Introduction: As the daily number of patients diagnosed with SARS-CoV-2 infection by PCR increases, the necessity to identify truly infectious cases becomes more significant. We aimed to identify a cut-off Ct value of the COVID-19 RT-PCR assay for likely infectivity by assessing the COVID-19 IgG status and investigating the utility of the Rapid Antigen Test (RAT) in identifying infectious cases among asymptomatic individuals. Methods: Nasopharyngeal/throat swabs were simultaneously taken for COVID-19 RT-PCR and RAT from 552 asymptomatic individuals at De Soysa Maternity Hospital, Colombo, from 23rd of November to 19th of December 2020. In addition, SARS-CoV-2 IgG (against nucleoprotein) status in PCR positive individuals was evaluated when simultaneously taken sera was available. Results: COVID-19 RT-PCR positive rate among asymptomatic individuals was 14.3% (n=79).  The overall sensitivity of RAT was 30.4% but increased to 73.9% when Ct values below 25 were considered. The COVID-19 IgG response was evaluated in 37 PCR positive subjects and the overall seropositivity was 40.5%.  The optimal Ct thresholds for discrimination of COVID-19 IgG status were 30.5 and 30.29 for the E and S gene respectively. There was a significant positive correlation between Ct values of the E gene and IgG ratio values (r=0.345, p<0.05). The Ct thresholds for RAT positivity were 26.5 and 26.06 for E and S genes respectively, with a significant negative correlation (p<0.001). Conclusion: We conclude that it is possible to define a cut off Ct value in SARS-CoV-2 PCR (with some error margin for practical purposes) for likely non-infectivity. It was also deduced that positive COVID-19 rapid antigen result seems to be more predictive of infectivity in comparison to positive PCR result.

AZD1222 (Covishield) COVID-19 vaccination response among healthy adults and chronic renal disease patients has been compared in this analysis. Compared to healthy individuals, vaccine induced immune response seems to be suboptimal after two doses of the vaccine in patients with chronic renal disease on hemodialysis. Infection following full course of vaccination can boost the vaccine induced antibody levels in both healthy individuals and patients with chronic renal disease.It is necessary to conduct comprehensive prospective research to further evaluate these conclusions made based on this preliminary data evaluation, and to find out its overall impact for future SARS-CoV-2 infection and its disease outcome.