A virological analysis of nine hospitalized patients with COVID-19 revealed active virus replication in upper respiratory tract tissues. High viral RNA levels were detected in throat swabs, peaking at 7.11 × 10⁸ RNA copies per throat swab on day 4. Infectious virus was isolated from throat and lung samples but not from stool samples, despite high viral RNA concentrations. Blood and urine samples tested negative for SARS-CoV-2. Active replication in the throat was confirmed by the presence of viral replicative RNA intermediates. Sequence-distinct virus populations were detected in throat and lung samples from one patient, indicating independent replication. Viral RNA shedding from sputum outlasted the end of symptoms, and seroconversion occurred in 50% of patients by day 7, with no rapid decline in viral load. The study highlights that COVID-19 can present as a mild upper respiratory tract infection, with active virus replication in the upper respiratory tract, which has implications for containment. The study also found that SARS-CoV-2 has a close genetic relationship with SARS-CoV, and that the predominant expression of ACE2 in the lower respiratory tract may have influenced the natural history of SARS. However, the study found that SARS-CoV-2 replicates in the upper respiratory tract, which is different from SARS. The study also found that SARS-CoV-2 can replicate in the gastrointestinal tract, as evidenced by the detection of subgenomic mRNA in stool samples. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum,A virological analysis of nine hospitalized patients with COVID-19 revealed active virus replication in upper respiratory tract tissues. High viral RNA levels were detected in throat swabs, peaking at 7.11 × 10⁸ RNA copies per throat swab on day 4. Infectious virus was isolated from throat and lung samples but not from stool samples, despite high viral RNA concentrations. Blood and urine samples tested negative for SARS-CoV-2. Active replication in the throat was confirmed by the presence of viral replicative RNA intermediates. Sequence-distinct virus populations were detected in throat and lung samples from one patient, indicating independent replication. Viral RNA shedding from sputum outlasted the end of symptoms, and seroconversion occurred in 50% of patients by day 7, with no rapid decline in viral load. The study highlights that COVID-19 can present as a mild upper respiratory tract infection, with active virus replication in the upper respiratory tract, which has implications for containment. The study also found that SARS-CoV-2 has a close genetic relationship with SARS-CoV, and that the predominant expression of ACE2 in the lower respiratory tract may have influenced the natural history of SARS. However, the study found that SARS-CoV-2 replicates in the upper respiratory tract, which is different from SARS. The study also found that SARS-CoV-2 can replicate in the gastrointestinal tract, as evidenced by the detection of subgenomic mRNA in stool samples. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum, which may contribute to its transmission. The study also found that SARS-CoV-2 has a prolonged viral shedding period in sputum,