This study primarily aims to verify whether the interval effect was evident in the beta coefficients (β) of the stock values of the companies listed on the Warsaw Stock Exchange (WSE) during the COVID-19 pandemic, and if so, whether this was due to the macrosectoral affiliation of these companies. The β coefficient is calculated using the ordinary least squares method (OLS) on a sample of issuers grouped in the following indices: WIG20, mWIG40, and sWIG80. When the β values are estimated for the above time horizons and analyzed for the COVID-19 pandemic years, the interval effect is observable for the finance, industrial, construction, and assembly production macrosectors. Interestingly, prior to the pandemic, the β values of the stock values in these industries did not significantly differ statistically when the time horizon was changed. The interval effect in the years preceding the COVID-19 pandemic is recorded for the healthcare and trade and services macrosectors. The research results show that the COVID-19 pandemic affected the sectoral differentiation of the interval effect. The statistical significance of the differences in β estimates affected other macrosectors more during the COVID-19 pandemic than it had in the years preceding it.