Long-term association between the COVID-19 pandemic and language development at 3 years of age

BACKGROUND: Few studies have examined the long-term association between the COVID-19 pandemic and language development in preschoolers or its effect modifiers. We therefore examined this association during health checkups of 3-year-olds in Japan. METHODS: This repeated cross-sectional study included 40,898 3-year-olds who underwent checkups in Okayama City, Japan. We compared children examined before (2017-2019) and during the pandemic (2020-2023). The primary outcome was physician-determined risk of language difficulties. The secondary outcome was children's infrequent questions using words such as "what," "who," and "where." Modified Poisson regression estimated RRs adjusted for confounders. Subgroup analyses examined effect modification by sex, childcare setting, and family size. RESULTS: The COVID-19 pandemic was associated with increased risk of language difficulties (RR 1.52, 95% CI 1.35-1.72). The association peaked in fiscal year 2021 and remained elevated throughout the pandemic. Secondary outcome showed elevated risk (RR 1.16, 95% CI 1.05-1.29). Girls and home-care children showed higher RRs than boys and preschool attendees. CONCLUSIONS: The COVID-19 pandemic was associated with risk of language difficulties among 3-year-old children, with associations varying by sex and childcare setting. These findings suggest the importance of maintaining social interaction opportunities for young children during public health crises. IMPACT: The COVID-19 pandemic was associated with increased risk of language difficulties in 3-year-old children. Using 7 years of health checkup data from 40,898 children, this study provides one of the largest and longest analyses of the pandemic's long-term association with language development in preschoolers. This study examined potential effect modifiers, including sex, daytime childcare setting, and family size. The pandemic's association varied by sex and childcare setting, with girls and home-cared children showing higher risk. These findings support the need for public health policies that prioritize maintaining social interaction opportunities for young children during health crises.