Background and Objectives Thailand has experienced the COVID-19 pandemic since mid-January 2020. Symptomatic COVID-19 patients are admitted for hospital inpatient treatment. At present, there is limited information on the association between treatment time and dynamics of viral load, as well as incidence of sero-conversion of household close contacts of COVID-19 patients after the patients are discharged and return home. Such information may have implications for stakeholders in COVID-19 prevention and control. The objectives of this study were: 1) to describe the association between treatment period and presence of viable virus in respiratory fluids of COVID=19 patients, and; 2) to describe the incidence of sero-conversion among household contacts of discharged COVID-19 patients. Methods For Objective 1, we conducted a retrospective cohort study among COVID-19 patients at Songklanagarind Hospital and an affiliated field hospital who were admitted in early 2020 and received at least two nasopharyngeal swab tests. We extracted data regarding patient characteristics and rRT-PCR results in the hospital information system, and sent leftover swab samples to the Department of Medical Sciences' central laboratory for viral culture with anonymized unique ID designated for each sample. We then merged culture results data with the rest of the data set and performed data analyses using descriptive statistics and regression analyses. For Objective 2, we conducted a prospective cohort study among household close contacts of COVID-19 patients who were discharged from the study hospital. Investigators contacted the patients and asked for the contact details of their adult household members, then contacted the mentioned household members and invited them to participate in the study. After giving informed consent, investigators briefly interviewed the household contacts (study participants) and collected blood samples on the day of the discharge or day after the discharge (Day 0), at 7 days after discharge (Day 7), and at 28 days after discharge (Day 28). Data were analyzed using descriptive statistics. Results For objective 1, there were 20 patients in the data set, 16 of whom met the study inclusion criteria with adequate data available (n=16 patients). The median length of stay was approximately 28 days, and more than half of the patients did not have positive viral culture in any sample. Among patients with at least one sample with positive culture, while the median length of time from admission to the date when viral culture became consistently negative was 10 days. There were large fluctuations in the patients' rRT-PCR cycle time during the course of treatment, but the rRT-PCR cycle time gradually increased and became plateaued after the first 15 days. Pattern of changes in cycle time differed between patients with viable virus in swab samples and patients without viable virus in swab samples. Discussions and Conclusion For objective 1, we found indications that patients might have unnecessarily overstayed at the hospital during their treatment. However, the study findings should be used selectively and cautiously, with due considerations for the study limitations including limited generalizability and lack of statistical power. For Objective 2, the absence of evidence on infection based on RT-PCR test results (which cannot be published) suggested that there was no new COVID-19 infection among the study participants, and thus there was no evidence that the 10-days stay guidelines for treatment of COVID-19 patients had become inadequate for prevention of intra-household COVID-19 transmission. Caveats regarding the lack of generalizability and potential effect of vaccination should be considered in the interpretation of the study findings.