Where index cases were not positive for Nabs, this may have been because the time from infection to testing was >6 weeks or low sensitivity of Nab detection tests. household contacts, and a no-contact group from the same area. == Results == A total of 770 participants were included (355 index cases, 103 household contacts, and 312 no contacts). All index cases were unvaccinated, but >90% of individuals in the household and no-contact groups had received 1 vaccine dose. SARS-CoV-2 neutralizing antibodies (Nabs) were present in >77% of unvaccinated index cases versus 64%/65.4% in the household/no-contact groups (p=0.001). Antibody concentrations in unvaccinated index cases were significantly higher than those in household contacts and no contacts, with no difference between the latter groups. In all cases, antibody levels declined markedly 6 weeks after contamination, and failed to persist beyond this time in the household and no-contact groups. == Conclusion == Community-based care may have helped to create community immunogenicity, but Nabs did not persist, highlighting a need for vaccination for all those individuals before, or from 6 weeks after, contamination with SARS-CoV-2. Keywords:Community, COVID-19, immunogenicity, pandemic, SARS-CoV2 == Graphical Abstract == == INTRODUCTION == Although the early response to the pandemic in Vietnam was excellent,1cases of coronavirus disease 2019 (COVID-19) began to increase dramatically in early July 2021.2This outbreak came when vaccination rollout was just beginning; and therefore, vaccination rates in the population were relatively low, despite good acceptance of the available vaccines.3Ho Chi Minh City was one of the worst affected areas, recording 8000 to 9000 new cases every day. 4The healthcare response included the establishment of hundreds of community centers and temporary hospitals. However, case numbers exceeded the capacity of both newly established facilities and crucial care hospitals. Therefore, in response to a call CY3 for help, the University of Medicine and Pharmacy at Ho Chi Minh City developed and implemented a community care model for the management of individuals with COVID-19 in District 10 of Ho Chi Minh City (area: 5.72 km2, populace: 234819). The community care model was based on three main principles: home care; providing monitoring and care at a distance; and providing timely emergency care if needed. One team supported cases at home with frequent contacts/remote monitoring, while a second team transferred and cared for cases requiring treatment at field emergency care facilities. This approach CY3 appeared to have some benefit due to the lower mortality rate in District 10 (0.43%) compared with Ho Chi Minh City over the same period (4.95%).4 Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a CY3 highly infectious computer virus.5The risk of infection in household members and other close contacts of COVID-19 cases was shown to be high,6while knowledge and implementation of preventive measures were low to moderate.7,8,9Therefore, one of the major concerns surrounding the application of the community care model was the transmission of SARS-CoV-2 from infected individuals (index cases) to uninfected individuals sharing the same house or living nearby during the lockdown period. Living quarters in the area of interest (District 10) were quite cramped, with as many as 45 people sharing a space of about 10 m2. Although some immune protection from close contact with individuals who have COVID-19 has been documented,10there is limited data around the seroprevalence of antibodies against SARS-CoV-2 in individuals who are in lockdown with confirmed COVID-19 cases. Seroprevalence data would help to better understand infectivity patterns and inform future vaccination rollouts, as well as pandemic responses/strategies. This study investigated the characteristics of immunogenicity against SARS-CoV-2 in household members and people who lived nearby to home-quarantined patients with COVID-19 in District 10, Ho Chi Minh City, Vietnam. The primary objective was to determine the prevalence of positive SARS-CoV-2 neutralizing antibody (Nab) and concentration of SARS-CoV-2 immunoglobulin G (IgG) in household contacts of positive COVID-19 cases. MCM7 Secondary objectives were: to determine the prevalence of positive SARS-CoV-2 Nab and SARS-CoV-2 IgG concentration in index cases; to compare SARS-CoV-2 Nab and SARS-CoV-2 IgG levels between index cases, household contacts, and no contacts; and to investigate associations between demographic factors, medical history and self-quarantine compliance for index cases and the prevalence of SARS-CoV-2 Nabs in the household contacts and no-contact groups. == MATERIALS AND METHODS == == Study design == This cross-sectional study was conducted in three blocks (V, X, Y) of Ngo Gia Tu apartment, District 10, Ho Chi Minh City from July to September 2021. All participants provided written informed consent prior to inclusion, and the study was approved by the Institutional Ethic Committee (number: 473/HDDD-DHYD, 21 September 2021). == Study participants == All individuals aged >18 years who lived in the specified apartment blocks and had the ability to understand the written informed consent document were eligible. Those with primary or secondary.