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COVID-19 pandemic and language development in children at 18 months: a repeated cross-sectional study over a 6-year period in Japan
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  1. Rumi Matsuo1,
  2. Naomi Matsumoto2,
  3. Toshiharu Mitsuhashi3,
  4. Takashi Yorifuji2
  1. 1Department of Epidemiology, Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, Okayama, Japan
  2. 2Department of Epidemiology, Faculty of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, Okayama, Japan
  3. 3Center for Innovative Clinical Medicine, Okayama University Hospital, Okayama, Japan
  1. Correspondence to Dr Rumi Matsuo, Department of Epidemiology, Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, Okayama 700-8558, Japan; pkjg52ww{at}s.okayama-u.ac.jp

Abstract

Objective To evaluate the impact of the COVID-19 pandemic experience on language development among children, we compared language development at 18 months of age, before and during the pandemic in Japan, where strict control measures continued over a long period.

Methods This was a repeated cross-sectional study and we included children who attended the 18-month health check-up provided by the Okayama City Public Health Center between January 2017 and December 2022 (n=33 484). We compared indicators of language development before (from January 2017 to February 2020) and during (from March 2020 to December 2022) the pandemic. Our primary outcome was the proportion of children who required follow-up for language development by the Public Health Center. The secondary outcome was the proportion of children who could not say three or more meaningful words. We estimated risk ratios (RRs) and their 95% CIs, adjusted for potential confounders.

Results The prevalence of the primary outcome was 33.5% before the pandemic and 36% during the pandemic. Compared with before the pandemic, increased RRs for the primary and secondary outcomes were observed during the pandemic, with RRs (95% CIs) of 1.09 (1.06–1.13) for the primary outcome and 1.11 (1.05–1.17) for the secondary outcome. Although the statistical interactions were not significant, the RRs were higher for children cared for at home than those in nursery schools and with ≤3 family members than those with ≥4 family members.

Conclusions The COVID-19 pandemic was associated with an increased risk of impaired language development in children at 18 months. More extensive support is needed for higher risk families, as well as follow-up of long-term language development in children affected by the COVID-19 pandemic.

  • Child Development
  • Child Health
  • Covid-19
  • Paediatrics

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WHAT IS ALREADY KNOWN ON THIS TOPIC

  • Previous studies examining the impact of the pandemic for infant and toddler communication development have provided inconsistent findings, and the number of studies is still limited. Moreover, these previous studies have evaluated the impact of the very early period of the COVID-19 pandemic, and the longer term studies are needed.

WHAT THIS STUDY ADDS

  • The present study showed that the experience of COVID-19 was associated with an increased risk of impaired language development at 18 months among children in Japan. Moreover, the effects were pronounced among children who were born after the COVID-19 pandemic had begun, especially those cared for at home and with fewer family members.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

  • Our study indicates that support is needed for children affected by the pandemic, especially those from high-risk families, and long-term follow-up of the language development of children is necessary.

Introduction

Three years have passed since the start of the COVID-19 pandemic, which has caused a large public health burden worldwide.1 2 COVID-19 affects children as well as adults. The effects of COVID-19 infection in children include acute severe illness as well as multisystem inflammatory syndrome in children, a severe post-COVID-19 complication.3–7 Moreover, there is concern regarding the impact of the pandemic experience on the neurodevelopmental outcomes of children.

One systematic review and meta-analysis that included articles published through March 2022 evaluated the associations of birth and being raised during the COVID-19 pandemic with impaired neurodevelopment among infants and suggested that there are negative impacts of the pandemic regarding the risk of communication delay.8 Studies from China and Ireland demonstrated some negative impacts of the pandemic on communication development at 1 year of age compared with the historical cohort before the pandemic.9 10 Huang et al targeted children born 1 year before the examination year (in March to May 2020)9 who had experienced the pandemic for only a couple of months and Byrne et al included those born between March and May 2020 who were part of a prospective pandemic birth cohort.10 However, another study in the USA did not identify any impact of the pandemic on communication development at age 6 months among children born between March and December 2020, in comparison with the historical cohort.11 However, the number of studies is still limited and the number of participants in published studies is relatively small; therefore, additional large studies are warranted to provide further insights.9 11–13 Moreover, most previous studies have evaluated the impact of the very early period of the COVID-19 pandemic; thus, longer term studies are needed to elucidate the long-term impact of the pandemic.

