Over 90% of the COVID-19 patients manifest mild/moderate symptoms or are asymptomatic. Although comorbidities and dysregulation of immune response have been implicated in severe COVID-19, the host factors that associate with asymptomatic or mild infections have not been characterized. We have collected serial samples from 23 hospitalized COVID-19 patients with mild symptoms and measured the kinetics of SARS-CoV-2 viral load in respiratory samples and markers of inflammation in serum samples. We monitored seroconversion during the acute phase of illness and quantitated the amount of total IgG against the receptor-binding domain of SARS-CoV-2 and estimated the virus neutralization potential of these antibodies. Viral load decreased by day 8 in all the patients but the detection of viral RNA in saliva samples did not correlate well with viral RNA detection in nasopharyngeal/oropharyngeal swab samples. 25% of the virus-positive patients had no detectable neutralizing antibodies in the serum and in other cases, the efficiency of antibodies to neutralize SARS-CoV-2 B1.1.7 strain was lower as compared to the circulating virus isolate. Decrease in viral load coincided with increase in neutralizing antibodies and interferon levels in serum. Most patients showed no increase in inflammatory cytokines such as IL-1β or IL-6, however, elevated levels of IL-7 and other inflammatory mediators such as TNF-α and IL-8 was observed. These data suggest that most mild infections are associated with absence of inflammation coupled with an active innate immune response, T-cell activation and neutralizing antibodies.
Over 95% of the COVID-19 cases are mild-to-asymptomatic who contribute to disease transmission whereas most of the severe manifestations of the disease are observed in elderly and in patients with comorbidities and dysregulation of immune response has been implicated in severe clinical outcomes. However, it is unclear whether asymptomatic or mild infections are due to low viral load or lack of inflammation. We have measured the kinetics of SARS-CoV-2 viral load in the respiratory samples and serum markers of inflammation in hospitalized COVID-19 patients with mild symptoms. We observed a bi-phasic pattern of virus load which was eventually cleared in most patients at the time of discharge. Viral load in saliva samples from a subset of patients showed good correlation with nasopharyngeal samples. Serum interferon levels were downregulated during early stages of infection but peaked at later stages correlating with elevated levels of T-cell cytokines and other inflammatory mediators such as IL-6 and TNF-alpha which showed a bi-phasic pattern. The clinical recovery of patients correlated with decrease in viral load and increase in interferons and other cytokines which indicates an effective innate and adaptive immune function in mild infections. We further characterized one of the SARS-CoV-2 isolate by plaque purification and show that infection of lung epithelial cells (Calu-3) with this isolate led to cytopathic effect disrupting epithelial barrier function and tight junctions. Finally we showed that zinc was capable of inhibiting SARS-CoV-2 infection in this model suggesting a beneficial effect of zinc supplementation in COVID-19 infection.
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