Dental caries, one of the most prevalent chronic diseases, have become a major public health burden worldwide (1, 2). Due to changes in dietary patterns, lower mineralization of deciduous teeth compared to permanent teeth, and weaker awareness of oral self-care among children, the prevalence of dental caries in preschool children remains persistently high (3). Almost half of preschool children globally are affected by dental caries (4). In China, the mean decayed, missing, or filled primary teeth index (dmft) among 5-year-old children has shown a significant increase over the past decade, rising from 3.5 in 2005 to 4.2 in 2015 (5, 6). During the same period, caries prevalence in this age group increased from 66.0% to 71.9%, reflecting a 5.9-percentage-point absolute increase (5, 6). Moreover, Dental caries in preschool children exhibit early onset, high prevalence, with a continuous upward trend, and low treatment rates.

Early childhood caries (ECC)is a clinically aggressive form of dental decay that typically appears shortly after the eruption of teeth. It primarily affects smooth enamel surfaces, progresses rapidly, and can lead to significant long-term consequences for dental health (7). In the early stages, children with dental caries often do not show obvious symptoms. If timely intervention is not provided, these children may experience pain and infection, which can escalate into more serious complications such as difficulties with feeding, sleep deprivation, chronic systemic infections, growth retardation, and psychological issues (7, 8). There is currently a significant gap between the preventive measures for ECC in China and those in developed countries such as Japan and the United States, indicating that China still faces substantial challenges in ECC prevention (8, 9). Significant variations in ECC epidemiology across different countries and regions necessitate investigating its risk factors and developing prevention strategies specifically tailored to China's unique context (9).

ECC is a multifactorial public health issue influenced by diet, oral hygiene, socioeconomic factors, and guardians’ attitudes, among others (10–12). Since there have been few longitudinal studies exploring the risk factors of ECC in China, the majority of research has concentrated on the prevalence and incidence of ECC. Wang et al. established a cohort of preschool children aged 3–4 years in Wenzhou, China, reporting a high incidence and prevalence of ECC (13). Similarly, Zhou et al. examined the early life influences in an 8-month-old cohort in Guangzhou, China (14). These gaps highlight an urgent need for longitudinal studies identifying ECC risk factors across key developmental stages, particularly 3–5 years old, in China.

Our previous study based on a one-year follow-up study revealed that residing in suburban areas, non-use of fluoride toothpaste, and frequent consumption of sugary snacks are significant risk factors for the onset of caries in preschool children (15). Building on these findings, this study conducted a two-year investigation among preschool children in Shanghai, China, further exploring risk factors for newly emerged ECC, identifying patterns of disease development and susceptible populations, and controlling the onset and progression of dental caries. Ultimately, the goal is to provide evidence and guidance for prevention strategies to reduce the incidence of ECC.

This study revealed a caries prevalence of 76.0% among senior-class preschool children in Shanghai, surpassing the 71.9% prevalence rate reported for 5-year-olds in the Fourth National Oral Health Survey in the Mainland of China (5). The higher prevalence may be that the subjects we included in this study were more from the suburbs of Shanghai. Moreover, the overall new caries incidence rate was 70.8% in this two-year follow-up study. The findings demonstrated a potentially more severe prevalence of dental caries among preschool children in Shanghai suburbs compared to previous estimates or national averages, indicating the urgent need for heightened awareness and targeted interventions.

The study investigated the association between socioeconomic backgrounds and ECC. Father's monthly income and education level, and mother's education level were inversely correlated with new caries incidence rates. Children from suburban areas exhibited significantly higher new caries incidence rates compared to urban children. Parents play a significant role in children's oral health behaviors and habit formation (16, 17). Previous studies have also reported significant relationships between minders’ attitudes, knowledge, oral health status, oral health habits and ECC development (18–20). Du et al. demonstrated that ECC prevalence was higher in children from low-income families. The main factors contributing to inequality were average household income, parents’ educational level, and living areas (21). Furthermore, the limited access to nutrient-dense foods among low-income families results in dependence on high-energy, low-cost, sugar-laden diets, which serve as a critical etiological factor in caries pathogenesis (22). These findings emphasized the importance of enhancing parents’ awareness of oral health care, which is crucial for fostering better oral health outcomes and laying a solid foundation for children's long-term dental well-being. Given that low paternal income and low parental education levels are well-established risk factors for ECC, leveraging external support to enhance parental health literacy is crucial. Furthermore, where feasible, governmental and community programs should supply oral health kits, like toothbrushes and dental floss, and offer subsidized dental care to individuals from low-income households, thereby alleviating financial barriers to oral healthcare access.

