Studies on the early introduction of SARS-CoV-2 in a naive population have important epidemic control implications. We report findings from the epidemiological investigation of the initial 135 COVID-19 cases in Brunei and describe the impact of control measures and travel restrictions. Methods Epidemiological and clinical information were obtained for all confirmed COVID-19 cases in Brunei, whose symptom onset was from March 9 to April 5, 2020 (covering the initial 5 weeks of the epidemic).

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... smission-related measures such as reproduction number (R), incubation period, serial interval were estimated. Time-varying R was calculated to assess the effectiveness of control measures. Results A total of 135 cases were detected, of which 53 (39.3%) were imported. The median age was 36 years (range = 0.5 to 72). 41 (30.4%) and 13 (9.6%) were presymptomatic and asymptomatic cases respectively. The median incubation period was 5 days (IQR = 5, range = 1 to 11), and the mean serial interval was 5.39 days (sd = 4.47; 95% CI: 4.25, 6.53). R0 was between 3.88 and 5.96, and the doubling time was 1.3 days. By the 13th day of the epidemic, the Rt was under one (Rt = 0.91; 95% credible interval: 0.62, 1.32 ) and the epidemic was under control. Conclusion Epidemic control was achieved through a combination of public health measures, with emphasis on a test-isolate-trace approach supplemented by travel restrictions and moderate physical distancing measures but no actual lockdown. To maintain suppression, regular and ongoing testing of high-risk groups can supplement the existing surveillance program.