Retrospective Wastewater Tracking of Measles Outbreak in Western Switzerland in Winter 2024

Abstract

Measles outbreaks remain a significant public health challenge, despite high vaccination coverage in many regions. Wastewater-based surveillance (WBS) offers a noninvasive and community-level approach to monitoring the circulation of pathogens, including the measles virus. Here, we retrospectively applied a duplex digital PCR assay to distinguish between wild-type and vaccine strains of the measles virus in wastewater samples available from an existing national WBS program. Samples originated from the wastewater treatment plant serving the Lausanne city catchment area, where an outbreak occurred before spreading to the broader Canton Vaud region. Despite high vaccination rates, viral loads of the measles wild type were detected during the first transmission event involving 21 cases identified within a week. However, viral loads were no longer detectable after the initial 21 cases, despite an additional 30 cases reported in the following 3 weeks, possibly due to lower incidence rate or location outside the catchment. Measles vaccine strain was not detected during the outbreak. Our results demonstrate the complementarity of WBS with clinical surveillance and suggest its potential as an early warning system for measles and other vaccine-preventable diseases. Further improvements in the assay sensitivity and integration with epidemiological data could enhance the utility of WBS for outbreak detection and control.

Keywords: Digital PCR; Measles; Outbreak detection; Strain differentiation; Wastewater surveillance.

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Temporal trends in measles viral load and rash onset and confirmed case counts from early January to late March 2024. (A–C) The primary vertical axis (left) shows the viral load in wastewater expressed as gene copies per day per person (gc d^–1 person^–1), represented by a black line with black open circles (below LoB, i.e. nondetected), black filled circles (above LoB, i.e. 95% confidence it is not random noise; at least three positive partitions), and black inverted triangles (above 2-replicate LoD but below LoQ, i.e. detected with 95% confidence when performing two replicates, >30% expected coefficient of variation) data points. The gray shaded region indicates the standard deviation around the mean of two replicate measurements of the viral load. The horizontal axis represents the date in 2024. (A) The secondary vertical axis (right) shows the number of people with rash onset observed and recorded by the hospitals in the area. (B) The secondary vertical axis (right) shows the number of confirmed measles cases by immunization type, depicted as stacked pink (not immunized), purple (partially immunized), and green (fully immunized) bars representing immunization status. (C) The secondary vertical axis (right) shows the number of confirmed measles cases by location, depicted as stacked purple (college #1), red (secondary school), orange (healthcare facility), dark purple (college #2), green (multinational), and blue (household). Clinical cases, their timing, and transmission chain analysis can also be viewed at https://www.cobuccio.me/measles_epicurve.html (last accessed: 14.03.2025) and https://www.cobuccio.me/transmission_chains_colorblind_proof (last accessed: 14.03.2025).