Serological response to a third dose of SARS-CoV-2 vaccine according to previous infection history

The IgG antibody titer against SARS-CoV-2 receptor binding protein (RBD) after mRNA vaccine were compared between those with and without previous infection (PI) for up to 48 weeks. Though sustained higher IgG-RBD were observed in the PI group after two doses of vaccines, both groups benefited from the booster shots of the third vaccine. This data supports the necessity of the booster shots to those with PI.


a b s t r a c t
The IgG antibody titer against SARS-CoV-2 receptor binding protein (RBD) after mRNA vaccine were compared between those with and without previous infection (PI) for up to 48 weeks. Though sustained higher IgG-RBD were observed in the PI group after two doses of vaccines, both groups benefited from the booster shots of the third vaccine. This data supports the necessity of the booster shots to those with PI. Antibody responses induced by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines are influenced by several host factors [1]. As recently reported by Pozo-Balado et al. [2], age is one of the most important determinants of the immune response; therefore, the need for a third vaccination of vulnerable individuals has been emphasized [3,4]. However, the effect of a third dose of vaccine on individuals with a history of previous infection, which strongly influences immune status [5], has not been sufficiently reported. As a result of the recent rapid spread of infection, an increased number of people have been exposed to SARS-CoV-2. Therefore, evidence regarding the need for a third dose of vaccine in those with a history of previous infection is urgently required.
We conducted a prospective cohort study of immune responses to the BNT162b2 vaccine among healthcare workers (HCWs) in Nagasaki, Japan from March 2021 to May 2022. Written informed consent was obtained from all participants. The second dose of vaccine was administered 3 weeks after the first dose. Blood samples were collected seven days before (T1), and seven (T2), and 14 days (T3) after the first dose of vaccine, and seven days (T4) after the second dose of vaccine, and at 12, 24, 36, and 48 weeks (T5 to T8). During the study period, the participants were provided the opportunity to receive a third dose of vaccine in accordance with the national policy of duty to endeavor to receive vaccination. Immunoglobulin G (IgG) against the SARS-CoV-2 receptorbinding domain (RBD) and the SARS-CoV-2 nucleocapsid (N) was measured using a chemiluminescence immunoassay ( titer was expressed as the mean titer in arbitrary units (AU)/mL. The significance of differences between groups was assessed using Tukey's multiple comparison test. Anti-N IgG was used to exclude participants who acquired SARS-CoV-2 infection during the study period. This study was approved by the institutional review board of Nagasaki University Hospital (approval number 21030401-2).
A total of 162 participants, including 49 previously infected (PI group) and 113 not previously infected (NI group), were included in the study. The mean ages were 42.7 for the PI group and 37.9 for the NI group. Among them, three participants in the NI group were excluded from the analysis, one because of suspected SARS-CoV-2 infection (indicated by an increase in anti-N IgG titer), one on immunosuppressive therapy, and one on hemodialysis. Fig. 1A shows the timeline of anti-RBD IgG titers in the NI and PI groups. The anti-RBD IgG titer in the PI group increased rapidly in the first 7 days after the first dose of vaccine (measured at T2) and then plateaued. In contrast, the anti-RBD IgG titer in the NI group increased more gradually and only attained a level comparable to that of the PI group 7 days after receiving a second dose of vaccine (at T4). After the anti-RBD IgG titer reached its peak, it declined gradually in both the groups, but the decline was more rapid in the NI group.
A third dose of vaccine was administered to all participants (110/110) in the NI group with a median of 289 days after the first dose, between T7 and T8, and 44 of the 49 participants in the PI group with a median of 273 days after the first dose, between T6 and T8. To assess the booster effect of the third dose of vaccine, we compared the anti-RBD IgG titers in the NI and PI groups before and after the third dose of vaccine (Fig. 1B) ). Notably, the anti-RBD IgG titer of the NI group after the third dose was significantly higher than that of the PI group before the third dose (P < 0.0001).
This longitudinal prospective cohort study showed that IgG RBD levels change according to vaccination status and PI status over 48 weeks. Several researchers have proposed that a reduced number of doses of vaccine could be considered for those with previous SARS-CoV-2 infection. [6,7] Our results suggest that individuals with and without PI benefit from receiving a third dose of vaccine, but the timing of vaccination should differ because higher IgG RBD titers were sustained for longer in the PI group after two doses of vaccine. Notably, we found that the mean antibody titer was lower in the non-boosted PI group than in the boosted NI group 1 week after the third dose, suggesting an inferior infection prevention effect of PI without booster. Based on this result, a history of previous infection should not preclude the administration of a third dose of vaccine.
This study has some limitations. The participants were relatively young, and the effect of age was not investigated. However, these results suggest that it is important to administer a third dose of vaccine to vulnerable individuals, who are likely to have a similar or greater decline in antibody titer. Another limitation is that we did not evaluate the effectiveness of the vaccine at preventing SARS-CoV-2 infection. Consistent with this result, a recent metaanalysis by Bobrovitz et al. [8] concluded that hybrid immunity contributes to the greatest magnitude and durability of protection, and that tailoring vaccination schedules according to infection history may be warranted. There are individual variations in the immune response; therefore, individual assessment based on measurement of antibody titers could play an important role in timing of booster vaccination.
In conclusion, this study demonstrated the anti-RBD IgG titer before and after vaccination in individuals with or without a previ-

Funding
This study was funded by Abbott Japan LLC, Tokyo, Japan. This work was supported by MEXT KAKENHI Grant Number JP21418576, a Grant-in-Aid for Early-Career Scientists.

Author contributions
KO analyzed and interpreted the data and made a major contribution to writing the manuscript. KI, DS, and TU obtained the samples and measured the antibody titers. SM, F-MK, KS, KK, HH, TT, KI, HM, and SK contributed substantially to the conception and design of the study, drafted the manuscript, and substantively revised it. AF, NA, YH, ST, and OM recruited participants and managed the study. KY contributed to funding, management, and supervision of the study.
All authors have read and approved the submitted version of the manuscript and have agreed to be accountable for their own contributions and to ensure that questions related to the accuracy or integrity of any part of the work, even those in which the author was not personally involved, are appropriately investigated and resolved and that the resolution is documented in the literature.

Data availability
Data will be made available on request.

Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: SM is an employee of Abbott Japan LLC.