Investigation Into the Effectiveness of Community Water Fluoridation and Dental Caries Experience in 4‐Year‐Old Aotearoa New Zealand Children: A National Repeated Cross‐Sectional Study From 2010 to 2022

OBJECTIVES: To provide contemporary national evidence on the effectiveness of community water fluoridation (CWF) for the prevention of early childhood caries (ECC) among 4-year-old children in Aotearoa New Zealand (NZ), and to examine potential effect modification by ethnicity and other sociodemographic characteristics. METHODS: A national repeated cross-sectional study was conducted using routinely collected B4 School Check data (2010-2022). Children aged 4 years were linked to a national spatial dataset of water distribution zones and council-reported fluoridation histories to classify exposure over the assessment year and preceding 3 years. ECC was identified via the 'lift-the-lip' screening. Multilevel mixed-effects modified Poisson regression models, with children nested within meshblocks, estimated incidence rate ratios (IRRs), adjusting for ethnicity, area-level deprivation, age, sex, rurality, and assessment year. Two-factor interactions were examined. RESULTS: Among 630 926 children, 14.2% had ECC. In crude analyses, ECC appeared more common in fluoridated areas; however, after adjustment for sociodemographic characteristics, CWF was associated with a lower rate of ECC (IRR = 1.13; 95% CI: 1.11-1.15 for non-fluoridated vs. fluoridated). A significant interaction between CWF and ethnicity was observed. The protective association was strongest among European/Other children (IRR = 1.25; 95% CI: 1.21-1.29) and attenuated among Māori and Asian children, with minimal effect among Pacific children. The ethnicity-weighted population attributable fraction was 4.4%, equating to approximately 282 ECC cases annually. CONCLUSION: Findings are consistent with CWF being an effective population-level oral health intervention. However, its association with caries experience varies by ethnicity, highlighting the importance of addressing structural inequities alongside universal preventive strategies.