Challenges in specifying parameter values for COVID-19 simulation models

A recent modelling paper on the coronavirus disease 2019 (COVID-19) epidemic in the US (Bartsch et al.) suggested that maintaining face mask use until a high vaccine coverage (70–90%) is achieved is generally cost-effective or even cost-saving in many of the scenarios considered. Their conclusion was based on the assumed effectiveness of continued face mask use, cited from a study that reported an 18% reduction in the effective reproduction number associated with the introduction of state-level mask mandate policies in the US in the summer of 2020. However, using this value implicitly assumes that the effect of face mask use in 2021 through 2022 is the same as that of summer 2020, when stringent nonpharmaceutical interventions were in place. The effectiveness of universal mask wearing in 2021–2022 is probably more uncertain than considered in Bartsch et al. and rigorous sensitivity analysis on this parameter is warranted.