In the face of a global COVID-19 vaccine shortage, an efficient vaccination strategy is required. Therefore, the immunogenicity of single or double COVID-19 vaccination doses (ChAdOX1, BNT162b2, or mRNA-1273) of SARS-CoV-2-recovered individuals was compared to that of unvaccinated individuals with SARS-CoV-2 infection at least one year post-convalescence. Moreover, the immunogenicity of SARS-CoV-2-naïve individuals vaccinated with a complete schedule of Ad26.CoV2.S, ChAdOX1, BNT162b2, mRNA-1273, or ChAdOX1/BNT162b2 vaccines was evaluated. Anti-SARS-CoV-2 S1 IgG antibody (S1-IgG), pseudotyped virus-neutralizing antibody titer (pVNT50), and IFN-γ ELISpot counts were measured. Humoral immune responses were significantly higher in vaccinated than in unvaccinated recovered individuals, with a 43-fold increase in the mean pVNT50 values. However, there was no significant difference in the pVNT50 and IFN-γ ELISpot values between the single- and double-dose regimens. In SARS-CoV-2-naïve individuals, antibody responses varied according to the vaccine type: BNT162b2 and mRNA-1273 induced similar levels of S1-IgG to those observed in vaccinated, convalescent individuals; in contrast, pVNT50 was much lower in SARS-CoV-2-naïve vaccinees than in vaccinated recovered individuals. Therefore, a single dose of ChAdOX1, BNT162b2, or mRNA-1273 vaccines would be a good alternative for recovered individuals instead of a double-dose regimen.

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The prevalence of varicella is especially high among children in the age group of 4–6 years in South Korea, regardless of vaccination. We investigated the immune status of healthy children enrolled in day-care centers and compared pre- and post-vaccination immunity. Antibody titers were measured using a glycoprotein enzyme-linked immunosorbent assay (gpEIA) kit, and the seroconversion rate was assessed using a fluorescent antibody to membrane antigen (FAMA) test. Among 541 vaccinated children, 109 (20.1%) had breakthrough varicella. However, 13 (72.2%) of the 18 unvaccinated children had a history of varicella. The gpEIA geometric mean titers (GMTs) of pre- and 5 weeks post-vaccination in 1-year-old children were 14.7 and 72 mIU/mL, respectively, and the FAMA seroconversion rate was 91.1%. The gpEIA GMTs of 2-, 3-, 4-, 5-, and 6-year-old children were 104.1, 133.8, 223.5, 364.1, and 353.0 mIU/mL, respectively. Even though the gpEIA GMT increased with age, the pattern of gpEIA titer distribution in 4- to 6-year-old vaccinees without varicella history represented both waning immunity and natural boosting immunity. These results suggest that some vaccinees are vulnerable to varicella infection. Therefore, it is necessary to consider a two-dose varicella vaccine regimen in South Korea.

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The currently used Japanese Oka and Korean MAV/06-attenuated varicella vaccine strains belong to clade 2 genotype varicella–zoster viruses (VZV). More than seven clades of VZV exist worldwide. In this study, we investigated the cross-reactivity of antibodies induced by clade 2 genotype vaccines against VZV strains belonging to clades 1, 2, 3, and 5 using a fluorescent antibody to membrane antigen (FAMA) test. Among 59 donors, 29 were vaccinated with the MAV/06 strain MG1111 (GC Biopharma, South Korea) and the other 30 were vaccinated with the Oka strain VARIVAX (Merck, USA). The sera were titrated using FAMA tests prepared with six different VZV strains (two vaccine strains, one wild-type clade 2 strain, and one each of clade 1, 3, and 5 strains). The ranges of geometric mean titers (GMTs) of FAMA against six different strains were 158.7–206.5 and 157.6–238.9 in MG1111 and VARIVAX groups, respectively. GMTs of the MG1111 group against all six strains were similar; however, GMTs of the VARIVAX group showed differences of approximately 1.5-fold depending on the strains. Nevertheless, the GMTs of the two vaccinated groups for the same strain were not significantly different. These results suggest that both MG1111 and VARIVAX vaccinations induce cross-reactive humoral immunity against other clades of VZV.