In China, measles, rubella, and mumps are notifiable diseases monitored through the National Notifiable Disease Reporting System (NNDRS), a web-based platform that requires reporting of both clinically diagnosed and laboratory-confirmed cases. This surveillance system provided the data used to determine the incidence rates of measles, rubella, mumps, and varicella in Jiangsu Province analyzed in this study. Although varicella is not officially classified as a statutory notifiable disease, it is routinely surveilled and reported using the same protocols in practice in Jiangsu province. All reported cases are recorded by date of symptom onset and classified based on clinical diagnosis, laboratory confirmation, or epidemiological linkage, in accordance with national guidelines. Incidence rates are calculated per 100,000 population using demographic data from the National Bureau of Statistics. Since 1965, China has introduced a series of measles vaccination policy adjustments. These include the implementation of a single−dose schedule in 1978; a two−dose measles or measles–rubella schedule at 8 months and 7 years in 1986; rescheduling of the second measles−only dose to 18 months in 2005; and, in 2020, optimizing the program to a two−dose measles–mumps–rubella (MMR) schedule at 8 and 18 months, thereby introducing the more advantageous combined vaccine. For varicella, Jiangsu Province began piloting a two-dose schedule in select cities in 2020, with province-wide implementation by January 2023. These policy changes aim to improve population immunity and reduce the incidence of vaccine-preventable diseases.

Serum samples were obtained from the Jiangsu Provincial Surveillance Program on Immunization Levels for Vaccine-Preventable Diseases in Healthy Populations. Suqian City served as the single-center study site. Residual serum samples collected from healthy individuals in 2019 (n = 541) and 2024 (n = 506) were analyzed, yielding a total of 1,047 samples. To ensure that seroprevalence reflected vaccine-induced immunity, individuals with recent vaccination (within six months) or a confirmed history of measles, rubella, mumps, or varicella infection were excluded. A stratified random sampling approach was used to enhance representativeness, with stratification based on age, sex, and geographic distribution across Suqian’s districts and counties. Age groups were defined as follows: 0–11 months, 12–23 months, 2–5 years, 6–18 years, 19–29 years, and ≥30 years. These corresponded to birth cohorts of approximately 2018-2019, 2017-2018, 2014-2017, 2001-2013, 1990-2000, and ≤1989 for the 2019 sample; and 2023-2024, 2022-2023, 2019-2022, 2006-2018, 1995-2005, and ≤1994 for the 2024 sample. Within each age group, samples were stratified by sex to ensure an approximately equal male-to-female ratio. Geographic stratification was conducted according to population distributions derived from the National Bureau of Statistics of China, with sample sizes allocated proportionally across administrative divisions. Random sampling was then performed within each stratum. Identical sampling procedures were used in both years to ensure comparability. For participants under 14 years of age, vaccination records were verified using official immunization documents or the Jiangsu Provincial Vaccination Integrated Service Management Information System. For individuals aged 14 years and older, vaccination history was primarily obtained through self-report, and participants with missing or unverified records were categorized as the “unknown” group. These participants were retained in the analysis to reflect real-world immunity levels, as their exclusion could introduce selection bias, especially in adult populations where vaccination records are often incomplete. Disease history was collected via participant recall.

Blood samples were collected from all participants. Serum was separated and stored at –20°C until analysis. IgG antibodies specific to measles, mumps, rubella, and varicella were measured using enzyme-linked immunosorbent assay (ELISA) kits (SERION ELISA classic, Institut Virion/Serion GmbH, Würzburg, Germany). All testing was performed at the central laboratory of the Affiliated Suqian First People’s Hospital of Nanjing Medical University. Quantitative analysis was conducted to determine seroprotection levels. Antibody concentrations were interpreted according to the manufacturer’s guidelines, which are based on internal validation and referenced literature. The cutoff values for seroprotection were as follows: Measles: ≥200 mIU/mL, Mumps: ≥100 IU/mL, Rubella: ≥20 IU/mL, Varicella: ≥50 mIU/mL.

