Background: To assess the effect of the COVID-19 pandemic on performance indicators in the population-based breast cancer screening program of Parc de Salut Mar (PSMAR), Barcelona, Spain. Methods: we conducted a before-and-after, study to evaluate participation, recall, false positives, the cancer detection rate, and cancer characteristics in our screening population from March 2020 to March 2021 compared with the four previous rounds (2012-2019). Using multilevel logistic regression models, we estimated the adjusted odds ratios (aORs) of each of the performance indicators for the COVID-19 period, controlling by type of screening (prevalent or incident), socioeconomic index, family history of breast cancer, and menopausal status. We analyzed 144,779 invitations from 47,571women. Results: during the COVID-19 period, the odds of participation were lower in first-time invitees (aOR = 0.90 [95% CI = 0.84-0.96]) and in those who had previously participated regularly and irregularly (aOR = 0.63 [95% CI = 0.59-0.67] and aOR = 0.95 [95% CI = 0.86-1.05], respectively). Participation showed a modest increase in women not attending any of the previous rounds (aOR = 1.10 [95% CI = 1.01-1.20]). The recall rate decreased in both prevalent and incident screening (aOR = 0.74 [95% CI = 0.56-0.99] and aOR = 0.80 [95% CI = 0.68-0.95], respectively). False positives also decreased in both groups (prevalent aOR = 0.92 [95% CI = 0.66-1.28] and incident aOR = 0.72 [95% CI = 0.59-0.88]). No significant differences were observed in compliance with recall (OR = 1.26, 95% CI = 0.76-2.23), cancer detection rate (aOR = 0.91 [95% CI = 0.69-1.18]), or cancer stages. Conclusions: the COVID-19 pandemic negatively affected screening attendance, especially in previous participants and newcomers. We found a reduction in recall and false positives and no marked differences in cancer detection, indicating the robustness of the program. There is a need for further evaluations of interval cancers and potential diagnostic delays
