Histological and Ultrastructural Changes in Lungs One Year Post Asymptomatic or Mild SARS-CoV-2 Infection: Evaluating the Potential for Active Virus Presence

Prior research on post-COVID-19 or long COVID primarily focused on the presence of SARS-CoV-2 mostly in symptomatic patients. This study aimed to investigate the persistence of SARS-CoV-2 after 1 year of asymptomatic or mild COVID-19. SARS-CoV-2 infected and control K18-hACE2 transgenic mice (n=25) were studied. Moderate and severe symptomatic subjects were sacrificed after eight days, while mild or asymptomatic mice were kept in BSL-III for twelve months. Analyses included general condition, histochemistry, immunohistochemistry, transmission electron microscopy, and qRT-PCR. Lungs from the twelve-month group showed thickening of alveolar walls, with some lungs exhibiting the recruitment of inflammatory cells, the presence of SARS- CoV-2 mRNA, immunopositivity for the SARS-CoV-2 spike protein, and TEM showed viruses (60-125 nm) within vesicles, indicating continued replication. Certain lung samples showed persistent SARS-CoV-2 presence in Club cells, endothelial cells, and macrophages. The eight-day group exhibited viral interstitial pneumonitis, SARS-CoV-2 immunopositivity, and mRNA. The eight-day hearts displayed viral mRNA, while the twelve-month hearts tested negative. Some asymptomatic twelve-month subjects presented reduced surfactant, basal membrane thickening, fibrosis, and mild autonomic nerve degeneration. In this study conducted on mice, findings indicate the potential for chronic persistence of SARS-CoV-2 in the lungs one year post initial mild or asymptomatic infection, which could suggest the possibility of recurrent episodes in similar human conditions. The observed thickening of alveolar walls and potential fibrotic areas in these mice may imply an increased risk of post-COVID fibrosis in humans. Furthermore, the presence of SARS-CoV-2-positive inflammatory cells in some asymptomatic murine cases could herald a progression toward ongoing inflammation and chronic lung disease in humans. Therefore, the necessity for further studies in human subjects and vigilant monitoring of high-risk human populations is underscored.