Data were analyzed using Microsoft Excel and R software. Seroprevalence estimates were calculated with 95% confidence intervals (CIs) using the Clopper–Pearson exact binomial method. Geometric mean concentrations (GMCs) of measles- and rubella-specific IgG antibodies were summarized using medians and interquartile ranges [M (IQR)]. For comparisons of seroprevalence across ordered categories, the trend chi-square test was applied; for comparisons among unordered groups, the standard chi-square test was used. The primary dependent variables included binary IgG serostatus (positive/negative) for each virus and GMCs of virus-specific IgG antibodies. Independent variables were age group, sex, vaccination history, and survey year. Because antibody concentrations are often skewed or contain outliers, non-parametric tests (Wilcoxon rank-sum and Kruskal–Wallis) were used to compare GMCs, ensuring robustness and validity without assuming normality. Overall comparisons of seroprevalence and GMCs between 2019 and 2024 were conducted. Subgroup comparisons by age, sex, or vaccination history were not performed due to limited sample sizes and the need to reduce multiple testing bias. However, subgroup results are presented descriptively for transparency. All statistical tests were two-sided, and p-values <0.05 were considered statistically significant.

As illustrated in Figure 1 , Jiangsu Province exhibited significant declines in reported cases and incidence rates of varicella, measles, rubella, and mumps (2005–2024). Varicella incidence decreased from 183.3 per 100,000 in 2019 to 59.3 per 100,000 in 2024 (67.6% reduction). Measles incidence declined from 11.2 per 100,000 to 0.01 per 100,000 (99.9% reduction). Rubella incidence dropped from 2.8 to 0.04 per 100,000 (98.6% reduction), and mumps incidence fell from 24.4 to less than 5.0 per 100,000 (79.5% reduction).

A total of 541 samples were collected in 2019 and 506 in 2024. The sex distribution was balanced in both years, with males and females each accounting for about half of the participants. In both years, participants were distributed across all age groups. The proportion of children aged 6–18 years was higher in 2024 (30.4%) than in 2019 (22.2%), while the proportion of those aged 0–5 years was slightly lower in 2024. MMR vaccination history improved over time. In 2019, 10.5% of participants had not received any MMR dose, while in 2024, all participants had received at least one dose. The proportion of participants with two or more MMR doses increased from 36.4% in 2019 to 48.0% in 2024 ( Table 1 ).

In 2019, the seropositivity rates for measles, rubella, mumps, and varicella were 85.9%, 65.1%, 77.6%, and 76.9%, respectively. By 2024, significant increases were observed in rubella (70.8%) and mumps (85.8%) seropositivity (p<0.05), while varicella also rose to 79.5%(p>0.05), indicating improved population immunity against these three diseases. Notably, measles seropositivity declined from 85.9% in 2019 to 79.3% in 2024 (p<0.05), suggesting potential immunity gaps in measles protection. Gender analysis revealed no significant differences in seropositivity rates between males and females in either 2019 or 2024 (P>0.05), indicating minimal gender-based variations in immune responses to these four diseases. Age-stratified analysis showed significant differences among age groups (P<0.05), with children aged 2–5 years demonstrating higher seropositivity rates for all four diseases in 2024, while those under 2 years (particularly 0–11 months) exhibited lower overall seropositivity. Regarding vaccination history, following the immunization strategy adjustment, the measles seropositivity rate among recipients of two doses of measles-containing vaccine decreased from 94.4% in 2019 to 88.1% in 2024, while significant improvements were observed for rubella (84.8%), mumps (91.4%), and varicella (77.9%) seropositivity rates ( Tables 2A, B , Figure 2 ).

GMC analysis revealed that from 2019 to 2024, measles antibody levels decreased from 577.9 mIU/ml (IQR: 298.2-1056.3) to 499.2 mIU/ml (IQR: 220.6-1181.1) (p<0.001), while antibody levels increased for rubella (31.5 to 42.2 IU/ml), mumps (277.8 to 350.6 IU/ml), and varicella (295.9 to 309.8 IU/ml). Gender analysis showed no significant differences in GMC for measles, rubella, mumps, and varicella between males and females in both 2019 and 2024 (Wilcoxon rank-sum test, p>0.05). Age group analysis indicated significant differences in GMC across different age groups for all four diseases (Kruskal-Wallis H test, p<0.001). Specifically, in 2024, children aged 2–5 years achieved the highest GMC values: 1367.2 mIU/ml (IQR: 649.8-2869.5) for measles, 74.9 IU/ml (IQR: 41.4-122.6) for rubella, 570.6 IU/ml (IQR: 288.7-1039.3) for mumps, and 133.8 mIU/ml (IQR: 35.4-262.5) for varicella. Vaccination history analysis also showed significant differences in GMC among different vaccination dose groups for the four diseases (Kruskal-Wallis H test, p<0.001). The GMC trends aligned with seropositivity patterns, showing noticeable increases for all three diseases except measles, which demonstrated a slight decline ( Tables 3A, B , Figure 2 ).