This systematic review examines the changes in healthcare service utilization during the COVID-19 pandemic, comparing pre-pandemic and pandemic periods across 20 countries. The review included 81 studies, covering over 11 million services pre-pandemic and 6.9 million during the pandemic. The primary outcome was a median 37% reduction in overall service utilization, with significant reductions in visits (42%), admissions (28%), diagnostics (31%), and therapeutics (30%). Secondary outcomes showed that 45% of studies reported larger reductions in utilisation among those with milder illness, while 55% reported no difference. The review highlights the need to address unmet needs and reduce unnecessary care, as well as the potential for learning from this "natural experiment" to improve healthcare systems post-pandemic.This systematic review examines the changes in healthcare service utilization during the COVID-19 pandemic, comparing pre-pandemic and pandemic periods across 20 countries. The review included 81 studies, covering over 11 million services pre-pandemic and 6.9 million during the pandemic. The primary outcome was a median 37% reduction in overall service utilization, with significant reductions in visits (42%), admissions (28%), diagnostics (31%), and therapeutics (30%). Secondary outcomes showed that 45% of studies reported larger reductions in utilisation among those with milder illness, while 55% reported no difference. The review highlights the need to address unmet needs and reduce unnecessary care, as well as the potential for learning from this "natural experiment" to improve healthcare systems post-pandemic.