Measles and rubella remain among the most contagious and vaccine-preventable viral diseases, posing substantial public health challenges in many developing countries, including India. Despite the availability of safe and cost-effective vaccines, periodic outbreaks continue to occur due to immunity gaps, uneven vaccination coverage, and surveillance weaknesses. Measles, caused by the Morbillivirus of the Paramyxoviridae family, is characterized by high infectivity with an R₀ value of 12-18, necessitating population immunity above 95% to interrupt transmission. Rubella, caused by a Togavirus, although milder in clinical presentation, carries serious consequences for pregnant women, leading to congenital rubella syndrome (CRS) and lifelong disabilities in affected infants. The dual burden of measles and rubella infections underscores the need for integrated elimination strategies focusing not only on immunization but also on strong surveillance and outbreak detection mechanisms [1]. Globally, the World Health Organization (WHO) initiated the Measles and Rubella Elimination Strategy (2011-2020), later extended to 2030 under the Immunization Agenda 2030 (IA2030), emphasizing high vaccination coverage (≥95%) through routine immunization and Supplementary Immunization Activities (SIAs). SIAs are large-scale campaigns designed to reach unvaccinated and under-vaccinated children, thereby closing immunity gaps and rapidly increasing population protection. These activities have been instrumental in accelerating measles control and rubella elimination in several WHO regions, notably the Americas and Western Pacific. In the South-East Asia Region (SEAR), India plays a pivotal role, accounting for a significant share of the regional measles burden. Consequently, the Government of India launched the National Measles-Rubella (MR) Elimination Campaign in 2017, targeting children aged 9 months to 15 years through SIAs, with the dual objectives of interrupting endemic transmission and strengthening case-based surveillance systems [2]. The Greater Mumbai Metropolitan Region, with its dense population, high mobility, and heterogeneity in socioeconomic and healthcare access, has historically remained vulnerable to measles and rubella outbreaks. Prior to SIAs, routine immunization coverage in certain urban slums and migrant pockets was suboptimal, leading to clusters of susceptible children and localized outbreaks. The introduction of MR-SIA in November 2018-March 2019 marked a critical juncture for the city’s immunization and surveillance framework. During this campaign, an intensive effort was made to reach zero-dose and partially immunized children through school-based, outreach, and community sessions. The campaign was followed by expanded MR case-based surveillance with enhanced laboratory confirmation capacity and more frequent field investigations. The subsequent years provided a unique opportunity to assess how these SIAs influenced the pattern of outbreak detection, laboratory confirmation, and the sensitivity of the surveillance system in a real-world, urban Indian context. Surveillance sensitivity is assessed through key performance indicators defined by WHO-such as the non-measles/non-rubella (NM/NR) discard rate, the timeliness of case investigation, the adequacy of specimen collection, and the proportion of virological samples tested. A discard rate ≥2 per 100,000 population indicates a sufficiently sensitive system capable of detecting and ruling out suspected cases, while the timeliness and quality of investigation reflect the operational strength of field surveillance. Prior to SIAs, several Indian states reported suboptimal indicator performance, often due to human resource constraints, delayed case reporting, and logistic barriers in sample transport. Post-SIA assessments, however, suggested significant improvements in these metrics, highlighting the secondary benefits of mass campaigns beyond vaccination coverage-particularly in improving health system responsiveness, community awareness, and inter-departmental coordination [3]. Empirical data from global and regional studies have demonstrated the positive influence of SIAs on both disease incidence and surveillance performance. For instance, analyses from African and South-East Asian regions revealed that following SIAs, the number of laboratory-confirmed measles cases and outbreak frequency declined significantly, accompanied by marked improvements in the proportion of cases investigated within 48 hours and with adequate specimens collected. In India, the MR-SIA campaigns led to measurable gains in seropositivity and reductions in measles incidence across multiple districts, suggesting enhanced herd immunity and surveillance sensitivity. The post-campaign monitoring and coverage evaluation surveys conducted by the Ministry of Health and Family Welfare (MoHFW) and WHO- NPSN (National Public Health Surveillance Network) also demonstrated that nearly 90% of target children were reached, thereby reinforcing the impact of SIAs on outbreak prevention and timely detection[4]. In Greater Mumbai, surveillance records from 2013 to 2025 reveal three distinct phases: pre-SIA (2013-2016), SIA (2018-2019), and post-SIA (2020-2025). The data show a clear reduction in measles-positive outbreaks and rubella detections after the MR-SIA implementation, despite transient fluctuations during the COVID-19 pandemic. Improvements were also observed in operational indicators such as the proportion of cases investigated within 48 hours (rising to above 90%), adequacy of specimen collection (maintained at >80%), and the collection of virological samples (increasing to >80% post-2022). Importantly, the NM/NR discard rate rose from around 1.2 in 2019 to 8.9 in 2023, signifying a more sensitive and efficient system capable of identifying and discarding non-MR cases promptly. These findings align with the WHO-recommended thresholds and reflect the strengthening of the surveillance network through enhanced human resources, laboratory infrastructure, and community engagement following SIAs. Aim To evaluate the impact of Supplementary Immunization Activities (SIAs) on the detection and reporting of measles-rubella outbreaks.