This study found that residential location significantly influenced new caries incidence, with children living in suburban areas exhibiting a higher rate than those in urban areas (p = 0.002), which was consistent with previous findings (15, 23, 24). This may be due to systemic health inequities of suburban areas characterized by poverty, lower education, and fewer oral healthcare providers. Interestingly, in the multiple regression model, location failed to demonstrate statistical significance as a predictor variable, which may be the result of the uneven distribution of participants across urban and suburban groups. This study suggests that suburban children need to pay more attention to oral health, which requires strengthening preventive measures and increasing access to dental healthcare services. Concurrently, key stakeholder groups including preschool teachers, community health workers, and primary care providers should be actively engaged to facilitate timely, community-based interventions for ECC. Recent studies suggest that insufficient knowledge and training in children's oral health and ECC management may exist even among pediatric specialists (25). Governments need to strengthen the capacity of primary dental care systems in suburban areas. This includes training community healthcare providers to conduct ECC risk screening, deliver oral health education, and establish streamlined referral pathways.

While dental visits are typically considered protective, the study observed that recent dental visits within 6 months showed elevated caries risk (p = 0.008), reflecting care-seeking behavior often triggered by symptoms of dental issues. Our findings align with the Fourth National Oral Health Epidemiological Survey of China, which revealed that 50.6% of preschoolers (3–5 years old)sought dental care for treatment purposes within 12 months, vs. merely 11.6% for preventive reasons (5). Such patterns underscore an urgent need for public education on routine check-ups and a shift from symptom-driven to prevention-oriented oral healthcare.

The study further confirmed that frequent sugar consumption strongly correlated with new caries development, aligning with our earlier 1-year follow-up results (15). In recent decades, dietary habits have shifted, including the popularity of energy-dense, low-nutrient-dense foods, which are often characterized by a high content of added sugar (26). Carbohydrates play a critical role in the caries development, acting as essential metabolic substrates and promoting the growth of cariogenic bacteria (23, 27). The WHO made a conditional recommendation for a further reduction of the intake of free sugars to <5% of total energy intake (28). As reported, restricting sugary food intake as well as avoiding unhealthy snacking habits are effective strategies for caries prevention (29). Limiting children's sugar intake should extend beyond the home to kindergarten settings, where concerted efforts are needed to reduce sugar-sweetened snacks and beverages while increasing the availability of healthier alternatives. For governments, multi-pronged environmental interventions, such as promoting healthier beverages in supermarkets, restricting the availability of sugar-sweetened beverages (SSB) in schools, and implementing fiscal measures, could reduce SSB consumption (30).

Additionally, consistent with our previous study, both the 1-year follow-up and current 2-year follow-up results demonstrated that fluoride toothpaste use effectively reduced new caries incidence, corroborating global evidence (15, 31). Fluoride enhances tooth remineralization and inhibits demineralization (32). Proper use of fluoride provides significant lifelong benefits for oral health. Topical fluoride therapies effectively reduce the incidence of ECC, but it is worth noting that children's home-use fluoride products should be used in smaller doses and more frequently (33). To avoid the side effects of fluoride, guidelines suggest using a rice-grain-sized amount for children under 2 years old and a pea-sized ease for those aged 3–6 years (34). Fluoride varnish demonstrates substantial caries-preventive efficacy in both permanent and primary dentition (35). Health education initiatives should advocate for use of fluoride toothpaste, while fluoride varnish applications ought to be integrated into routine pediatric healthcare protocols.

The VPI assessment revealed that poor oral hygiene status significantly increased Δdmft and Δdmfs, and overall caries incidence. This highlights the critical role of oral hygiene maintenance in preventing caries. Poor brushing habits is significantly related to caries development (36). Given preschool children's limited cognitive abilities and weak self-care awareness, promoting parents to help their children brush their teeth is the essential method to improve tooth brushing in children and to prevent childhood dental caries. Instead of directly instructing preschool children about oral health, multiple-level dental knowledge instruction of guardians can be more effective to prevent ECC. Healthcare professionals need to pay more attention in guiding parents to improve children's brushing practices, including proper technique, adequate duration, and a twice-daily brushing routine (37). Community-level interventions should incorporate not only regularly scheduled workshops delivering step-by-step oral hygiene protocols, but also scalable digital tools (38). Platforms like Douyin/TikTok have already proven instrumental in disseminating chronic disease education offering a strategic channel to amplify ECC prevention messaging (39).

There are some limitations of this study. Firstly, the generalizability of the findings is limited due to its focus on a single city. These samples were exclusively drawn from Shanghai, an economically advanced coastal city in eastern China. Given its unique socioeconomic profile and substantial regional disparities, the findings may not represent China's national average status of ECC. Secondly, the assessment of oral hygiene knowledge of caregivers is incomplete, such as the lack of knowledge about caries prevention. Future research should balance urban and suburban ratios, and improve questionnaires to completely assess caregivers’ knowledge and behaviors.

The present study provides new insights into ECC-related risk factors in Shanghai. Several factors contribute to ECC disparities, including low father's income, mother's education below high school, high sugar consumption, non-use of fluoridated toothpaste, and poor oral hygiene. There is an urgent need to enhance parents’ awareness of oral health, particularly among caregivers with limited education. Children's oral health would benefit from reducing sugar intake, promoting fluoride use, and establishing good oral hygiene habits. Effective ECC prevention in China requires coordinated actions across multiple levels, from individuals and families to schools, communities, and government, tailored to local realities.