Policy window period: analysis of HPV vaccination status among age-eligible girls in China (before national immunisation programme implementation)

Abstract

Objective

To analyse the human papillomavirus (HPV) vaccination status among age-eligible girls in Fujian Province, China, during 2022–2023; to assess the relationship between vaccination rates and economic levels; and to provide scientific evidence to advance HPV vaccination programmes.

Method

The subjects of this study were girls aged 13-14.5 years in Fujian Province, China, from 2022 to 2023. The HPV vaccination information was collected from the Chinese Immunization Information System. The data on Gross Domestic Product (GDP) per capita and per capita disposable income were from the official statistical yearbook of Fujian Province.

Results

A total of 499,188 girls aged 13-14.5 years were vaccinated during the study period. The HPV vaccine coverage rate was 83.55%. The vaccination rate varied across 9 cities in Fujian province. The free vaccination rate was negatively correlated with GDP per capita and per capita disposable income. The self-funded vaccination rate was positively correlated with GDP per capita and per capita disposable income. For both free and self-paid vaccines, the strength of association was stronger for the first dose than for the second. For every 1 yuan increase in per capita disposable income, the free first-dose HPV vaccination coverage rate decreased by 0.569%. In contrast, the first-dose self-funded HPV vaccination coverage rate increased by 0.666%. Similarly, the second-dose free HPV vaccination coverage rate decreased by 0.509%, and the second-dose self-funded HPV vaccination coverage rate increased by 0.519%.

Conclusion

During the free domestic bivalent vaccine policy period, both GDP per capita and per capita disposable income influenced HPV vaccination rates, and the direction of this association showed opposite trends according to the type of vaccine (free or self-paid). Future strategies should transition from universal coverage to stratified precision approaches, implementing differentiated vaccine subsidy and supply policies based on regional economic development levels.

Introduction

Globally, cervical cancer is the second most common cancer among women [1]. The latest age-period cohort model indicates that the incidence and mortality rates of cervical cancer in China have increased over the past three decades [2]. There is detailed evidence-based research that supports the assertion that persistent infection with high-risk human papillomavirus (HPV) is the primary cause of cervical cancer [3], with HPV 16 exhibiting the highest carcinogenic potential [4]. The successful development of HPV vaccines demonstrates the feasibility of eradicating cervical cancer through vaccination [5]. Increasing HPV vaccination rates among girls can prevent multiple cancers and related diseases caused by high-risk HPV infection, thereby reducing cervical cancer incidence and mortality rates, improving population health, and promoting health equity.

In its 2020’ Global Strategy to Accelerate the Elimination of Cervical Cancer’, the World Health Organization (WHO) proposed three key measures for cervical cancer prevention and control: vaccination, screening, and treatment. This strategy aims to achieve the ‘90-70-90’ targets by 2030: the first 90% represents the proportion of girls who should be vaccinated against HPV before the age of 15. In its updated position paper from 2022, the WHO recommends that the primary target group for HPV vaccination should be girls aged 9–14 who are not yet sexually active [6]. Some countries have implemented HPV vaccination programmes for extended periods, with several achieving high coverage rates among target populations. For example, more than 90% of adolescent girls in Brunei, Norway, Mexico, Rwanda, and Turkmenistan have been vaccinated [7]. As of May 2024, 138 countries worldwide had incorporated preventive HPV vaccines into their national immunization programs [8].

Since HPV vaccines were approved for market release in China in 2017, vaccination policies have varied across regions, with some areas piloting free vaccination programmes for eligible girls. Taking into account regional disparities, cost-effectiveness, public acceptance, and organisational feasibility, the 2023 edition of China’s Comprehensive Cervical Cancer Prevention and Control Guidelines recommends prioritising HPV vaccination for 13- to 14-year-old girls in provinces capable of providing free immunisation [9].

In 2022, Fujian Province initiated a free vaccination program with a domestically produced bivalent HPV vaccine for age-eligible girls. Specifically, the domestic bivalent HPV vaccine is provided free of charge, while all other HPV vaccines are administered at full cost to recipients. This study analyzed factors associated with HPV vaccination during the 2022–2023 policy implementation period to inform policy adjustments and targeted interventions.

Materials and methods

Study population

The study population included girls aged 13 to 14.5 years in Fujian Province, China, during 2022–2023. The inclusion criteria were household registration or school registration in Fujian province. No exclusion criteria were used in this study.