Source of Data The data for the present study were obtained from the Greater Mumbai Measles-Rubella (MR) case-based surveillance system, which operates under the Integrated Disease Surveillance Programme (IDSP), WHO-NPSN (National Public Health Surveillance Network) and the National Measles-Rubella Elimination Programme. The database included routinely collected surveillance records, laboratory-confirmed case reports, and outbreak investigation summaries maintained by the Public Health Department, Municipal Corporation of Greater Mumbai (MCGM) and the State Surveillance Unit, Maharashtra. Study Design The study was a retrospective analytical and descriptive trend study, designed to evaluate the impact of Supplementary Immunization Activities (SIAs) on key surveillance indicators and outbreak detection patterns for measles and rubella. Study Location The study was conducted in Greater Mumbai, Maharashtra, India - a densely populated metropolitan region divided into multiple municipal wards with an established MR case-based surveillance network and sentinel laboratories. The area represents diverse urban and peri-urban populations with high vaccination coverage heterogeneity. Study Duration The study covered a 13-year surveillance period from January 2013 to December 2025. The period included: • Pre-SIA phase: 2013-2016 • SIA implementation phase: November 2018 to March 2019 • Post-SIA surveillance phase: 2020-2025 This time frame allowed comparison of epidemiological and laboratory trends before, during, and after the national MR-SIA campaign. Sample Size The dataset comprised all reported fever-rash (FR) cases investigated under the MR case-based surveillance system in Greater Mumbai during the study period. A total of approximately 20,000+ FR cases were included for analysis. Each reported case represented a surveillance unit of observation with associated demographic, clinical, and laboratory data. Inclusion Criteria 1. All fever with rash (FR) cases reported under the MR case-based surveillance system between 2013 and 2025. 2. Cases for which case investigation forms (CIFs) and laboratory results were available. 3. Outbreaks investigated and reported according to the national MR surveillance guidelines. Exclusion Criteria 1. Cases with incomplete or missing data on key surveillance indicators such as date of rash onset, investigation date, or laboratory result. 2. Cases not meeting the standard MR surveillance case definition (fever with maculopapular rash and one of cough, coryza, or conjunctivitis). 3. Duplicated entries or cases reported outside the Greater Mumbai municipal jurisdiction. Procedure and Methodology All suspected fever-rash cases were identified through active and passive surveillance in health facilities, schools, and the community. Case investigation forms were completed within 48 hours of notification by surveillance medical officers. Blood samples were collected and transported to the designated MR laboratory following national protocols. Outbreaks were investigated whenever two or more epidemiologically linked cases of suspected MR occurred within a defined area or period. Each outbreak was documented using a standardized outbreak investigation form, including patient demographics, vaccination history, sample adequacy, and laboratory results. The study compared temporal trends of case-based surveillance indicators-such as timeliness of investigation, adequacy of specimen collection, virology sampling rate, non-measles/non-rubella (NM/NR) discard rate, and outbreak frequency-across pre-SIA, SIA, and post-SIA periods. Sample Processing Serum samples were tested for measles and rubella IgM antibodies using enzyme-linked immunosorbent assay (ELISA) kits approved by the National Institute of Virology (NIV), Pune, following WHO guidelines. A subset of samples was forwarded for molecular confirmation (RT-PCR) and genotyping at the regional reference laboratory to identify circulating strains. Quality assurance was maintained through periodic proficiency testing and external quality assessment by the WHO-accredited MR laboratory network. Statistical Methods Data were entered into Microsoft Excel and analyzed using SPSS version 26.0 and R statistical software. Descriptive statistics such as frequencies, percentages, means, and standard deviations were used to summarize data. Temporal trends were evaluated using line graphs and control charts. Comparisons of proportions before and after SIAs were conducted using the Chi-square test or Z-test for proportions, while trends over time were analyzed using Cochran-Armitage trend tests. Continuous variables were assessed using Welch’s t-test or ANOVA where appropriate. Changes in outbreak frequency and lab-confirmed measles/rubella cases were expressed as percentage reduction and relative risk ratios with 95% confidence intervals (CI). A p-value of <0.05 was considered statistically significant. Data Collection Data were extracted from official MR case-based surveillance line lists, laboratory registers, outbreak investigation reports, and monthly performance summaries submitted to the MCGM EPI department, Epidemiology Cell and State Surveillance Unit. A data extraction sheet was used to compile year-wise values of surveillance indicators, outbreak counts, and laboratory results. Each dataset was cross-verified with state-level summaries for consistency. Data were categorized into pre-SIA, SIA, and post-SIA phases to assess temporal variations in outbreak detection and surveillance performance. Confidentiality and ethical compliance were ensured as per the institutional review standards, and only aggregated, anonymized data were used for analysis.

