The study on varicella-zoster virus (VZV) immune response in the elderly is critical due to the heightened risk of severe complications associated with VZV reactivation in this age group. As individuals age, their immune system undergoes immunosenescence, reducing the ability to maintain immunity against latent infections. This makes older adults more susceptible to herpes zoster (HZ), a painful condition caused by VZV reactivation, and its complications, including postherpetic neuralgia and neurological sequelae. The necessity for such research is particularly acute in developing nations, where immunization coverage for VZV and other preventable diseases is frequently inadequate. Limited access to healthcare, lack of public health infrastructure, and financial constraints contribute to lower vaccination rates, leaving many elderly individuals unprotected. Understanding the immune mechanisms involved in VZV reactivation and evaluating vaccine efficacy in resource-limited settings can inform the development of cost-effective interventions tailored to these populations. In this study, we examined the immune profile of elderly individuals residing in high-density populated areas in Thailand, employing multiple immunological assays to evaluate humoral immune response (HIR) and cell-mediated immune response (CMIR) to VZV infection.

In this cohort, 98.6% of the elderly population exhibited seropositivity for VZV, and all VZV-seropositive individuals demonstrated seropositivity for CMV. The VZV seropositivity rate was higher in elders than in adults, with binding antibody (BAb; VZV gp-specific IgG and gH/gL-specific IgG) levels increasing with age. This concurs with other studies showing that the percentage of VZV seropositivity and antibody levels increase with age^19,20. A recent study in Thailand reported a VZV seropositivity rate of 95.9% in individuals aged 50–59 years and reached 100% in those aged ≥ 70 years, with higher antibody levels in older adults²¹. We found that males had higher VZV-BAb levels than females, agreeing with a study on healthcare workers in Taiwan²⁰. However, the reason for increased BAb levels in the elderly remains unclear and requires further investigation.

Although BAb is useful for VZV seroepidemiology, it may not reflect the antibody functionality. This study measured neutralizing antibody (NAb) levels and found that only a subset of VZV-seropositive individuals exhibited neutralizing activity. While BAb levels increased with age, absolute NAb levels remained stable, suggesting that the majority of antibodies in the elderly were BAb. This indicated an increase in the proportion of non-NAb in older individuals, coinciding with the increased ratio of BAb to NAb, particularly for VZV gH/gL-specific IgG to NAb. This study revealed an age-dependent decline in the proportion of NAb against VZV, indicating that the elderly had a response less focused on VZV-neutralizing domains, which may have led to diminished protective immunity and contributed to a higher incidence of VZV reactivation in the elderly. Further research is required to investigate the role of non-NAb in controlling VZV reactivation. This could provide deeper insights into the mechanisms through which the immune system regulates viral latency and reactivation. Furthermore, the ratio of binding to neutralizing antibodies may potentially be utilized to indicate whether an older individual requires vaccination.

Several studies have shown a decline in VZV-specific CMIR with age^4,8,22,23; however, our findings indicated no significant differences in VZV-specific T cell phenotype and function across elderly age groups, although differences were noted between elderly and adult individuals. We observed a marginal increase in the percentage of VZV-specific IFN-γ-secreting CD4⁺ T cells in the elderly compared with in adults. This inconsistency might be due to variations in the study populations or antigens used for stimulation. Similarly, a study conducted in Japan revealed no age-related decrease in VZV-specific CMIR as measured by IFN-γ ELISPOT assay; however, it observed a decline when assessed by skin test²⁴.

We noted a reduction in the naïve subset and an increase in T_CM and T_EM cells among CD4⁺ and CD8⁺ T cells, along with an increase in T_EMRA CD8⁺ T cells, in the elderly. These cells exhibited higher dysfunction levels, as evidenced by increased expression of an exhaustion marker (PD-1) in CD4⁺ T cells and a senescent marker (CD57) in CD8⁺ T cells, supporting previous studies^8,9. This highlights the onset of immunosenescence, suggesting that older adults may be more vulnerable to severe HZ and may have a weaker response to HZ vaccination. Notably, males had a lower percentage of VZV-specific IFN-γ-secreting CD4⁺ and CD8⁺ T cells than females. This observation suggests that elderly males may exhibit a higher susceptibility to HZ reactivation compared to females in our cohort; however, this finding contradicts previous reports of higher HZ incidence in females²⁵. This discrepancy may be attributed to factors beyond immunological considerations²⁶.

While CCR7 is a valuable and widely used marker for studying T cell phenotypes^27,28,29,30, our findings suggest potential limitations in its application for T cell subset analysis. Specifically, our observations indicated that CCR7 expression exhibited sensitivity to in vitro stimulation, demonstrating a decreasing trend in the CD4⁺ T cell subsets and an increasing trend in the CD8⁺ T cell subsets (Supplementary Fig. S5). This phenomenon may potentially result in an underestimation of CCR7⁺ cells within the CD4⁺ T cell subsets and a corresponding overestimation within the CD8⁺ T cell subsets. To mitigate potential bias, it is advisable to incorporate additional markers, such as CCR7 with CD27 following stimulation, or to consider utilizing non-stimulation-based assays, such as the MHC-tetramer assay, for a more comprehensive assessment in subsequent studies.

