Persistent nucleic acids from endemic human coronaviruses in adenoids: Do they enhance children's immune response to SARS-CoV-2 infection?

• In this study, we investigated the presence of nucleic acids in endemic HCoVs (HCoV-OC43, HCoV-HKU1, HCoV-NL63, and HCoV-229E) in patients without symptoms of acute respiratory infection. • Therfor we collected a total of 99 adenoid tissue samples from children and young adults up to 25 years of age. The samples were examined by RT-nested PCR. • Among the 99 samples, 28 (28.3 %) tested positive for at least one endemic HCoV. Throat swabs were collected from 67 participants immediately before surgery. Endemic HCoV RNA was rarely detected in positive tissue samples (4/67). • Our findings suggest the frequent presence of endemic HCoV nucleic acids in the lymphatic tissue of the Waldeyer's ring, especially in children. This observation supports the concept of an early immune trigger which could enhance children's immune response to SARS-CoV-2 infection. Human Coronaviruses (HCoVs), including HCoV-OC43, HCoV-HKU1, HCoV-NL63, and HCoV-229E, are endemic viruses that circulate globally and infect humans. In contrast to SARS-CoV-2, which causes more severe symptoms, these endemic HCoVs typically induce mild cold symptoms and confer only temporary immunity [ 1 , 2 ]]. Understanding the presence and persistence of HCoVs in the human population, particularly in asymptomatic individuals, is crucial for understanding their potential role in immunity and viral dynamics. This study aimed to investigate the presence of nucleic acids of endemic HCoVs (HCoV-OC43, HCoV-HKU1, HCoV-NL63, and HCoV-229E) in individuals without symptoms of acute respiratory infection. A total of 78 adenoid tissue samples were collected from children (up to 10 years old) without current symptoms of airway infection. The samples were analyzed using RT-nested PCR to detect viral RNA. Of the 78 adenoid samples, 24 (30,8 %) tested positive for at least one type of endemic HCoV. Additionally, throat swabs were collected from 56 participants immediately before surgery. Endemic HCoV RNA was rarely detected in these throat swabs, with only 3 out of 56 samples testing positive. Our findings suggest the frequent presence of endemic HCoV nucleic acids in the lymphatic tissue of Waldeyer's ring, especially in children. This observation supports the concept of these viruses acting as immune triggers. The persistence of viral remnants in the adenoid tissue may contribute to continuous immune surveillance, which could have implications for immunity to future infections and for understanding viral dynamics in asymptomatic individuals.