Vaccine information

The free vaccine during the study period was a domestic bivalent vaccine. Self-paid vaccines include bivalent, quadrivalent, and nine-valent HPV vaccines imported from abroad.

Study setting and data sources

This observational study used cross-sectional designs. Individual HPV vaccination records from 2022 to 2023 were sourced from the Chinese Immunization Information System, with each recipient uniquely identified by a vaccination number. The records contained personal information, including age at vaccination, vaccination site, vaccination date, and vaccine type. Following identity verification via “ID-Person Matching”, the vaccination data were entered into the immunization system. The economic indicators used in this study were extracted and collated from the official China Statistical Yearbook. Specifically, we queried and recorded the GDP per capita and per capita disposable income of all counties and districts in Fujian Province during the study period.

Specifically, we queried and recorded the GDP per capita and per capita disposable income of all counties and districts in Fujian Province during the study period. Vaccination rates were calculated at the county or city level. Data were pooled at the county level. Each observation represented a county district, and the unit of analysis was the county district. Linear regression was used to analyze the ecological association between socioeconomic indicators (GDP per capita and per capita disposable income) and each county’s vaccination rate. Because this was an ecological analysis, the results reflect population-level associations and cannot be directly generalized to individuals. This study primarily analyzed the relationship between vaccine coverage and regional economic levels. Its secondary outcome concerned the distribution and variation of vaccine coverage across the nine cities.

Statistical analysis

Statistical analysis was performed using SPSS 29.0 software. Count data were expressed as n (%), and the differences in HPV vaccination rates across regions were compared using the χ² test, corrected χ² test, or Fisher’s exact probability method. To identify the specific sources of differences in vaccination rates among the 9 prefecture-level cities, we conducted post hoc multiple comparisons using the Bonferroni correction after the overall chi-square test indicated statistically significant differences. Since this study involved 9 prefecture-level cities, pairwise post-hoc comparisons were required a total of 36 times. Therefore, the originally set two-sided significance level α was adjusted to α’ = α/36. The Pearson or Spearman method was used to analyze the correlation between the free first-dose HPV vaccination coverage rate, the first-dose self-funded HPV vaccination coverage rate, the second-dose free HPV vaccination coverage rate, the second-dose self-funded HPV vaccination coverage rate, and GDP per capita and per capita disposable income, respectively. The absolute value of the Pearson or Spearman coefficient indicates the degree of correlation, with larger values indicating stronger correlation. We performed multivariate linear regression analyses using region-level aggregated data. The dependent variables were the free first-dose HPV vaccination coverage rate, the first-dose self-funded HPV vaccination coverage rate, the free second-dose HPV vaccination coverage rate, and the self-funded second-dose HPV vaccination coverage rate. GDP per capita and per capita disposable income were included as independent variables (predictors). Separate regression models were fitted for each vaccination outcome to examine the association between vaccination uptake and these socioeconomic indicators. The regression coefficient βrepresents the linear mean difference in the dependent variable: a one-unit increase in each independent variable is associated with a β-unit change in the dependent variable, holding the other variables constant. All statistical tests were two-sided, with α = 0.05. Two independent personnel entered and verified the original database twice. External source data were entered and verified twice using EPi Data 4.6.

Results

Overall HPV vaccination status among age-eligible girls in Fujian Province

A total of 597,440 girls in Fujian province met the inclusion criteria for the 2022–2023 period, of whom 499,188 received HPV vaccination. The overall HPV vaccination rate was 83.55%. The HPV vaccination coverage ranged from 44.14% to 100% across counties/districts. The interquartile range for HPV vaccination coverage was 86.19% (81.32%, 91.32%).

A chi-square test of independence examined differences in vaccination rates across the nine cities. The analysis revealed a statistically significant association between city and vaccination rate (df = 8, p < 0.001). This result indicates that vaccination rates were not uniformly distributed. To assess the strength of this association, we calculated Cramér’s V, yielding an effect size of 0.23. According to Cohen’s (1988) guidelines for df = 8, where small, medium, and large effects correspond to V = 0.06, 0.17, and 0.27, respectively, this value represents a medium-to-large effect.

The distribution of HPV vaccination rates across various prefecture-level cities is presented in Table 1. Pairwise comparisons following Bonferroni correction (α’=0.05/36 = 0.00139) revealed that the vaccination rates in the nine cities could be stratified into four distinct tiers:

Table 1.

