A comparison of using online market panel members and traditional methods in control recruitment for a case-control study during a national outbreak of STEC O145, United Kingdom.

National foodborne outbreaks of gastrointestinal disease often require rapid case-control investigations to identify the source. Online market panels offer a potential alternative to traditional control recruitment. We compared market panel controls to traditional controls in case-control recruitment during a 2024 UK outbreak of STEC O145. Two case-control studies were conducted for two different control groups (a) Salmonella cases as case-controls and (b) online market panel members. Timeliness, cost, and resources were compared, and a logistic regression compared findings in a control-control analysis. In total, 43 cases of STEC O145, 63 Salmonella case-controls, and 93 panel controls were recruited. Neither control group reached the recruitment target for the younger adult age group. Salmonella case-controls had a ninefold greater staff time to recruit and cost five times more than panel controls (£25.82 vs. £4.99 per control), partially due to interviewer-administered questionnaires compared with self-completion by panel controls. Both analytical approaches identified the same outbreak source, with no significant differences in exposures between the control groups. The cost and resource savings associated with panel controls justify their use as a standard procedure in outbreak investigations. We recommend exploring engagement with age groups that are difficult to recruit and assessing alternative strategies to reach them.