At present, Thailand offers two types of HZ vaccines; the live-attenuated zoster vaccine (ZVL), recommended for individuals aged ≥ 60 years, and the recombinant zoster vaccine (RZV) for those aged ≥ 50 years¹⁴. Both vaccines are intended to reduce the incidence and severity of HZ in the Thai elderly population. However, these vaccines remain optional and are associated with significant costs. Our analysis revealed a significant reduction in the proportion of NAb in individuals aged ≥ 70 years. In conjunction with the existing evidence, these findings demonstrated consistent levels of functional T cells across the elderly population. This observation may suggest considering the inclusion of the HZ vaccination in the Thai Expanded Program on Immunization (EPI) for individuals aged ≥ 70 years, which could potentially enhance cost-effectiveness and more precisely target the population at greatest risk in Thailand.

Our results showed a correlation between VZV gp-specific IgG and gH/gL-specific IgG results, with only gH/gL IgG levels correlated with the NAb titer. VZV contains several glycoproteins (gp), with gH/gL being the third most abundant and highly immunodominant³¹ which could explain the correlation between VZV gH/gL-specific IgG and gp-specific IgG levels. The correlation between VZV gH/gL-specific IgG and NAb is consistent with studies suggesting that antibodies targeting the gH/gL domain can inhibit VZV infection^32,33,34. Antibodies targeting the VZV gH/gL domain exhibited the strongest neutralizing capacity³², explaining the correlation between the gH/gL-specific IgG level and NAb titer. In contrast, antibodies targeting other gp domains demonstrate minimal or no neutralizing capacity³², which may account for the lack of correlation between gp-specific IgG levels and NAb titers. Therefore, VZV gH/gL-specific IgG may be a good adjunct marker for VZV-NAb. Additionally, we observed an inverse correlation between the percentage of VZV-specific IFN-γ-secreting CD4⁺ T cells and VZV gp-specific IgG levels. This is consistent with previous findings showing a declining trend in VZV-specific IFN-γ ELISPOT response as VZV gp-specific IgG increases in the elderly²⁴.

A recent report identified an association between VZV-specific antibodies and chickenpox history in a Thai population²¹. However, our study did not observe a similar association with the history of HZ or HZ vaccination. This discrepancy may be attributed to recall bias, as participants self-reported their history of HZ and HZ vaccination. Furthermore, the small number of individuals who experienced HZ and received the HZ vaccine may have limited our ability to detect a significant association. Our investigation demonstrated that VZV-specific BAb, NAb, and IFN-γ-secreting T cells were not associated with frailty status in Thai elderly individuals. These findings are consistent with those of previous studies, which similarly did not observe an association between these immune parameters and frailty^35,36. It is expected that VZV-specific CMIR and CMV-specific CMIR will produce similar results, as both viruses belong to the same Herpesviridae family and may induce similar T cell changes through similar mechanisms. We observed a correlation between VZV gp-specific IgG and CMV-specific IgG levels in this cohort. This finding supports previous studies indicating that CMV infection is associated with VZV reactivation^11,12. These studies observed a co-elevated level of VZV-specific antibodies and CMV-specific antibodies in patients with HZ. CMV infection contributes to immunosenescence, resulting in less T cell responsiveness to VZV compared to CMV-seronegative individuals³⁷. This impairs VZV control and leads to VZV reactivation, which prompts the production of antibodies and leads to increased levels of antibodies against both VZV and CMV.

Our study had some limitations. First, we focused solely on NAb as a functional antibody without exploring the roles of non-NAb, such as those involved in antibody-dependent cellular cytotoxicity (ADCC). Investigating these functions would provide a more comprehensive understanding of the antibody-mediated immune response to VZV³⁸. Second, our analysis relied exclusively on VZV-specific T cells secreting IFN-γ, whereas assessing polyfunctional T cell responses involving a broader range of Th1 cytokines could offer deeper insights into CMIR to VZV^9,39. Third, the study included a limited number of VZV- and CMV-seronegative samples, and unequal sample sizes between comparison groups may have reduced the statistical power of our analysis. Future studies with equal sample sizes and greater representation of seronegative individuals are needed to validate these findings. Lastly, further exploration of the immune response to VZV vaccination among the Thai elderly could provide valuable insights.

In conclusion, our study revealed that VZV-BAb levels increased with age, while the proportion of NAb declined, which was significantly lower in elderly aged ≥ 70 years. In the elderly population, these antibodies are predominantly BAb rather than NAb, reflecting non-neutralizing domain-focused antibody responses. T cell phenotype and function against VZV were consistent across the elderly age groups; however, there was a notable decline in naïve T cells and an accumulation of aged T cells expressing markers of exhaustion and senescence, indicative of immune dysfunction. This immune profile suggests a progressive decline in immunity to VZV in older adults, potentially contributing to an increased susceptibility to VZV reactivation and more severe cases of HZ. These insights could guide the development of age-specific HZ vaccination campaigns.