**1. Introduction**

Immunization has been considered one of the most cost-effective public health interventions worldwide [1]. Its efficacy and transparency have already been marked for eradicating smallpox globally. The launch of the Expanded Program of Immunization (EPI) by the World Health Organization (WHO) has configured the Universal Immunisation Program (UIP) which aims at achieving coverage of vaccines for all neonates, children, and pregnant women [1]. However, some vaccine-preventable diseases (VPDs) may equally affect the adult population. Moreover, owing to the current demographic transition in lowand middle-income countries (LMICs) such as India, the adult population is on the rise [1]. An increase in the adult population highlights the urgent need for their immunization to achieve universal health coverage (UHC). Adult immunization has become a major concern, especially in LMICs such as India where this group is more susceptible to acquiring diseases during outbreaks or various other conditions associated with non-communicable diseases (NCDs) [1,2]. Additionally, waning immunity and age-related factors (including immunosenescence) also highlight the need for adult vaccination [2,3].

The measles and rubella (MR) vaccine and consideration of the human papillomavirus (HPV) for potential inclusion in the UIP under the ambit of public vaccination efforts have brought the transition of childhood vaccination programs towards adolescents. Vaccines such as tetanus toxoid (TT) vaccines for pregnant ladies and Japanese encephalitis (JE) vaccines for adults residing in endemic districts are also provided free of cost by the

**Citation:** Singh, D.; Sinha, A.; Kanungo, S.; Pati, S. Disparities in Coverage of Adult Immunization among Older Adults in India. *Vaccines* **2022**, *10*, 2124. https:// doi.org/10.3390/vaccines10122124

Academic Editors: Ahmad Reza Hosseinpoor, M. Carolina Danovaro, Devaki Nambiar, Aaron Wallace and Hope Johnson

Received: 26 November 2022 Accepted: 6 December 2022 Published: 12 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

government. Moreover, TT and JE have the highest coverage among adult vaccines [1]. However, other adult vaccines such as influenza, pneumococcal, hepatitis B, typhoid, and diphtheria and tetanus vaccines are also available but are often underutilized. Still, adult vaccination remains a challenge, especially amongst deprived groups [2,4]. Additionally, various groups such as adults with multimorbidity (simultaneous occurrence of two or more chronic conditions in an individual without considering index disease) are at a higher risk of contracting VPDs and may immediately need vaccination [2,3].

Adult immunization is one of the keys to empowering a life course initiative for health care services. The sustainability of vaccines depends on availability and affordability. However, India does not have a clear mandate for providing universal adult vaccination which makes these vaccines underused. This becomes grave with the under-recognition of outbreaks and a deficiency of data on the real burden of VPDs among adults [1,5]. Further, this points toward an urgent need to generate evidence on the present adult vaccination coverage in India. This would help in planning future policies for transitioning conventional childhood and recent adolescent immunization towards adults to avert mortality and morbidity in this age group. Additionally, there is a dearth of literature on adult immunization coverage in India. The available studies represent either a city or some particular region with no national-level data. Hence, this study was conducted to estimate the coverage of the influenza vaccine, pneumococcal vaccine, hepatitis B vaccine, typhoid vaccine, and diphtheria and tetanus (DT) vaccine among adults in India using nationally representative data.
