Epidemiological surveillance based on monitoring pathogens in sewage has become an effective tool for monitoring diseases in the population. This strategy makes it possible to detect viral genomes before the onset of symptoms or in asymptomatic infections, functioning as an early warning and complement to traditional surveillance. SARS-CoV-2, the etiologic agent of the COVID-19 pandemic, is a single-stranded, enveloped RNA virus. This study evaluated the presence of the SARS-CoV-2 genome in raw sewage from the largest wastewater treatment plant in Porto Alegre, which serves about 50% of the population, relating viral load, detection frequency, population events, and vaccination coverage. Composite raw sewage water samples were collected, and the genomic viral load present was compared over the same period in consecutive years: between September and April 2023–2024 (n=65) and 2024–2025 (n=55). For genomic load detection, samples were concentrated by ultracentrifugation. RNA was extracted automatically using the RSC Viral Total Nucleic Acid kit. Quantification in genomic copies per liter was performed by RT-qPCR (N1 target). The frequency of SARS-CoV-2 detection fell from 92.3% (60/65) to 83.6% (46/55). Despite this, there was an increase in viral load at two specific moments: October 2024 and April 2025, indicating greater circulation of the virus. These peaks coincide with crowding events, such as cultural holidays and religious festivals. Porto Alegre has high vaccination coverage with monovalent immunizers: 100% of the population received at least two doses, 77% three and 32% four. However, only 36% received the bivalent vaccine, aimed at recent variants. Even with widespread vaccination, detection of high viral loads in sewage suggests that, although monovalent vaccines reduce severe cases, their effectiveness is limited in containing and transmitting infection by current SARS-CoV-2 variants, especially in contexts of crowding. Environmental surveillance demonstrated that, even with reduced detection frequency, the virus continues to circulate, especially during critical periods of crowding. Low coverage of the bivalent vaccine reinforces the importance of expanding access to it and maintaining environmental epidemiological surveillance as a strategic public health tool.
MATTOS et al. (Sun,) studied this question.