous intervention comparisons.	Vaccine acceptance HPV vaccination status and intentions HPV vaccination knowledge	Fair quality
Lau et al [67]	Study design: RCT Vaccine type: SIV Australia	University students and staff Age: 26.2 (SD 9.07) years 57% female N=742	Healthy.me: a web-based personally controlled health management system on the uptake of seasonal influenza vaccine and primary care service utilization among university students and staff.	6-month waitlist	Uptake of vaccines compared with baseline Uptake of seasonal influenza vaccine	Fair quality
Chodick et al [73]	Study design: RCT Vaccine type: HPV Israel	MHSk,l members who were mothers to 14-year-old daughters in the 2019 school year (who were born between October 2004 and December 2005) Age: 44.6 (SD 5.2) years Only female N=21,592	Facebook Targeted campaign	The control group (20%) did not receive targeted campaign messages.	Uptake of vaccines compared with baseline HPV immunization history among the eighth-grade daughters of the study participants	Poor quality
Ortiz et al [74]	Study design: online survey experiment Vaccine type: HPV United States	Adolescents who had not completed the HPV vaccine series Age: 15.6 (SD 1.68) years 60.2% female N=108	Facebook: providing relevant health information from a credible health source via a commonly used social media platform.	No intervention, just another email to complete a second survey questionnaire.	Vaccine acceptance Improve adolescents’ knowledge about vaccination against HPV	Poor quality
Osborne et al [75]	Study design: RCT Vaccine type: SIV United States	Undergraduates at a large midwestern public university Age: >18 years 70% female N=702	Twitter: following a Twitter account that posted near-daily tweets (1.24 tweets per day) promoting flu vaccination. In addition to direct tweet exposure, campaign engagement was incentivized with prize raffle entries. For each month of the study, an intervention group member could receive 1 raffle entry (up to 7 over the study) by retweeting 1 of the promotional tweets, or by constructing their own tweet containing a hashtag that was unique to the campaign.	Following a Twitter account that tweeted no content.	Uptake of vaccines compared with baseline Vaccine rates	Poor quality

^aRisk of bias was assessed by RKH and EG using the Cochrane Risk-of-Bias Tool for Randomized Controlled Trials.

^bDaley et al [63], Glanz et al [64], O’Leary et al [65] belong to the same protocol in ClinicalTrials.com 24.

^cRCT: randomized controlled trial.

^dKPCO: Kaiser Permanente Colorado.

^eTdap: tetanus, diphtheria, pertussis.

^fSIV: seasonal influenza vaccine.

^gN/A: not applicable.

^hHPV: human papillomavirus.

ⁱMMR: measles, mumps, and rubella.

^jDynata (Research Now) maintains a large panel of American adults recruited via verified sources, uses multiple layers of authentication, and periodically invites the panel to take part in studies.

^kState-mandated health organization in Israel (MHS).

^lMHS: Maccabi Healthcare Services.