Population-based data on COVID-19 are urgently needed. We report on three rounds of probability sample household surveys in the state of Rio Grande do Sul (Brazil), carried out in nine large municipalities using the Wondfo lateral flow point-of-care test for immunoglobulin M and G antibodies against SARS-CoV-2 (https://en.wondfo.com.cn/product/wondfo-sars-cov-2-antibody-test-lateral-flow-method-2/). Before survey use, the assay underwent four validation studies with pooled estimates of sensitivity (84.8%; 95% confidence interval (CI) = 81.4–87.8%) and specificity (99.0%; 95% CI = 97.8–99.7%). We calculated that the seroprevalence was 0.048% (2/4,151; 95% CI = 0.006–0.174) on 11–13 April (round 1), 0.135% (6/4,460; 95% CI = 0.049–0.293%) on 25–27 April (round 2) and 0.222% (10/4,500; 95% CI = 0.107–0.408) on 9–11 May (round 3), with a significant upward trend over the course of the surveys. Of 37 family members of positive individuals, 17 (35%) were also positive. The epidemic is at an early stage in the state, and there is high compliance with social distancing, unlike in other parts of Brazil. Periodic survey rounds will continue to monitor trends until at least the end of September, and our population-based data will inform decisions on preventive policies and health system preparedness at the state level.

迫切需要基于人口的COVID-19数据。我们报告了在南里奥格兰德州（巴西）进行的三轮概率样本家庭调查，该调查在9个大型城市中使用了Wondfo侧向流动即时点检验来针对SARS-CoV-的免疫球蛋白M和G抗体2（https://en.wondfo.com.cn/product/wondfo-sars-cov-2-antibody-test-lateral-flow-method-2/）。在使用调查之前，该测定经历了四项验证研究，汇总了敏感性（84.8％； 95％置信区间（CI）= 81.4–87.8％）和特异性（99.0％； 95％CI = 97.8–99.7％）的估计值。我们计算得出，4月11日至13日（第1轮）的血清阳性率为0.048％（2 / 4,151; 95％CI = 0.006-0.174），25-25年的血清阳性率为0.135％（6 / 4,460; 95％CI = 0.049-0.293％） 4月27日（第2轮）和5月9日至11日（第3轮）的0.222％（10 / 4,500； 95％CI = 0.107-0.408），在调查过程中呈明显上升趋势。在37位阳性个体的家庭成员中，有17位（35％）也为阳性。该流行病处于该州的早期阶段，与巴西其他地区不同，人们对社会隔离的遵守程度很高。定期调查将至少在9月底之前持续监测趋势，而我们基于人口的数据将为州一级的预防政策和卫生系统准备工作提供决策依据。