Fig 1.
Illustrative example of the conservative and liberal approach to estimate the importation of SARS-CoV-2 variants of concern (VoC) to Switzerland.
A: In the liberal approach, each subsequent subtree of only Swiss sequences is considered as a separate importation event. In this example, there would be two imports to Switzerland. B: In the conservative approach, subtrees with mixed Swiss and non-Swiss sequences are considered as one importation. Non-Swiss sequences are assumed to be exports from Switzerland or to originate from parallel evolution outside of Switzerland. In this example, there would be one import into Switzerland. More details are provided in the Method in S1 File.
Fig 2.
Scheme of the SARS-CoV-2 transmission model.
The model includes individuals that are susceptible (S), exposed to the pre-circulating variant (E1) or the imported VoC (E2), infected with the pre-circulating variant (I1) or the imported VoC (I2), tested (C1, C2), and recovered from infection (R1, R2). ωt denotes the importation of VoC as derived from the phylogenetic analysis.
Fig 3.
SARS-CoV-2 epidemic in Switzerland from October 2020 to September 2021.
A: Number of laboratory-confirmed SARS-CoV-2 cases per day. B: Effective reproduction (Re) number of SARS-CoV-2 in Switzerland.
Fig 4.
Dynamics of Alpha and Delta importation to Switzerland.
A, B: Number of imports estimated with the phylogenetic analysis. C, D: Number of laboratory-confirmed SARS-CoV-2 cases per day (gray). The black lines show the overall simulated number of reported cases. The blue and purple line show the simulated number of VoC cases. E, F: Proportion of reported Alpha and Delta among all SARS-CoV-2 infections. Gray: Genomic surveillance data. Blue: Liberal approach. Purple: Conservative approach.
Table 1.
Parameter values of the SARS-CoV-2 transmission model.
Fig 5.
Counterfactual scenarios for mitigating the spread of VoCs in Switzerland.
A: Existing border closure. B: Implemented border closure after warning about VoC. C: Increased surveillance of travelers.