On the misuse of time-dependent models in assessing mask usage and excess mortality

TL;DR

This paper critically examines the flawed use of time-dependent models in assessing mask effectiveness and excess mortality, demonstrating that previous associations are likely spurious and not driven by reverse causality.

Contribution

It identifies methodological flaws in prior ecological analyses and introduces a new longitudinal approach that challenges the reverse causality hypothesis.

Findings

Previous associations are likely spurious due to flawed models.

Reverse causality has a minor role in observed mask-mortality links.

Methodological flaws invalidate causal interpretations in earlier studies.

Abstract

The effectiveness of face masks as a population level intervention against respiratory viral transmission remains contested. While a large observational literature published during the COVID-19 pandemic reported beneficial effects, randomized controlled trials have consistently shown limited or no impact. An ecological analysis of European countries reported that average mask usage during the years 2020 and 2021 is positively associated with excess mortality in that same period in 24 European countries (Tausk and Spira, 2025). Such association remains after several attempts at controlling for confounding variables. This finding was later challenged by other authors and attributed to reverse causality (Cerqueira-Silva et al., 2026). In this paper, we reassess those criticisms in detail. We show that their analysis is fundamentally flawed, as the time-dependent regression framework used to refute the original findings yields spurious results partly due to the use of cumulative excess mortality as an outcome variable, thereby incorporating pre-intervention deaths and producing statistically significant effects even at impossible negative time lags. Diagnostic analyses further demonstrate that key assumptions of the model are violated, invalidating any association or causal interpretation. Finally, we present an original longitudinal analysis of mask usage designed to directly test the reverse causality hypothesis. By constructing multiple indices that capture mask adoption during distinct phases of pandemic waves, including interwave periods characterized by low mortality, we show that the association between mask usage and excess mortality persists and is not driven by reactive increases in masking. These findings provide substantial evidence that reverse causality provides, at most, a minor contribution to the observed association.