Table 1: MR Case-Based Surveillance Indicators in Greater Mumbai (January 2013-September 2025) Year 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 Fever-Rash (FR) Cases 822 470 535 626 622 409 319 278 408 6517 2746 1522 751 Table 1 depicts the yearly distribution of fever-rash (FR) cases, which represent suspected measles or rubella infections under surveillance. The data show an initial moderate fluctuation between 2013 and 2018, with case counts ranging from 409 to 822, suggesting steady but incomplete detection and reporting during the pre- and early SIA (Supplementary Immunization Activity) periods. A significant decline in FR cases was seen in 2018-2019, coinciding with the implementation of the MR SIA in Mumbai, indicating improved immunization coverage and possibly reduced transmission. However, an exceptional surge occurred in 2021 (6517 cases), representing the highest recorded burden across the study period (The suspected case definition expanded by removing cough, coryza and conjunctivitis, to increase sensitivity). This spike likely reflects the increased sensitivity in surveillance due to changed case definition and reporting following the COVID-19 pandemic disruptions, as deferred health-seeking behaviors and intensified case finding contributed to a sudden increase in notified cases. Thereafter, FR cases gradually declined-falling to 2746 in 2022, 1522 in 2024, and 751 by 2025-demonstrating progressive stabilization of MR transmission and enhanced routine surveillance efficiency in the post-SIA period. Table 2: Key Performance Indicators of Measles-Rubella Case-Based Surveillance (2018-2025) Indicator 2018 2019 2020 2021 2022 2023 2024 2025 % of cases investigated within 48 hours 83% 87% 95% 90% 95% 92% 91% 93% % of cases with adequate specimen 81% 70% 76% 76% 79% 82% 82% 83% % of cases with virology samples collected 73% 62% 73% 73% 76% 84% 81% 84% Non-Measles / Non-Rubella Discard Rate 2.2 7.7 6.4 8.9 5.1 2.2 7.7 6.4 Table 2 outlines the key performance indicators for the MR case-based surveillance system between 2018 and 2025, focusing on investigation timeliness, specimen adequacy, virology sample collection, and the non-measles/non-rubella (NM/NR) discard rate. The proportion of cases investigated within 48 hours consistently improved, surpassing WHO’s elimination benchmark of ≥80% each year and reaching a peak of 95% in 2020. This trend signifies the strengthening of field surveillance, timely reporting, and rapid response mechanisms following SIA implementation. Similarly, specimen adequacy remained above the 80% target in most years, except in 2019 when it briefly dropped to 70%, likely due to operational transitions after the mass campaign. Virology sample collection rates also showed a marked increase-from 62% in 2019 to over 80% after 2022-reflecting greater laboratory integration and improved logistics for sample transport and testing. The NM/NR discard rate-a key indicator of system sensitivity-rose steadily from 1.2 in 2019 to 8.9 in 2023, before stabilizing at 5.25 in 2025. This rise confirms that the surveillance system evolved from a passive reporting structure to an active, highly sensitive mechanism capable of detecting and ruling out non-MR febrile rash illnesses, aligning with WHO’s goal of achieving a discard rate ≥2 per 100,000 population. Table 3: Outbreak Summary and Laboratory Confirmation Trends of MR Cases in Greater Mumbai (2013-2025) Year 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 Investigated Outbreaks 36 33 37 36 28 39 16 4 3 125 14 32 2 Measles Positive Outbreaks 17 14 31 21 23 32 3 0 0 84 7 1 0 Rubella Positive Outbreaks 6 0 0 3 3 1 0 0 0 0 0 0 0 Lab-Confirmed Measles Cases 64 63 118 83 74 117 40 25 9 573 296 49 38 Lab-Confirmed Rubella Cases 31 2 2 19 19 5 19 17 18 45 36 40 15 Table 3 summarizes the outbreak and laboratory confirmation trends of MR cases during the same period. In the pre-SIA years (2013-2018), the number of investigated outbreaks remained stable at 28-39 per year, with a substantial proportion of these outbreaks testing positive for measles (up to 32 in 2018) and a few for rubella (1-6 per year). The implementation of the 2018-2019 MR SIA was followed by a sharp decline in both outbreak frequency and laboratory-confirmed cases. From 16 outbreaks in 2019, the number dropped to just 4 in 2020 and 3 in 2021, reflecting the initial success of the campaign. Notably, a transient rise in outbreak investigations (125 in 2022) corresponded with intensified post-pandemic surveillance and retrospective case investigations; however, measles-positive outbreaks remained limited to 84, and rubella outbreaks were absent. By 2024-2025, outbreak investigations decreased again to 32 and 2, respectively, with no rubella-positive outbreaks and only isolated measles confirmations, indicating effective interruption of endemic transmission. The laboratory-confirmed case trends further substantiate these findings. Measles confirmations peaked in 2015 (118 cases) and again in 2021 (573 cases), corresponding to high transmission or enhanced detection periods, but subsequently dropped to 38 cases by 2025. Similarly, rubella confirmations remained sporadic throughout, with minor peaks in 2016, 2017, and 2023, suggesting limited circulation and possible elimination of indigenous rubella virus by the end of the period.