HPV vaccination rates across different cities in Fujian Province

	Vaccination rate (%)	P-values
		Fuzhou	Xiamen	Putian	Sanming	Quanzhou	Zhangzhou	Nanping	Longyan	Ningde
Fuzhou	79.52	--	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001
Xiamen	62.40	< 0.001	--	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001
Putian	93.01	< 0.001	< 0.001	--	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001
Sanming	89.37	< 0.001	< 0.001	< 0.001	--	0.012	< 0.001	< 0.001	< 0.001	0.423
Quanzhou	88.91	< 0.001	< 0.001	< 0.001	0.012	--	< 0.001	< 0.001	< 0.001	< 0.001
Zhangzhou	84.12	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	--	0.008	0.003	< 0.001
Nanping	84.72	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	0.008	--	0.695	< 0.001
Longyan	84.82	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	0.003	0.695	--	< 0.001
Ningde	89.53	< 0.001	< 0.001	< 0.001	0.423	< 0.001	< 0.001	< 0.001	< 0.001	--

1. High vaccination rate region: Putian, whose vaccination rate was significantly higher than all other regions (all P < 0.00139).

2. Relatively high vaccination rate regions: Sanming, Quanzhou, and Ningde. The vaccination rates in these three cities were lower than in high-coverage areas but higher than in medium- and low-coverage areas. There was no significant difference in vaccination rates between Sanming and Quanzhou, whereas the vaccination rate in Ningde was significantly higher than that in Quanzhou (P < 0.00139).

3. Medium vaccination rate regions: This tier comprises Zhangzhou, Nanping, and Longyan. There was no significant difference in vaccination rate among the three cities (P > 0.00139), but significant differences were observed compared to vaccination rates in other levels (all P < 0.00139).

4. Low vaccination rate regions: This tier includes Fuzhou and Xiamen. Their vaccination rates were significantly lower than those in the high, medium, and relatively high tiers (all P < 0.00139). Furthermore, a significant difference in vaccination rate was also observed between Fuzhou and Xiamen (P < 0.00139).

Relationship between vaccination rates and economic levels

The coverage of the free first dose of HPV vaccine was negatively correlated with GDP per capita and per capita disposable income, with Pearson correlation coefficients of -0.301 and − 0.569, respectively (both p < 0.01). In contrast, the coverage rate of the HPV vaccine at one’s own expense was positively correlated with these economic indicators, with Pearson’s correlation coefficients of 0.335 and 0.666, respectively (all p < 0.01).

Similarly, the coverage of the free second dose of HPV vaccine was negatively associated with GDP per capita and per capita disposable income (Pearson correlation coefficients, 0.276 and − 0.509, respectively; Both p < 0.01), while the coverage rate of the second dose of HPV vaccine at their own expense was positively correlated (Pearson correlation coefficients were 0.247 and 0.519, respectively; both p < 0.01).

In absolute terms, the first dose was more strongly associated with GDP per capita and per capita disposable income than the second dose for both free and self-paid vaccines.

See Figs. 1 and 2.

Fig. 1.

Relationship between HPV vaccination and GDP per capita. A Scatter plot of the relationship between the free first-dose HPV vaccination coverage rate and GDP per capita. B Scatter plot of the relationship between the first-dose self-funded HPV vaccination coverage rate and GDP per capita. C Scatter plot of the relationship between the second-dose free HPV vaccination coverage rate and GDP per capita. D Scatter plot of the relationship between the second-dose self-funded HPV vaccination coverage rate and GDP per capita

Fig. 2.

Relationship between HPV vaccination and per capita disposable income. A Scatter plot of the relationship between the free first-dose HPV vaccination coverage rate and per capita disposable income. B Scatter plot of the relationship between the first-dose self-funded HPV vaccination coverage rate and per capita disposable income. C Scatter plot of the relationship between the second-dose free HPV vaccination coverage rate and per capita disposable income. D Scatter plot of the relationship between the second-dose self-funded HPV vaccination coverage rate and per capita disposable income

Multivariate linear regression analyses of vaccination rates

1. The dependent variable was the free first-dose HPV vaccination coverage rate. Multiple stepwise linear regression analysis showed that the overall regression model was statistically significant (p < 0.001). The adjusted R² was 0.315, indicating that the model explained 31.5% of the variance in the dependent variable. Per capita disposable income hurt the coverage rate of the first dose of the free vaccine (β = -0.569, p < 0.001); specifically, for every one-yuan increase in per capita disposable income, the coverage rate of the first dose of the free vaccine decreased by 0.569%.

