Impact of cross-border-associated cases on the SARS-CoV-2 epidemic in Switzerland during summer 2020 and 2021

Martina L Reichmuth; Emma B Hodcroft; Julien Riou; Richard A Neher; Niel Hens; Christian L Althaus

doi:10.1016/j.epidem.2022.100654

Impact of cross-border-associated cases on the SARS-CoV-2 epidemic in Switzerland during summer 2020 and 2021

Epidemics. 2022 Dec:41:100654. doi: 10.1016/j.epidem.2022.100654. Epub 2022 Nov 17.

Authors

Martina L Reichmuth¹, Emma B Hodcroft², Julien Riou³, Richard A Neher⁴, Niel Hens⁵, Christian L Althaus⁶

Affiliations

¹ Institute of Social and Preventive Medicine, University of Bern, Bern, Switzerland. Electronic address: martina.reichmuth@ispm.unibe.com.
² Institute of Social and Preventive Medicine, University of Bern, Bern, Switzerland; Swiss Institute of Bioinformatics, Lausanne, Switzerland.
³ Institute of Social and Preventive Medicine, University of Bern, Bern, Switzerland; Federal Office of Public Health, Liebefeld, Switzerland.
⁴ Swiss Institute of Bioinformatics, Lausanne, Switzerland; Biozentrum, University of Basel, Basel, Switzerland.
⁵ Interuniversity Institute for Biostatistics and statistical Bioinformatics, Data Science Institute, Hasselt University, Hasselt, Belgium; Centre for Health Economics Research and Modelling Infectious Diseases, Vaccine and Infectious Disease Institute, University of Antwerp, Antwerp, Belgium.
⁶ Institute of Social and Preventive Medicine, University of Bern, Bern, Switzerland.

Abstract

During the summers of 2020 and 2021, the number of confirmed cases of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections in Switzerland remained at relatively low levels, but grew steadily over time. It remains unclear to what extent epidemic growth during these periods was a result of the relaxation of local control measures or increased traveling and subsequent importation of cases. A better understanding of the role of cross-border-associated cases (imports) on the local epidemic dynamics will help to inform future surveillance strategies. We analyzed routine surveillance data of confirmed cases of SARS-CoV-2 in Switzerland from 1 June to 30 September 2020 and 2021. We used a stochastic branching process model that accounts for superspreading of SARS-CoV-2 to simulate epidemic trajectories in absence and in presence of imports during summer 2020 and 2021. The Swiss Federal Office of Public Health reported 22,919 and 145,840 confirmed cases of SARS-CoV-2 from 1 June to 30 September 2020 and 2021, respectively. Among cases with known place of exposure, 27% (3,276 of 12,088) and 25% (1,110 of 4,368) reported an exposure abroad in 2020 and 2021, respectively. Without considering the impact of imported cases, the steady growth of confirmed cases during summer periods would be consistent with a value of R_e that is significantly above the critical threshold of 1. In contrast, we estimated R_e at 0.84 (95% credible interval, CrI: 0.78-0.90) in 2020 and 0.82 (95% CrI: 0.74-0.90) in 2021 when imported cases were taken into account, indicating that the local R_e was below the critical threshold of 1 during summer. In Switzerland, cross-border-associated SARS-CoV-2 cases had a considerable impact on the local transmission dynamics and can explain the steady growth of the epidemic during the summers of 2020 and 2021.

Keywords: Cross-border-associated cases; Import; SARS-CoV-2; Summer; Travel.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

COVID-19* / epidemiology
Epidemics*
Humans
Public Health
SARS-CoV-2
Switzerland / epidemiology
Travel