Estimation of the Actual Incidence of Coronavirus Disease (COVID-19) in Emergent Hotspots: The Example of Hokkaido, Japan during February-March 2020.

Andrei R Akhmetzhanov,Hiroshi Nishiura,Ryosuke Omori,Sung-Mok Jung,Natalie M Linton,Kenji Mizumoto

doi:10.3390/jcm10112392

Abstract

Following the first report of the coronavirus disease 2019 (COVID-19) in Sapporo city, Hokkaido Prefecture, Japan, on 14 February 2020, a surge of cases was observed in Hokkaido during February and March. As of 6 March, 90 cases were diagnosed in Hokkaido. Unfortunately, many infected persons may not have been recognized due to having mild or no symptoms during the initial months of the outbreak. We therefore aimed to predict the actual number of COVID-19 cases in (i) Hokkaido Prefecture and (ii) Sapporo city using data on cases diagnosed outside these areas. Two statistical frameworks involving a balance equation and an extrapolated linear regression model with a negative binomial link were used for deriving both estimates, respectively. The estimated cumulative incidence in Hokkaido as of 27 February was 2,297 cases (95% confidence interval (CI): 382–7091) based on data on travelers outbound from Hokkaido. The cumulative incidence in Sapporo city as of 28 February was estimated at 2233 cases (95% CI: 0–4893) based on the count of confirmed cases within Hokkaido. Both approaches resulted in similar estimates, indicating a higher incidence of infections in Hokkaido than were detected by the surveillance system. This quantification of the gap between detected and estimated cases helped to inform the public health response at the beginning of the pandemic and provided insight into the possible scope of undetected transmission for future assessments.

Highlights

In December 2019, a cluster of 41 patients with atypical pneumonia of unknown etiology was reported in the city of Wuhan, China [1,2]
Cases were predominantly linked to the Sapporo Snow Festival, but overall, the geographic distribution of COVID-19 cases was widespread
Our estimate of the incidence in Sapporo in February–March 2020 is in the range of 1000–10,000 cases and resembles early estimates of COVID-19 incidence in Wuhan city, China that used data on the first cases among international travelers [3,52]

Summary

Introduction

In December 2019, a cluster of 41 patients with atypical pneumonia of unknown etiology was reported in the city of Wuhan, China [1,2]. The number of atypical pneumonia cases in Wuhan rapidly increased in early January and cases began appearing across China and in other countries [3,4]. The cause of the pneumonia was recognized as a newly emerged coronavirus called severe acute respiratory syndrome coronavirus 2 (SARS-CoV2), and the disease it causes was given the name coronavirus disease 2019 (COVID-19). Because of the limited testing capacity in the first months of the outbreak [6] and the specific contact-based, centered surveillance of SARS-CoV-2 infections in Japan [7], a substantial fraction of infected individuals may have remained undetected during the first months of the pandemic

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