Analysis of sewage water shows a steady rise in the spread of the delta variant.
The research is being carried out by the Federal Polytechnics in Lausanne and Zurich.
Regular samples are being taken from treatment plants across the country. The delta variant accounts for 33% of all samples in Zurich and nearly half in Bern, but lower in other areas, say researchers.
Before May 20th, there was no sign of the new virus. Experts say the delta variant is significantly more contagious than the previous strain and they expect it to become dominant.
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