Assessing Community and Social Media Influence to Increase Influenza Vaccine Uptake among Youth in Soweto, South Africa (The Bambisana Study): Protocol for a Mixed Methods Pretest-Posttest Intervention Study - Janan Dietrich; Catherine Hill; Gugulethu Tshabalala; Tshepiso Msibi; Stefanie Vermaak; Mulalo Mashamba; Nellie Myburgh; Sarah Malycha; Isabella Goldstein; Elliot Grainger; Prima Alam; Kimberley Gutu; et al. (JMIR Research Protocols)