Objective – To systematically review evidence on effectiveness of contact tracing apps (CTAs) for SARS-CoV-2 on epidemiological and clinical outcomes Design – Update of a systematic review (https://doi.org/10.1136/bmjopen-2021-050519) Data sources - EMBASE (OVID), MEDLINE (PubMed), BioRxiv, and MedRxiv were searched up to June 9^th 2021 Study selection – Studies, empirical or model-based, assessing effect of CTAs for SARS-CoV-2 on quarantine rate, reproduction number (R), total number of infections, hospitalization, mortality, and other epidemiologically and clinically relevant outcomes, were eligible for inclusion. Data extraction – Empirical and model-based studies were both critically appraised based on dedicated quality and risk of bias assessment checklists. Data on type of study (i.e., empirical or model-based), sample size, (simulated) time horizon, study population, CTA type (and associated interventions), comparator, and outcomes assessed, were extracted. Key findings were extracted and narratively summarized. Specifically for model-based studies, characteristics and values of important model parameters were collected. Results – 5123 studies were identified, of which 27 studies (five empirical, 22 model-based studies) were eligible and included in this review. All empirical studies were observational (non-randomized) studies and either at unclear or high risk of bias, mostly due to uncontrolled confounding. Risk of bias of model-based studies was considered high for 7 of 22 studies. Most studies demonstrated beneficial effects of CTAs on R, total number of infections, hospitalization, and mortality. Effect size was dependent on other model parameter values (e.g., proportion of asymptomatic individuals, testing delays), but in general a beneficial effect was observed at CTA adoption rates of 20% and over. Conclusions – CTAs are potentially effective at reducing SARS-CoV-2 related epidemiological and clinical outcomes, though effect size depends on other model parameter values. Methodologically sound comparative empirical studies on effectiveness of CTAs are lacking and would be desirable to confirm findings from model-based studies.