This study intervention was performed 7.8?months after the second inoculation of BNT162b2 (Figure 1(a)). to the KD-414 group as a secondary outcome. A single dose of KD-414 induced lower serum neutralizing activity against the wild-type virus within 7?days compared to after the primary series of BNT162b2 but significantly induced anti-SARS-CoV-2-S1-receptor-binding domain-binding immunoglobulin G (IgG) antibodies and SARS-CoV-2-S peptide-specific CD4+ and CD8+ T cell responses. Local or systemic symptoms were significantly lower in the participants who received KD-414 than in those who received BNT162b2 as the third COVID-19 vaccine dose. The present data indicate that a single booster dose of KD-414 induces a substantial immune response in BNT162b2-primed individuals and has a good safety profile, thereby supporting further clinical trials to identify rational targets. KEYWORDS: COVID-19, SARS-CoV-2, COVID-19 vaccine, inactivated vaccine, side effects, adverse events, vaccine safety, neutralizing antibody, KD-414 Introduction The rapid spread of the novel coronavirus (severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]) disease 2019 (COVID-19) has resulted in the most serious global public health and socioeconomic disaster in this century.1 To act against the pandemic, more than 160 COVID-19 vaccines are still under development2 using multiple platforms.3 Among these, whole-virion inactivated vaccines directed toward Japanese encephalitis, hepatitis A, rabies, and poliovirus,4 represent one traditional approach. Unlike natural infection or live vaccines, inactivated vaccine-induced immunity Procaine HCl is generally not long-lasting, and multiple doses are required to boost the effect over time.5 On the other hand, inactivated vaccines contain the intact pathogen. Thus, the immune response is likely to target viral structural proteins such as the matrix, envelope, and nucleoprotein,3 and to stimulate toll-like receptors and Procaine HCl induce type 1 interferon (IFN).6 KD-414 is a purified inactivated whole SARS-CoV-2 virion-containing vaccine adjuvanted with aluminum hydroxide and developed by KM Biologics Co., Ltd. (Kumamoto, Japan).7 A double-blind, randomized, placebo-controlled phase I/II trial of KD-414 involving 210 healthy Nr4a1 adults in Japan with no prior COVID-19 vaccination or history of COVID-19 showed good tolerability for all ages tested.7 A high dose (10 ug/dose) used in the phase 1/2 study to evaluate the efficacy of three doses induced significant neutralizing antibody activity in age-dependent manner. This was especially evident in participants under 40?years of age, when the first two doses were administered intramuscularly 28?days apart.7 Administration of the third dose of KD-414 after 6 or more months from the second dose was also well tolerated and induced a higher and longer-lasting neutralizing antibody response compared with after the second dose.7 More than 80% of Japanese people in their 20s and 30s have already completed the primary series of vaccinations, mostly using an mRNA platform. 8 It is therefore important to evaluate the security and immunogenicity of booster vaccinations. Here, we statement the immunogenicity and security of a single booster vaccination with KD-414 after main vaccination with two doses of the BNT162b2 mRNA vaccine in Japanese volunteers. The aim was to determine whether a single booster dose of KD-414 after a primary series of BNT162b2 induces considerable SARS-CoV-2-specific humoral and cellular responses with a strong security profile. Materials and methods Intervention; COVID-19 inactivated vaccine, KD-414 KD-414 is an inactivated vaccine comprising purified inactivated whole SARS-CoV-2 virions cultured in Vero cells (RRID: CVCL_0059)9 that provides significant immunogenicity following two-dose main series vaccination,7 especially in adults under 40?years of age. The vaccine is definitely adjuvanted with aluminium hydroxide to boost the immune response. A SARS-CoV-2 strain isolated in Japan in 2020 was utilized for vaccine production.7,9 One dose of KD-414 (0.5?mL) contains 10?g of inactivated whole SARS-CoV-2 virions. The concentration of aluminium hydroxide Procaine HCl in KD-414 is definitely 0.40?mg/mL (mainly because aluminium). The aluminium hydroxide used in KD-414 is the same as that used in the hepatitis B (HB) vaccine (Bimmugen) licensed in Japan. Study design and participants This open-label, non-randomized, single-arm study was carried out using 100 volunteer staff members of the National Center for Global Health and Medicine (NCGM) in Tokyo, Japan, from November 2021 to January 2022 (Qualified Review Table of NCGM authorization No. NCGM-C-004374, Japan Clinical Trial Registry No.: jRCTs031210388), as previously described.10 In brief, 100 individuals who participated inside a clinical trial to evaluate the antigenicity and safety of the primary series of BNT162b2 (Ethics evaluate committee of NCGM approval No. NCGM-A-004175)11 and adopted from March to June 2021 were selected as potential candidates (Number 1(a)). Blood samples were collected at 7 (PD7) and 40 (PD40) days after the primary series of BNT162b2. This study treatment was performed 7.8?weeks after the second inoculation of BNT162b2 (Number 1(a)). Blood samples were collected from individuals confirmed to be eligible and who agreed to participate in the present clinical study. All.