The wave of the SARS-CoV-2 Omicron variant resulted in a rapid spike and decline as highlighted by municipal wastewater surveillance
Francesca Cutrupi, Maria Cadonna, Serena Manara, Mattia Postinghel, Giuseppina La Rosa, Elisabetta Suffredini, Paola Foladori
Abstract
This paper highlights the extraordinarily rapid spread of SARS-CoV-2 loads in wastewater that during the Omicron wave in December 2021–February 2022, compared with the profiles acquired in 2020–21 with 410 samples from two wastewater treatment plants (Trento+suburbs, 132,500 inhabitants). Monitoring of SARS-CoV-2 in wastewater focused on: (i) 3 samplings/week and analysis, (ii) normalization to calculate genomic units (GU) inh−1 d−1; (iii) calculation of a 7-day moving average to smooth daily fluctuations; (iv) comparison with the ‘current active cases’/100,000 inh progressively affected by the mass vaccination. The time profiles of SARS-CoV-2 in wastewater matched the waves of active cases. In February–April 2021, a viral load of 1.0E＋07 GU inh−1 d−1 corresponded to 700 active cases/100,000 inh. In July–September 2021, although the low current active cases, sewage revealed an appreciable SARS-CoV-2 circulation (in this period 2.2E＋07 GU inh−1 d−1 corresponded to 90 active cases/100,000 inh). Omicron was not detected in wastewater until mid-December 2021. The Omicron spread caused a 5–6 fold increase of the viral load in two weeks, reaching the highest peak (2.0–2.2E＋08 GU inh−1 d−1 and 4500 active cases/100,000 inh) during the pandemic. In this period, wastewater surveillance anticipated epidemiological data by about 6 days. In winter 2021–22, despite the 4–7 times higher viral loads in wastewater, hospitalizations were 4 times lower than in winter 2020–21 due to the vaccination coverage >80%. The Omicron wave demonstrated that SARS-CoV-2 monitoring of wastewater anticipated epidemiological data, confirming its importance in long-term surveillance.