key: cord-0990435-h78857zs
authors: Wong, Justin; Abdul Aziz, Anita B Z; Chaw, Liling; Mahamud, Abdirahman; Griffith, Matthew M; Ying-Ru, L O; Naing, Lin
title: High proportion of asymptomatic and presymptomatic COVID-19 infections in travelers and returning residents to Brunei
date: 2020-05-05
journal: J Travel Med
DOI: 10.1093/jtm/taaa066
sha: 4487fe1352d36faeb110c055d0ad8a3bb464b5b9
doc_id: 990435
cord_uid: h78857zs

We report early findings from COVID-19 cases in Brunei suggesting a remarkably high proportion of asymptomatic (12%) and presymptomatic (30%) cases. This proportion was even higher in imported cases. These have implications for measures to prevent onward local transmission and should prompt reconsideration of current testing protocols and safe de-escalation of social distancing measures.

Conflict of interest: None to be declared

We report early findings from COVID-19 cases in Brunei suggesting a remarkably high proportion of asymptomatic (12%) and presymptomatic (30%) cases. This proportion was even higher in imported cases. These have implications for measures to prevent onward local transmission and should prompt reconsideration of current testing protocols and safe deescalation of social distancing measures.

Asymptomatic transmission of SARS-CoV-2 could complicate disease control, yet robust evidence on such transmission is lacking. Reports have described clusters arising from asymptomatic transmission. 1 Estimates of the asymptomatic proportion of COVID-19

infections range from 17.9% to 30.8%. 2, 3 Given the urgency of knowing the proportion of admitted and isolated until viral clearance. We can thus distinguish between patients who remain asymptomatic until viral clearance (hereafter referred as 'asymptomatic') and those who were asymptomatic at diagnosis but later developed symptoms (hereafter referred as 'presymptomatic').

Brunei detected its first case on March 5. As of April 24, Brunei has reported 138 cases. We found 16 (12%) cases were asymptomatic and 42 (30%) were presymptomatic. Figure 1 indicates that all local transmissions had a relevant contact history, therefore short serial intervals suggest presymptomatic transmission. We estimate Serial Interval (SI) (defined as the difference in symptom onset dates between secondary and index case) for 53 symptomatic infector-infectee pairs. 21 (39.6%) had a SI of ≤3 days and 6 (11.3%) had zero or negative SI values, suggesting potential infectivity when asymptomatic (minimum SI is -4 days). The median incubation period is 4.5 days.

Since the start of outbreak in Brunei, 55 cases were imported. Of these 19 (35%) were presymptomatic, and 7 (13%) were asymptomatic. From March 21 (when mass testing for all arrivals to Brunei commenced) until April 24, 1,396 arrivals were tested and isolated for 14 days. Of these 30 (2.15%) tested positive for SARS-CoV-2, 11 (0.8%) were presymptomatic, and 3 were asymptomatic (0.3%) suggesting that nearly half of all imported cases (or 1% of all travelers) do not have symptoms at time of diagnosis. Our finding of a 2.15% infected rate in all travelers is lower than the 3.6% -6.3% observed in repatriation flights to Greece, 4 but higher than the 0.87% observed in flights from Wuhan at the peak of the epidemic there. 5 These findings have several implications. First, the high level of asymptomatic and presymptomatic imported cases suggests that symptom-based screening at points of entry may miss silent chains of transmission. To account for this, a number of countries have implemented 14-day quarantine period of travelers, however these may be unsustainable in the long-term. 6 In the absence of widespread vaccination, the development of an accurate

rapid test kit that can identify early disease and its widespread adoption at points of entry for travelers from high-risk areas regardless of symptoms should be considered as an interim measure.

Second, many countries that have implemented enhanced physical distancing measures, are now preparing for relaxation. Our findings suggest that this cannot be safely accomplished until the relevant infrastructure is in place for large-scale community-based testing, including the capacity for testing individuals without symptoms. 

Presumed Asymptomatic carrier transmission of COVID-19

Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship

Estimation of the asymptomatic ratio of novel coronavirus infections (COVID-19)

High prevalence of SARS-CoV-2 infection in repatriation flights to Greece from three European countries

Estimates of the severity of coronavirus disease 2019: a model-based analysis

Reducing onward spread of COVID-19 from imported cases: quarantine and "stay at home" measures for travellers and returning residents to