Structural Changes in IgG4 Antibodies Induced by Repeated mRNA Vaccination and Their Association with Breakthrough Infections
Background
During the early stages of the coronavirus disease 2019 (COVID-19) pandemic, messenger ribonucleic acid (mRNA) vaccine platforms contributed significantly to pandemic control through rapid development and strong neutralizing antibody induction. However, the continuous emergence of viral variants has led to the routine administration of multiple booster doses. Existing studies have primarily focused on the quantitative titer of neutralizing antibodies to assess immune efficacy. The long-term effects of structural changes in antibodies or the detailed composition of immunoglobulin subclasses on the human immune system remain insufficiently understood. In particular, research on the direct relationship between the qualitative changes in antibodies induced by repeated vaccinations and the efficacy of infection prevention is still limited.
Key Findings
The research team used liquid chromatography-mass spectrometry (LC-MS) to precisely track antibody structural changes in relation to the number of vaccine doses. A Swedish healthcare worker cohort of 104 individuals participated in up to six doses, while an 18-member pediatric cohort from Singapore received up to three doses, supporting a comparative analysis of vaccine-induced antibody characteristics across age groups. Analysis revealed that repeated mRNA vaccination strongly induced class switching to immunoglobulin G4 (IgG4), a subclass known for not inducing inflammatory responses. This IgG4 response was observed to persist in the adult cohort for at least six doses. This subclass switch was distinctly observed in individuals without prior SARS-CoV-2 infection history before the first vaccination, in contrast to those who had already been exposed to the virus and experienced immune priming. A similar IgG4 structural profile was also observed in the pediatric cohort after the third dose. Notably, among 41 individuals without prior infection history and lacking mucosal IgA responses, those with higher vaccine-induced IgG4 switching showed a significantly increased risk of breakthrough infection (Hazard Ratio, HR 1.83, p=0.028). Additionally, prior to the full onset of IgG4 class switching, a unique biomarker pattern was observed in the early stage, characterized by high fucosylation levels in the Fc region of immunoglobulin G1 (IgG1).
Implications and Outlook
This study demonstrates that prior natural infection status before vaccine development can influence the nature of subsequent immune responses induced by vaccination. IgG4, which does not activate the complement system, tends to suppress immune responses. Therefore, the increase in IgG4 due to repeated vaccinations may contribute to increased vulnerability to breakthrough infections, independent of neutralizing capacity. However, the research team clearly emphasized that these findings do not question the efficacy or safety of the widely used mRNA vaccine platform itself. mRNA vaccines still contribute to neutralizing antibody formation and the prevention of severe disease. Nevertheless, to better understand the qualitative changes in the immune system caused by repeated vaccinations, long-term follow-up studies across diverse age groups, including children and adults, are necessary. Future research should also aim to verify the long-term impact of IgG4 increases on in vivo immune responses.
Repeated administration of mRNA vaccines against SARS-CoV-2 has been associated with qualitative changes in antibody responses, including the emergence of non-inflammatory IgG4 subclasses and changes in Fc glycosylation. While neutralizing antibody titers remain key correlates of protection, potential functional implications of these structural antibody features warrant further investigation across age groups and multiple mRNA vaccine boosters. We characterized spike-specific IgG responses-including subclasses and Fc glycosylation patterns-using a liquid chromatography-mass spectrometry-based approach across six mRNA vaccine doses in a Swedish healthcare worker cohort (n = 104) and across three mRNA doses in a Singaporean pediatric cohort (n = 18). Repeated mRNA vaccination induced an IgG4 class switch which was sustained across at least six doses. This response associated with an increased risk of breakthrough infection (HR = 1.83, p = 0.028) in infection-naïve individuals (n = 41) lacking mucosal IgA responses that would potentially confound the interpretation of systemic IgG4-related infection outcomes. Moreover, the IgG4 class switch was preceded with high early IgG1 Fc fucosylation signatures. Notably, these IgG structural changes were observed primarily in individuals who were naïve to SARS-CoV-2 at the time of first vaccination. Similar IgG structural features were observed across three mRNA doses in the small infection-naive pediatric cohort. Our results illustrate an IgG4-dominated immunological signature in both adults and children and highlight that immune priming by prior infection may shape subsequent mRNA vaccine-induced antibody responses. While these findings do not call into question the efficacy or safety of the widely adapted mRNA vaccine platform, they may have implications that merit further investigation.
These research findings suggest that future public health vaccination strategies should adopt a more personalized approach, taking into account individual immune histories rather than simply increasing the number of doses. In particular, individuals who have undergone immune priming through natural infection prior to vaccination and those who have not show different antibody production pathways, necessitating more precise adjustments in the timing and frequency of booster vaccinations for different groups. Furthermore, these results may serve as a basis for incorporating more detailed structural indicators, such as subclass switching ratios and Fc glycosylation patterns, into the evaluation criteria for new vaccine candidates. In the future, when designing mRNA vaccines for diseases requiring repeated booster doses or for therapeutic applications, these findings could provide useful baseline data for minimizing unwanted immune suppression and maximizing protective efficacy.