In the present study, we compared neurodevelopment at 18 months of age before (2017–2019) and during the COVID-19 pandemic (2020–2022) in Japan, where strict control measures continued over a long period. We focused on language development in children on the basis of a previous review.8

Methods

Participants

We included children aged 18–23 months who lived in Okayama City, Japan, and attended the 18-month health check-up provided by the Okayama City Public Health Center between 1 January 2017 and 31 December 2022 (n=33 484). In Japan, the Maternal and Child Health Act stipulates that municipalities must provide health check-ups for all children aged 18 months and older but under age 2 years. We provide the details of health check-ups in online supplemental method 1.

COVID-19 pandemic

To examine the impact of the COVID-19 pandemic experience on language development in children, we compared indicators of language development before (from 1 January 2017 to 29 February 2020) and during (from 1 March 2020 to 31 December 2022) the pandemic. We chose this study period because the first COVID-19 case in Okayama Prefecture was reported on 22 March 2020. Additionally, because we considered that the impact of the COVID-19 pandemic differed in each year, we divided the pandemic period into the following three periods: ‘pandemic 2020’ from March 2020 to February 2021; ‘pandemic 2021’ from March 2021 to February 2022; and ‘pandemic 2022’ from March 2022 to December 2022. The COVID-19 epidemic and infection control measures in Japan are described in online supplemental method 2 and figure 1.

Figure 1

Time trend of 3-month moving average for the proportion of the primary outcome (ie, language development) per month according to daytime childcare location (home, nursery school) or number of family members (≤3, ≥4). Part (A) shows the overall results; (B) shows the results stratified by daytime childcare location (home, nursery school); (C) shows the results stratified by number of family members (≤3, ≥4).

Outcome measures

We focused on the language development of children at the 18-month health check-up and defined the proportion of children who required follow-up by the Public Health Center as the primary outcome. As mentioned, public health nurses conduct a brief interview and check-up based on the questionnaire filled out by children’s parents or guardians. Then, public health nurses determine whether each child needs follow-up on the basis of the responses to three questions: (1) Can your child say three or more meaningful words such as ‘wan wan’ (meaning a dog in Japanese)? (2) Does your child follow simple commands such as ‘Please throw away the trash’? (3) Does your child point to objects that they know, such as when asked to point to a ‘wan wan’? As for question (3), public health nurses further check whether the child can perform pointing using picture cards. If the child does not satisfactorily meet the conditions represented by any of three questions, then the public health nurse considers that the child requires follow-up for language development. We defined the secondary outcome as the proportion of children who cannot say three or more meaningful words (the first question). These three questions were based on the guidance for health check-ups in infants and children provided by the Ministry of Health, Labour and Welfare and the National Center for Child Health and Development in Japan.14

After excluding 539 children with missing information for the primary or secondary outcome (326 children before the pandemic and 213 children during the pandemic), we included 32 945 children in the final analysis (18 217 children before the pandemic and 14 728 children during the pandemic).

Statistical analyses

We first compared baseline characteristics of children who were enrolled in the health check-up before and during the COVID-19 pandemic. We then plotted the proportions of each outcome per month as a 3-month moving average during the study period in children overall and according to the location of daytime childcare (home or nursery school) as well as number of family members (≤3 or ≥4).

To assess the association of the COVID-19 pandemic with the primary and secondary outcomes, we conducted Poisson regression with robust error variance and estimated risk ratios (RRs) and their 95% CIs, adjusted for the child’s age in months (continuous), sex, daytime childcare location (home, nursery schools), number of family members (≤3, ≥4) and calendar month. We used the period before the COVID-19 pandemic (ie, from January 2017 to February 2020) as a reference and estimated the RRs for the period during the pandemic (from March 2020 to December 2022) as well as for each pandemic year (ie, pandemic 2020, pandemic 2021 and pandemic 2022).

Moreover, we conducted subgroup analysis according to location of daytime childcare (home, nursery school) as well as number of family members (≤3, ≥4) to examine the effect modification of these variables. We conducted a test of statistical interaction by entering multiplicative terms between the respective factors and COVID-19 pandemic into the model. We considered p values less than 0.05 as statistically significant.

