Author’s Response

Dr. Sarman Singh has raised important considerations regarding surrogate markers of vaccine protection and the broader challenges associated with assessing vaccine efficacy using immunological endpoints.¹ His observations highlight the continuing difficulty in identifying reliable immunological correlates of protection for several vaccines.

I agree that immunological markers, including antibody titres, cellular immune responses and other laboratory indicators, are important tools for understanding vaccine-induced immunity and for informing research, surveillance and programme evaluation. The issues raised in the letter underscore the need for continued investigation into such correlates of protection, particularly for vaccines where definitive immunological endpoints remain uncertain.

It may be useful to clarify that the objective of the YUVA/YAII article was not to examine these immunological questions in detail.² Rather, the article sought to outline a policy-oriented framework aimed at strengthening life-course immunization in India, with particular emphasis on the young adult population. The focus of the framework was therefore on programmatic and structural aspects of immunization systems, including policy alignment, delivery platforms, coverage expansion and the integration of adult vaccination within existing public health infrastructure. The intention was to position adult immunization within a broader public health and systems perspective, while recognizing that evolving scientific evidence, the including immunological correlates of protection, will continue to inform programme refinement over time.

Within such a systems-oriented framework, insights from immunological research such as those related to vaccine non-responders, waning antibody titres or surrogate markers of protection, can contribute to programme monitoring and targeted surveillance. Such information may help identify groups that could benefit from additional follow-up or booster strategies, thereby strengthening the effectiveness of adult immunization efforts.