A negative serum control was run each time with the assay. years were compared. The quantitative level of anti-spike IgG was significantly lower in individuals aged 60 and men aged 18C59 years. There were 7.5% of poor or non-responders among the 18C59 years and 11.7% of poor or non-responders in the 60 years using RAF709 a four-fold increase parameter. There were 37.0C58.1% with low lymphocyte count (<1000/mm3), 33.3C45.2% with low CD4 cell counts (<500/mm3), and 74.1C96.8% with low B cell counts (<100/mm3) in the non-seroconversion group. An individual with an anti-SARS-CoV-2 spike IgG titer below 50 BAU/mL might be considered a poor or non-responder between 14 and 90 days after the last vaccine dose. Booster vaccination or additional protective measures should be recommended to poor or non-responders as soon as possible to reduce disease severity and mortality. Keywords: COVID-19, RAF709 vaccine, non-responders, international standard, B cell immunity, BAU/mL, cut-off, IgG 1. Introduction Coronavirus disease (COVID-19), caused by severe acute respiratory syndrome coronavirus RAF709 2 (SARS-CoV-2), has been declared a pandemic. The virus has infected more than 640 million people and caused more than 6.6 million deaths [1]. Since December 2020, the World Health Organization (WHO) recommends vaccination against COVID-19, nine types of COVID-19 vaccines have been included in the emergency use list [2]. Vaccination against COVID-19 is especially important in reducing severe illnesses and mortality. According to the data from the Centers for Disease Control and Prevention (CDC) in 2016C2017, the mortality rate caused by influenza virus was 0.13% [3]. To bring the COVID-19 pandemic under control as soon as possible and ensure that the mortality rate is close to that caused by the influenza virus, the prevention and treatment of children, as well as older adults and immunocompromised people, has emerged RAF709 as a top priority [4,5,6,7,8]. Sun et al. have reported that hospitalization and severe outcomes were similar in unvaccinated healthy individuals and immunocompromised patients who received two doses of SARS-CoV-2 vaccination in the United States, suggesting that COVID-19 breakthrough infection after vaccination is associated with immune dysfunction. Hospitalization and severe outcomes were 21.1% and 1.9%, respectively, in unvaccinated healthy individuals, and 20.7% and 2.1% in patients with immune dysfunction after 14 days following full vaccination, indicating that an immune barrier is not well established. Therefore, post-vaccination serologic testing (PVST) is necessary to identify immunocompromised individuals without specific immunity so that they can be given additional prophylaxis after full vaccination [8]. This study suggests that PVST helps reduce mortality, demonstrating the importance and urgency of PVST using an international standard. To date, more than CDX4 5 billion people have been vaccinated against COVID-19 [9]. In clinical trials associated with COVID-19 vaccination, the efficacy of COVID-19 vaccination to elicit specific B cell or antibody responses has been reported. Effective immunogenicity or humoral immune response is defined as a 4-fold increase in antibody titers from baseline and is considered the gold standard for assessing specific B cells or antibody protection in vaccinated recipients in clinical studies [10,11,12]. In contrast, a poor or nonresponder is an individual who demonstrates no effective immunogenicity or humoral immune response despite the completion of the suggested vaccination procedure [13,14]. Non-responders to the hepatitis B vaccine have been described. Szmuness et al. have reported that 7.4% of immunized individuals fail to elicit detectable specific antibodies after two doses of the hepatitis B vaccine, suggesting that there are nonresponders in the population [15]. Roome et al. have found that 11.9% of individuals with the hepatitis B vaccine had no or inadequate levels of antibody, suggesting that PVST should be performed at intervals of 30 to 90 days after the last vaccine dose [16]. Repeated poor or non-responders to a third or fourth dose of the SARS-CoV-2 vaccine were first observed in transplant recipients. Caillard et al. reported a cohort of 92 renal transplant recipients who did not have effective seroconversion after the third dose of mRNA vaccines [17]. Furthermore, there were 52.9% (18/34) of poor or non-responders to BNT162b2 vaccine and 48.3% (28/58) of poor or non-responders to the mRNA-1273 vaccine after the fourth dose of mRNA vaccines, showing that PVST should be performed after the third or fourth vaccine dose. Poor or non-responders to the SARS-CoV-2 vaccine are vulnerable populations with.