We used Stata SE V.17 (StataCorp, College Station, Texas, USA) for all analyses.

Results

Table 1 shows the baseline characteristics of the enrolled children before and during the COVID-19 pandemic. The mean age of children was 18.7 months before the pandemic and 18.9 months during the pandemic. Compared with before the pandemic, children who were enrolled in the 18-month health check-up during the pandemic were cared for in a nursery school during the daytime.

Table 1

Demographic characteristics of study participants (n=32 945)

Figure 1 shows the 3-month moving average for the proportion of the primary outcome per month in children overall and separately. The proportion of the primary outcome increased from April 2020 (figure 1A), especially among children cared for at home (figure 1B) and those with three or fewer family members (figure 1C). A similar trend was observed for the secondary outcome (figure 2).

Figure 2

Time trend of 3-month moving average for the proportion of the secondary outcome (ie, speech with meaningful words) per month according to daytime childcare location (home, nursery school) or number of family members (≤3, ≥4). Part (A) shows the overall results; (B) shows the results stratified by daytime childcare (home; nursery schools); (C) shows the results stratified by number of family members (≤3, ≥4).

We present the results from Poisson regression in table 2. Compared with before the COVID-19 pandemic, the prevalence of each outcome increased slightly during the pandemic: 2.5% for the primary outcome and 1.1% for the secondary outcome. Moreover, increased RRs for the primary and secondary outcomes were observed and the adjusted RRs were 1.11 (95% CI 1.08 to 1.15) for the primary outcome (ie, language development) and 1.14 (95% CI 1.08 to 1.20) for the secondary outcome (ie, speech with meaningful words). For each pandemic year, although the estimated RRs were elevated in most periods, the point estimates were highest during the pandemic 2022 period for both outcomes.

Table 2

Crude and adjusted RRs and 95% CIs for language development among children compared with before and during the COVID-19 pandemic (n=32 945)

Figure 3 and online supplemental table 1 show RRs for the associations between the COVID-19 pandemic and both outcomes according to daytime childcare location and number of family members. Although p values for statistical interaction were not significant, point estimates tended to be higher for children cared for at home than those in nursery schools and with ≤3 family members than those with ≥4 family members (figure 3A). The same tendency was observed for the secondary outcome (figure 3B), but when stratified by daytime childcare location, increased RRs were found only for children cared for at home.

Figure 3

Adjusted risk ratios and 95% CIs for primary and secondary outcomes compared with before the pandemic (January 2017 to February 2020) as a reference, and during the pandemic (March 2020 to December 2022), stratified by daytime childcare location (home, nursery school) and number of family members (≤3, ≥4). Adjusted for age (months), sex, daytime childcare location, number of family members and calendar month. The p values for statistical interaction are shown. Part (A) shows the results for the primary outcome (ie, language development); (B) shows the results for the secondary outcome (ie, speech with meaningful words).

Discussion

In this study, we compared language development in children at 18 months of age before (2017–2019) and during (2020–2022) the COVID-19 pandemic in Japan, where strict control measures were continued over a long period. We found that the experience of the pandemic was associated with an increased risk of requiring follow-up for language development as well as the inability to say three or more meaningful words. Moreover, the effects were greater for those who were enrolled in the check-ups during the pandemic 2022 period (ie, from March 2022 to December 2022) than those who enrolled in the preceding 2 years. Furthermore, the effects were more pronounced among children cared for at home and those with fewer family members than other groups.

The results of this study are consistent with those previous studies reporting negative effects of the very early pandemic period on children’s language development.8–10 These findings could be owing to the following two reasons. First, fear of COVID-19 infection, social isolation and a lack of various supports during the pandemic could have increased maternal depressive and anxiety symptoms, which could decrease reciprocal exchanges, which support language development in early childhood.15–19 Second, children may have missed various opportunities outside the home to promote language and social development, such as attendance at a childcare centre.20–23 By examining the impact over a longer period of the pandemic, our study findings provide further evidence regarding the negative impact of the pandemic on language development in young children.

