The authors were interested in the shedding pattern of SARS-CoV-2 and patient antibody responses before and after the onset of symptoms. They used the Diamond Princess cruise ship as a simulation of the basic functions of a city. Adult passengers from Hong Kong who had been onboard the cruise ship docked in Yokohama, Japan in February 2020 were recruited. All participants had been found to be negative for SARS-CoV-2 by RT-PCR 4 days before disembarking and were then transferred to further quarantine in a public estate in Hong Kong. Participants were prospectively screened using quantitative RT-PCR (RT-qPCR) of nasopharyngeal and throat swabs, and serum IgG and IgM against internal nucleoprotein and the surface spike receptor-binding protein (RBD) of SARS-CoV-2 at baseline and on days 4, 8 and 12 of quarantine. 215 adults were recruited, of whom 4% were positive for SARS-CoV-2 by RT-qPCR or serology and were hospitalised. Of these nine patients, RT-qPCR of nasopharyngeal swabs were positive in 89% at baseline. All nine patients were positive for anti-RBD IgG by day 8. 89% of patients were simultaneously positive for nasopharyngeal swab RT-PCR and anti-RBD IgG. One patient who was positive for anti-RBD IgG and had a negative viral load was found to have multifocal peripheral ground-glass changes on high-resolution CT. 56% of patients with ground-glass changes on high-resolution CT were found to have higher anti-nucleoprotein-IgG OD values on days 8 and 12, and anti-RBD IgG OD value on day 12, than patients without ground-glass changes. 67% of patients remained asymptomatic throughout the 14-day quarantine period. Therefore patients with COVID-19 can develop asymptomatic lung infection with viral shedding and those with evidence of pneumonia on imaging tend to have a higher antibody response.
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