Cryptic Circulation and Co-Infections of Endemic Human Coronaviruses During the First Years of the COVID-19 Pandemic in Brazil.

During the COVID-19 pandemic, the global focus on SARS-CoV-2 overshadowed the epidemiology of other respiratory pathogens. This study aimed to characterize the circulation of endemic human coronaviruses (HCoVs) in Brazil. We retrospectively analyzed results from 22,472 PCR tests for HCoVs (from 5183 patients) and 601,278 tests for SARS-CoV-2 (from 475,856 patients) between November 2019 and June 2021. HCoVs were detected in 160 patients (3.09%), with HCoV-NL63 as the most frequent species. HCoV circulation was intermittent, with positivity peaks up to 4% but also periods of up to six months with an absence of detections in 2020, contrasting with the sustained high positivity of SARS-CoV-2 (22.37%). Co-infections were frequent: 26.25% of HCoV-positive patients were co-infected with at least one other respiratory pathogen, most commonly Rhinovirus/Enterovirus, and cases involving up to five pathogens were observed, seven patients had co-infections between HCoVs and SARS-CoV-2. These findings reveal the persistent, often cryptic, circulation of HCoVs during the pandemic and highlight their role as key components in complex multi-pathogen infections. This underscores the critical importance of implementing comprehensive molecular diagnostic panels in routine respiratory surveillance to ensure accurate etiology, guide appropriate clinical management, and fully assess the public health burden of non-SARS-CoV-2 coronaviruses.