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Background: Multiple studies are evaluating how to prevent SARS-CoV-2 infection. Interventions are wide ranging and include vaccines, prophylactic drugs, public health safety measures, and behavioural interventions. Heterogeneity in the outcomes measured and reported is leading to research waste and inefficiency, slowing worldwide identification and implementation of effective methods to prevent infection. A core outcome set (COS) for studies of interventions to prevent SARS-CoV-2 infection has recently been developed, identifying infection as a critical outcome to measure. This paper examines how SARS-CoV-2 infection outcomes are measured in registered COVID-19 prevention trials and considers how this can be improved. Methods: We searched the Cochrane COVID-19 Study Register to identify and review SARS-CoV-2 infection outcomes in prevention trials, including the rationale for choice of outcome measurement. We included phase 3 and 4 trials of COVID-19 prevention interventions. Early phase trials and studies relating to the transmission, treatment or management of COVID-19 were excluded. Results: We identified 430 entries in the register, of which 199 unique prevention trials were included across eight settings and 12 intervention types. Fifteen (8%) trials did not include any SARS-CoV-2 infection outcomes. The remaining 184 (92%) studies included a total of 268 SARS-CoV-2 infection outcomes, of which 32 (17%) did not specify how infection would be measured. Testing (i.e. formal diagnostic test) as a standalone method for determining infection was used in 57 (31%) trials, whereas defining infection by symptoms alone was used in 16 (9%) trials. All other trials (n=79, 43%) included multiple infection outcomes, defined in different ways. Discussion: There is considerable variation in how SARS-CoV-2 infection is measured within and across different interventions and settings. Furthermore, few studies report the rationale for outcome selection and measurement. Better transparency and standardisation of SARS-CoV-2 infection measurement is needed for the findings from prevention trials to inform decision-making.