We found that the adverse effects on language development were greater for children who were enrolled in the health check-ups during the pandemic 2022 period (ie, from March 2022 to December 2022) than those in the preceding 2 years. All children who were examined in this period were born after the COVID-19 pandemic began, and their mothers had experienced the pandemic from the early stages of pregnancy. For example, children who received the 18-month check-up in March 2022 were born in September 2020 and the first trimester of pregnancy would have been during the period at the start of the pandemic. Therefore, in these children, prenatal maternal stress may have affected development of the fetus in utero.17 24 Moreover, children born after the beginning of the pandemic may have experienced negative impacts through the mechanisms mentioned above since the very beginning of their life. In addition, higher numbers of children were infected in 2022 due to the Omicron strain, which may have affected outcomes (online supplemental figure 1). These factors may explain the larger effect estimates observed among children enrolled in the health check-ups during the pandemic 2022 period.

Our study also showed that negative effects of the pandemic on language development tended to be higher for children cared for at home than those cared for in nursery schools and with ≤3 family members than those with ≥4 family members. While caregivers in nursery schools always wore masks during the pandemic, the effect estimates were greater for children cared for at home. This finding suggests that the effect of reduced interaction with others on language development was probably greater than the effect of masks worn by caregivers. Although the information on the birth order was not e-coded in the present dataset, according to the partial data available for the birth order in the current health check-up dataset, about 80% of children with ≤3 family members were the first child, and about 80% of children with ≥4 family members were the second or later child. Therefore, our study supports a previous study reporting that the effects of the pandemic on neurodevelopment at age 1 year were observed only in first-born children and not in second or later born children.9 These findings suggest that decreased interaction with others would have an important impact on language development in children.25 Because of a lack of interaction with siblings and maternal postpartum depression tends to be more common among first-born children,26 27 they may be more susceptible to the negative effects of the pandemic, which highlights the need for more extensive support for these higher risk families.

Our study has several strengths. First, the Japanese child health check-up system is mandatory for local municipalities and has a high rate of attendance at approximately 95%. Second, the number of children eligible for the health check-up in Okayama City is large. Third, we analysed 6 years of data, which is sufficiently long to make comparisons before and during the pandemic.

Our study also has several limitations. First, we used the proportion of children who required follow-up as the primary outcome and did not use more validated developmental outcomes, such as the Ages & Stages Questionnaire, Third Edition, as used in previous studies. However, the three questions included in our questionnaire were based on guidelines of the Ministry of Health, Labour and Welfare and both the primary and the secondary outcomes showed the same tendency, which validate our outcome definitions. Second, we used years to indicate periods before and during the pandemic; however, there is a possibility that long-term changes before the pandemic owing to other factors (eg, lifestyle changes) may have influenced language development during the pandemic, and the pandemic itself did not have a direct impact on language development in children. However, the primary and secondary outcomes did not change within the period before the pandemic (figures 1 and 2); thus, other factors would not explain the present findings. Third, there were missing data in this study; however, the characteristics did not differ substantially between the groups with missing data and those with complete data (data not shown). Fourth, residual confounding is still possible. As noted, we could not use the birth order information in the statistical adjustment; however, we adjusted for the number of family members in the model.

In conclusion, the present study showed that the experience of COVID-19 was associated with an increased risk of impaired language development at 18 months among children in Japan. Moreover, the effects were pronounced among children who were born after the pandemic had begun, especially those cared for at home and with fewer family members. Our study indicates that support is needed for children affected by the pandemic, especially those from high-risk families. Additionally, it is necessary to conduct follow-up for long-term language development in children affected by the pandemic.

Ethics statements

Patient consent for publication

Ethics approval

This study was approved by the Ethics Committee of Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences and Okayama University Hospital (No K2301-032), which waived the requirement for informed consent because this was a retrospective study with complete anonymity ensured.

Acknowledgments

We thank Hiroaki Matsuoka, Chiyori Satou, Eriko Sasaki, Takako Bessho, Aiko Teraoka, Mayumi Sanehira, Saori Irie and Yoko Oka for valuable support in collecting data. We thank Analisa Avila, MPH, ELS, of Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.

References

Supplementary materials

  • Supplementary Data

    This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.

Footnotes

  • Contributors RM analysed the data and wrote the first draft. NM, TM and TY contributed to the interpretation of the data and revised the manuscript. TY is the guarantor of the article.

  • Funding This work was supported by a grant from Okayama Prefecture for investigating the COVID-19 outbreak (Grant No 7402300018).

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.