ba‐Fu 2013.

Study characteristics
Methods	Case‐control study. Amongst children in Guangzhou aged 8 months to 12 years during 2006 to 2012
Participants	Case participants 8 months to 12 years of age were randomly selected from 2 electronic databases in Guangzhou: the Notifiable Disease Reporting System and the Children’s Expanded Programmed Immunization (EPI) Administrative Computerised System. Controls were randomly selected amongst children aged 8 months to 12 years listed in the Children’s EPI Administrative Computerised System, which was designed to manage the immunisation records of children less than 7 years of age in Guangzhou in 1997. Controls were accepted if they did not have prior history of mumps, as confirmed by a phone call by physicians from the Guangzhou Center for Disease Control and Prevention. A list of potential controls with sequence number for each case participant was then created and matched by birth date, gender, and residence (living area, in the same community or village, and residence was categorised into urban, rural, and rural‐urban continuum area). A random number was used to select the potential control. If the potential control declined to participate or had prior history of mumps disease, or both, a control candidate with the next‐closest date of birth to the case participant was enrolled to participate.
Interventions	The EPI system allows healthcare workers to easily record, retrieve, and analyse all children’s vaccination information; registration of vaccination information in the system is required. Vaccines MMR or measles‐rubella
Outcomes	A mumps case was defined as having acute onset of unilateral or bilateral tender swelling of the parotid of salivary gland lasting 2 or more days without any other apparent cause. Bacterial infection was excluded by the absence of an increase in white blood cell count.
Funding Source	Government
Notes	Only mumps vaccinations received at least 30 days before the onset of mumps disease were considered valid. For controls, we considered only doses administered up to 30 days before the date of symptom onset in the corresponding case participant.
Risk of bias
Bias	Authors' judgement	Support for judgement
CCS ‐ case selection	Low risk	Adequate ‐ from 2 electronic databases
CCS ‐ control selection	Low risk	Adequate ‐ community
CCS ‐ comparability	Low risk	Birth date, gender, and residence (living area, in the same community or village, and residence was categorised into urban, rural, and rural‐urban continuum area)
CCS ‐ exposures	Unclear risk	The type of vaccine administered is missing in a high percentage of vaccinated.
Summary Risk of Bias assessment	Unclear risk	We had concerns regarding at least 1 domain such that some doubt is raised about the results.