The wave of the SARS-CoV-2 Omicron variant resulted in a rapid spike and decline as highlighted by municipal wastewater surveillance

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 [Formula: see text] 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.