2. The dependent variable was the first-dose self-funded HPV vaccination coverage rate; the regression model was statistically significant (p < 0.001), with an adjusted R² of 0.436, explaining 43.6% of the variance. Per capita disposable income had a positive effect on the coverage rate of the first dose of the self-paid vaccine (β = 0.666, p < 0.001); that is, for every one-yuan increase in per capita disposable income, the coverage rate increased by 0.666%.

3. The dependent variable was the second-dose free HPV vaccination coverage rate, the regression model was statistically significant (p < 0.001), and the adjusted R² was 0.250, indicating that the model explained 25.0% of the variance. Per capita disposable income was negatively associated with the coverage rate of the second dose of the free vaccine (β = -0.509, p < 0.001); each one-yuan increase in per capita disposable income was associated with a 0.509% decrease in the coverage rate of the second dose of the free vaccine.

4. The dependent variable was the second-dose self-funded HPV vaccination coverage rate. The regression model was statistically significant (p < 0.001), with an adjusted R² of 0.260, explaining 26.0% of the variance. Per capita disposable income had a positive effect on the coverage rate of the second dose of the self-paid vaccine (β = 0.519, p < 0.001); that is, for every one-yuan increase in per capita disposable income, the coverage rate of the second dose of the self-paid vaccine increased by 0.519%.

See Table 2.

Table 2.

Multivariate stepwise linear regression of vaccination rates and economic levels

	Unstandardized Coefficients		Standardized Coefficients Beta	t	p	95% Confidence Interval for β
	β	Std. Error				Lower Bound	Upper Bound
Panel A: Free first-dose HPV vaccination coverage rate (%)
(Constant)	111.748	5.644
Per capita disposable income	−0.001	< 0.001	−0.569	−6.220	< 0.001	−0.001	−0.001
Panel B: First-dose self-funded HPV vaccination coverage rate (%)
(Constant)	-14.332	2.672
Per capita disposable income	0.000	0.000	0.666	8.032	< 0.001	0.000	0.001
Panel C: Second-dose free HPV vaccination coverage rate (%)
(Constant)	70.669	5.078
Per capita disposable income	-0.001	0.000	-0.509	-5.321	< 0.001	-0.001	0.000
Panel D: Second-dose self-funded HPV vaccination coverage rate (%)
(Constant)	-16.111	4.676
Per capita disposable income	0.001	0.000	0.519	5.458	< 0.001	0.000	0.001

Discussion

This study utilized the Fujian Provincial Immunization Information System to investigate HPV vaccination among girls aged 13-14.5 years in Fujian Province between 2022 and 2023, and to evaluate the association between vaccination coverage and economic indicators. The data showed that the overall HPV vaccination coverage rate was 83.55%. Coverage of the free vaccine was negatively correlated with both GDP per capita and per capita disposable income. In contrast, coverage of the self-paid vaccine was positively correlated with these economic indicators. For both vaccine types, the first dose showed a stronger correlation with GDP per capita and per capita disposable income than the second dose. Specifically, for every 1-yuan increase in per capita disposable income, the free first-dose HPV vaccination coverage rate decreased by 0.569%. In comparison, the first-dose self-funded HPV vaccination coverage rate increased by 0.666%. Similarly, the second-dose free HPV vaccination coverage rate decreased by 0.509%, while the second-dose self-funded HPV vaccination coverage rate increased by 0.519%.

Previous studies indicate that national financial support and health policies are crucial factors that enable the implementation and promotion of HPV vaccination coverage [10]. High-income countries typically exhibit higher HPV vaccination rates, benefiting from organized vaccination programs and substantial investments in healthcare resources [11]. In low- and middle-income countries, economic burdens and insufficient healthcare resources remain significant barriers to achieving higher HPV vaccination rates [12]. A retrospective cohort study in Belgium observed that socioeconomic disparities in HPV vaccine coverage increased over time and with age during periods when co-payments were required outside of school-based programs. Conversely, vaccination rates were substantially higher when the vaccine was offered free of charge in school settings [13].

