COVID-19: An updated comprehensive review

More than six years after the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), coronavirus disease 2019 (COVID-19) remains a complex global health challenge shaped by continued viral evolution, heterogeneous population immunity, and unequal access to prevention and care. The transition from the acute pandemic phase has changed the relationship between infection, hospitalization, and mortality, while reduced testing and surveillance complicate assessment of the contemporary burden. Ongoing diversification of Omicron descendants, waning protection against infection, and recurrent exposure have introduced additional uncertainties regarding transmissibility, immune escape, and clinical risk. Beyond acute respiratory illness, COVID-19 encompasses multisystem complications and persistent postinfectious symptoms, requiring an integrated appraisal of virological, immunological, clinical, and public health evidence. The aim of this narrative review is to critically synthesize current knowledge of SARS-CoV-2 epidemiology, genomic evolution, transmission, pathogenesis, host immune responses, and the spectrum of clinical manifestations. It also examines advances and limitations in diagnostic testing, severity-stratified therapeutics, vaccination, nonpharmaceutical interventions, and the recognition and management of long COVID. Particular attention is given to evidence gaps surrounding reinfection, immune durability, variant-specific risk, postacute disease mechanisms, equitable access to countermeasures, and sustainable surveillance. By connecting these domains, the review seeks to provide an updated framework for interpreting COVID-19 evidence and identifying priorities for clinical practice, public health policy, and future research.