With the world facing a public health emergency due to the Coronavirus disease (COVID-19) in a global pandemic, this paper provides insight about how a simulation model was used to determine the impact of headcount variability during lockdown on fab performance. To create a robust simulation model, operator loading time was introduced as one of the input parameters. An existing and well validated Discrete Event Fab simulation model was extended with operator modelling, and was used to conduct case studies, evaluating the impact of different operator availability scenarios including work disruptions for several shifts within a week. The studies provide implications for operation to derive mitigation strategies, weighing the trade-off between cost demand and speed loss due to operator resources.