During Fujian Province’s implementation of free domestic bivalent HPV vaccination, the policy adhered to the principle of “vaccinate all eligible individuals voluntarily.” The government publicly listed accredited vaccination clinics and established dedicated free vaccination sites near school clusters, with schools organizing student vaccination at centralized sites. Experience indicates that incorporating HPV vaccination into government public health services and implementing free vaccination policies offers advantages, including comprehensive coverage, effective mobilization, streamlined management, and orderly implementation. It provides a concrete model for other regions to implement free HPV vaccination programs.

However, our data revealed that, despite the development of a unified policy and management approach to HPV vaccination across the province, there remained regional differences in vaccination rates. Previous studies in similar contexts suggest that regional economic status significantly influences HPV vaccine uptake among individuals who pay for the vaccine themselves. For example, an analysis of HPV vaccination among Japanese girls aged 14–19 years revealed that a higher socioeconomic status and greater availability of medical facilities were associated with higher cumulative HPV vaccination rates [14]. Analysis of Brazilian adolescent HPV vaccination data from 2014 to 2022 revealed that socioeconomic factors, region-specific factors, and their interactions may influence vaccination rates [15]. A geospatial analysis of Australia found that the 2016 SEIFA economic resources index was the most significant predictor of HPV vaccination rates, accounting for 72% of the geographic variation in vaccination rates [16]. A systematic review and meta-analysis of HPV vaccination-related determinants in Europe revealed that low socioeconomic status was an additional barrier to adherence to HPV vaccination [17].

The study found that during the same period, the relationship between regional economic indicators (GDP per capita and per capita disposable income) and the free HPV vaccination rate and the self-paid HPV vaccination rate showed an inverse trend, indicating economic stratification in the implementation of public health policy. Residents may also differ in their vaccine choice behavior. Free HPV vaccination coverage was higher in areas with lower GDP per capita and lower per capita disposable income. It is speculated that due to the price threshold of self-paid vaccines, residents in these areas usually mainly rely on free vaccination, indicating that the free HPV vaccination policy for age-eligible girls implemented in Fujian Province of China during the study period protected the interests of those groups who could not be vaccinated before due to economic reasons. The government’s special funding has played an important role in ensuring access to basic vaccines in these areas, and also reflects the equity of the free vaccination policy. It could provide empirical evidence for future policy-making. For example, the basic medical and public health system should be prioritized in less developed areas, and coverage should be adjusted according to regional economic changes. By contrast, in regions with higher GDP and disposable income, the demand for self-funded, optional, or high-value vaccines becomes more prominent.

Regarding resource accessibility, the density of healthcare infrastructure is often linked to regional economic development. The distribution and service radius of vaccination clinics directly influence the convenience and accessibility of services, thereby affecting out-of-pocket vaccination rates [18–19]. For instance, a study in Guangzhou found that optimizing the spatial layout of vaccination sites reduced average travel time by 15% and significantly increased HPV vaccination intent [20]. This disparity in resource allocation creates a positive correlation between regional economic status and convenient access to vaccination services, which, in turn, reinforces the observed statistical association between economic level and self-paid vaccination rates. Consequently, areas with higher economic development and resident income tend to have a denser network of self-paid HPV vaccination sites and a greater likelihood of vaccination uptake.

Our data show that the first dose of vaccination is more strongly associated with regional economic levels than the second dose. This phenomenon reveals that the initiation phase of vaccination is the most sensitive to economic factors. Compared with full vaccination, the first dose of HPV vaccination involves only four steps: knowing about the HPV vaccine, willingness to accept it, going to the vaccination site, and paying the fee or signing up for the free vaccination. The main economic constraint is the vaccine’s cost. However, it is necessary to take into account additional factors, such as the schedule of subsequent doses, changes in vaccination sites, and the ongoing ability to pay. As the number of intermediate links increases, the influence of the economic level becomes more dispersed. This phenomenon indicates that economic conditions have a strong short-term effect on the first dose. From a policy perspective, focusing on promoting the first dose of vaccination may improve overall vaccination coverage with half the effort.

The results of this study provide important empirical evidence for further optimizing HPV vaccination policies. We found that the operation mode of the free vaccine differed from that of the self-funded vaccine, suggesting that the “one-size-fits all policy” intervention cannot meet the actual needs of residents with different economic levels simultaneously. Therefore, it is necessary to promote flexible, differentiated strategies to develop intervention measures and policy guidance that are easy to implement.

