Key points are not available for this paper at this time.
Funding Information: *Received by the editors June 21, 2022; accepted for publication (in revised form) November 18, 2022; published electronically March 16, 2023. https: //doi. org/10. 1137/22M1504433 Funding: This work was supported by Project 105572 NordicMathCovid as part of the Nordic Programme on Health and Welfare, funded by NordForsk. Department of Mathematics, Stockholm University, SE-106 91 Stockholm, Sweden (tom. britton@math. su. se, https: //staff. math. su. se/tom. britton/). Department of Mathematics and Systems Analysis, Aalto University, FI-02015 Espoo, Finland (lasse. leskela@aalto. fi, http: //math. aalto. fi/ lleskela/). Publisher Copyright: © 2023 SIAM.
Britton et al. (Wed,) studied this question.