Surveillance bias in the assessment of the size of COVID-19 epidemic waves: a case study.

ABSTRACT

OBJECTIVES: To estimate the size of COVID-19 waves using four indicators across three pandemic periods and assess potential surveillance bias. STUDY DESIGN: Case study using data from one region of Switzerland. METHODS: We compared cases, hospitalizations, deaths, and seroprevalence during three periods including the first three pandemic waves (period 1 Feb-Oct 2020; period 2 Oct 2020-Feb 2021; period 3 Feb-Aug 2021). Data were retrieved from the Federal Office of Public Health or estimated from population-based studies. To assess potential surveillance bias, indicators were compared to a reference indicator, i.e. seroprevalence during periods 1 and 2 and hospitalizations during the period 3. Timeliness of indicators (the duration from data generation to the availability of the information to decision-makers) was also evaluated. RESULTS: Using seroprevalence (our reference indicator for period 1 and 2), the 2nd wave size was slightly larger (by a ratio of 1.4) than the 1st wave. Compared to seroprevalence, cases largely overestimated the 2nd wave size (2nd vs 1st wave ratio 6.5), while hospitalizations (ratio 2.2) and deaths (ratio 2.9) were more suitable to compare the size of these waves. Using hospitalizations as a reference, the 3rd wave size was slightly smaller (by a ratio of 0.7) than the 2nd wave. Cases or deaths slightly underestimated the 3rd wave size (3rd vs 2nd wave ratio for cases 0.5; for deaths 0.4). The seroprevalence was not useful to compare the size of these waves due to high vaccination rates. Across all waves, timeliness for cases and hospitalizations was better than for deaths or seroprevalence. CONCLUSIONS: The usefulness of indicators for assessing the size of pandemic waves depends on the type of indicator and the period of the pandemic.