From a fiscal affordability perspective, establishing an HPV vaccine subsidy mechanism aligned with local fiscal capacity to reduce the actual vaccination cost for the target population is one of the key pathways to optimizing HPV vaccination strategies. Existing research has shown that government subsidies can significantly enhance parents’ willingness to pay for and acceptance of high-priced HPV vaccines. For example, the subsidized vaccination program for girls aged 13–14 in Chengdu significantly increased vaccination rates in this age group [21]. Furthermore, innovative payment models, such as the “pay-it-forward” mechanism, are effective in improving HPV vaccination coverage among girls aged 15–18 in China [22].

From the perspective of vaccine supply, it is necessary to regularly optimize the number and proportion of free and self-funded vaccination sites to improve vaccine accessibility and selectivity. To reduce barriers caused by imbalances in public health resources, vaccination site locations should be carefully evaluated and planned, with emphasis on coverage in less developed areas. At the same time, according to the regional economic level, traffic accessibility, population density, and demographic characteristics of local permanent residents, the layout of HPV vaccination sites should be adjusted to reduce the time and transportation costs of residents to vaccination sites, and enable residents to choose vaccination programs flexibly according to their own economic conditions. In terms of policy formulation, it is necessary not only to ensure the equality of basic public health services but also to give full consideration to the individualized needs of residents, and to promote the expansion of HPV vaccination from equity to diversity and precision.

From a broader perspective of policy evolution, starting from November 2025, China has officially incorporated the HPV vaccine into the national immunization program, providing free domestic bivalent HPV vaccination for eligible girls (aged 13) [23]. This national-level institutional arrangement is expected to advance the popularization of HPV vaccines significantly. In this context, the conclusions of this study can not only serve as a reference for implementing the national immunization program but also provide practical data support for the parallel scheme implemented in Fujian Province starting in 2025, which focuses on free domestic bivalent vaccination, supplemented by moderate subsidies for other vaccine types. Looking ahead, it is recommended to promote the systematic transformation of HPV vaccination policies from a “universal benefit type” to a “stratified and precise type.” By establishing a differentiated intervention mechanism that balances equity and efficiency, the depth and breadth of vaccine coverage can be further enhanced, contributing to the public health goal of primary prevention of cervical cancer.

Advantages

This study included all age-eligible girls in this age group in Fujian province, and the population was well represented without bias. Vaccination information was obtained from the National Immunization Planning System, and economic data from the official statistical yearbook. Therefore, the data are authoritative and reliable, providing high-quality data support for real-world research.

Limitations

This study faces several limitations. First, long-term dynamic tracking of HPV vaccine uptake has not been conducted due to the relatively short study period. With the progress of urbanization in China, the relationship between vaccination coverage and regional economic level observed in this study may change over time, so extrapolation of the findings to future time periods needs to be cautious. Second, the design of the ecological study prevents the drawing of individual behaviors.

Conclusions

This study used big data to systematically analyze regional differences in HPV vaccination rates among girls in Fujian Province under the policy of free domestic bivalent HPV vaccination. This study revealed that before the Chinese government fully introduced HPV vaccination into the national immunization program, the HPV vaccination rate among age-eligible girls in Fujian Province was associated with regional economic level, and the direction of this association differed by vaccine type (free or self-paid). A stronger association was observed for the first vaccine dose. These findings provide an empirical basis for developing more targeted interventions in the future. Our findings provide real-world evidence and decision-making support for optimizing HPV immunization strategies and for evidence-based formulation of relevant public health policies in China.

Authors’ contributions

Xin-xin Huang and Peng-ming Sun wrote the main manuscript text, Peng Yao and Qin Wang provided the raw data for this study. All authors reviewed the manuscript.

Funding

No funding or grant support was used to support this work.

Data availability

The data are available upon reasonable request. The authors are not authorized to share unauthorized data with a third party. However, the statistical analysis data are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

This study was conducted in accordance with the ethical principles outlined in the Declaration of Helsinki. The Human Subjects Ethics Committee of the study hospital approved the study. Parents were informed of the study’s purpose and content. Written informed consent was obtained from parents and caregivers. To ensure patient privacy, all participants’ personal or sensitive information was assigned an alphabetical or numerical code. All the authors agree with the content of the manuscript.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.