Low seroprevalence of COVID-19 in Lao PDR, late 2020.

Background: In 2020 Lao PDR had low reported COVID-19 cases but it was unclear whether this masked silent transmission. A seroprevalence study was done August - September 2020 to determine SARS-CoV-2 exposure. Methods: Participants were from the general community (n=2433) or healthcare workers (n=666) in five provinces and bat/wildlife contacts (n=74) were from Vientiane province. ELISAs detected anti- SARS-CoV-2 Nucleoprotein (N; n=3173 tested) and Spike (S; n=1417 tested) antibodies. Double-positive samples were checked by IgM/IgG rapid tests. Controls were confirmed COVID-19 cases (n=15) and pre-COVID-19 samples (n=265). Seroprevalence for the general community was weighted to account for complex survey sample design, age and sex. Findings: In pre-COVID-19 samples, 5.3%, [95% CI=3.1-8.7%] were anti-N antibody single-positive and 1.1% [0.3-3.5%] were anti-S antibody single positive. None were double positive. Anti-N and anti-S antibodies were detected in 5.2% [4.2-6.5%] and 2.1% [1.1-3.9%] of the general community, 2.0% [1.1-3.3%] and 1.4% [0.5-3.7%] of healthcare workers and 20.3% [12.6-31.0%] and 6.8% [2.8-15.3%] of bat/wildlife contacts. 0.1% [0.02-0.3%] were double positive for anti-N and anti-S antibodies (rapid test negative). Interpretation: We find no evidence for significant SARS-CoV-2 circulation in Lao PDR before September 2020. This likely results from early decisive measures taken by the government, social behavior, and low population density. High anti-N /low anti-S seroprevalence in bat/wildlife contacts may indicate exposure to cross-reactive animal coronaviruses with threat of emerging novel viruses. Funding: Agence Francaise de Developpement. Additional; Institut Pasteur du Laos, Institute Pasteur, Paris and Luxembourg Ministry of Foreign and European Affairs ("PaReCIDS II").