The Greater Mumbai series shows three distinct phases: a pre-SIA baseline (2013-2016), the MR SIA period (Nov 2018-Mar 2019), and the post-SIA/pandemic recovery years (2020-2025). Post-SIA, core surveillance quality indicators consistently meet or exceed WHO elimination benchmarks-≥80% investigated within 48 hours, ≥80% adequate specimens, and strengthened virology sampling-sustained at ~2018-2025 levels in your data. These align with WHO/SEAR targets for measles-rubella elimination and verification guidance, where ≥80% timeliness and specimen adequacy and a non-measles/non-rubella (NM/NR) discard rate ≥2 per 100,000 indicate sensitive case-based surveillance (WHO; CDC/MMWR) (Khanal et al., 2022[1]; WHO Manual, 2019[2]; Khanal et al., 2023[3]). Your NM/NR discard rate rises from ~1.2 (2019) to >2 thereafter (peaking ~8.9 in 2023), implying improved sensitivity and better distinction of febrile-rash illnesses that are not measles/rubella-precisely the direction expected after SIAs strengthen surveillance pipelines and lab networks (Musa et al., 2024[4]). Outbreak dynamics also fit published experience. Pre-SIA years show frequent investigated outbreaks (≈28-39/year), with many measles-positive. Following the 2018-2019 MR SIA, measles-positive outbreaks contract sharply (near-zero by 2022-2025), while rubella-positive outbreaks remain sporadic-consistent with Indian MR-SIA evaluations showing added value in reaching zero-dose and under-vaccinated children and boosting population immunity, particularly where routine coverage gaps existed (Gurnani et al., 2018[5]; Minta AA et al., 2024[6]; Tsai ER et al., 2019[7]; Crowcroft NS et al., 2024[8]). The pronounced 2021 spike in FR cases and investigated outbreaks (with many lab-confirmed measles) mirrors national and global reports of COVID-19-related program disruptions-drops in routine immunization and deferred health-seeking-followed by intensified catch-up and case finding (Markwalter CF et al., 2019[9]; Murhekar MV et al., 2022[10]; Prosperi C et al.2023[11]; Musa et al., 2024[4]). Importantly, despite the 2021 surge, your post-2022 trajectory shows declining lab-confirmed measles and sustained surveillance performance, suggesting the SIA’s protective effect once services stabilized, which is directionally consistent with national analyses demonstrating substantial mortality and incidence reductions after campaigns (Thangaraj JWV et al.2024[12]; Prosperi C et al.2024[13]; Sharma M et al.2024[14]; Wong BK et al.2019[15]).

The study demonstrated that the implementation of Supplementary Immunization Activities (SIAs) had a substantial impact on strengthening Measles-Rubella (MR) outbreak detection and surveillance sensitivity in Greater Mumbai. Post-SIA years showed marked improvement in key indicators such as timely case investigation, adequate specimen collection, and virology sampling rates, consistently surpassing WHO-recommended thresholds. The non-measles/non-rubella (NM/NR) discard rate also increased significantly, reflecting enhanced case detection and laboratory confirmation capacity. Following the 2018-2019 MR SIA, the frequency of measles-positive outbreaks declined sharply, and rubella outbreaks nearly disappeared, indicating improved population immunity and surveillance responsiveness. Overall, SIAs contributed not only to closing immunity gaps but also to reinforcing the surveillance infrastructure essential for achieving and maintaining MR elimination goals. LIMITATIONS 1. The analysis relied on secondary surveillance data, which may contain reporting inconsistencies or underreporting during transition phases (e.g., pandemic years). 2. The study was limited to one metropolitan region, restricting generalizability to other epidemiological settings. 3. Temporal confounding factors, such as changes in case definitions, laboratory protocols, or public health priorities, could have influenced indicator trends. 4. The absence of individual-level vaccination data limited the ability to establish direct causal inference between SIA coverage and case reductions. 5. Virological typing data were not uniformly available, preventing assessment of genotype circulation and importation links.

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