ABSTRACT
Objective
Infant weight gain predicts childhood obesity, yet the effect of pandemic‐related social isolation on first‐year weight gain is unclear. We assessed its impact and implications for obesity risk.
Methods
We analysed a nationwide birth cohort of South Korean children born 2014–2021, excluding prematurity, low birth weight or neonatal intensive care admission. Anthropometrics at 9–12 months were used. Interrupted time‐series regression compared pre‐pandemic (January 2015–January 2020) and pandemic (February 2020–December 2021) periods, adjusting for confounders. Relative risks (RRs) were estimated using modified Poisson regression with robust standard errors. The primary outcome was accelerated weight gain, defined as a change in weight‐for‐age Z‐score (ΔWAZ) > 1.0. The secondary outcome was rapid weight gain, ΔWAZ > 0.67, encompassing excessive (ΔWAZ > 0.67 to ≤ 1.0) and accelerated (ΔWAZ > 1.0) gain.
Results
Among 1 809 054 infants (51.2% male), accelerated gain rose immediately after social isolation began (RR, 1.173; 95% confidence interval, CI, 1.154–1.193) and remained elevated (RR, 1.093; 95% CI, 1.063–1.125). Excessive gain likewise increased immediately (RR, 1.173; 95% CI, 1.154–1.192) and persisted (RR, 1.040; 95% CI, 1.025–1.055). Birth weight was stable (mean 3.24 kg, SD 0.36), while breastfeeding rates declined, socioeconomic levels rose and perinatal abnormalities increased. Infants born during the pandemic gained an additional 244 g (95% CI, 214–275) by 9–12 months.
Conclusions
Infants born during pandemic‐related social isolation gained more weight in the first year than pre‐pandemic cohorts. Policies that sustain breastfeeding, promote healthy feeding and active play, support caregiver mental health and ensure routine early growth monitoring may help mitigate these risks.
Keywords: COVID‐19 pandemic, infant, newborn, paediatric obesity, weight gain
1. Introduction
Widespread social isolation and community‐level restrictions markedly affected global health, daily life and economic stability. These conditions were most pronounced in recent years, with downstream effects on physical activity, diet quality, psychosocial stress and financial hardship [1]. Children, whose growth is rapid and dietary patterns more variable, were especially susceptible to lifestyle disruptions stemming from prolonged school closures, curtailed outdoor play and reduced access to community services [2]. Although severe illness rates in children were lower than in adults and older individuals, they were more vulnerable to the collateral lifestyle changes associated with mitigation measures [3]. Beyond somatic outcomes, pervasive social isolation was accompanied by developmental delays, learning difficulties, mental health concerns, declines in physical fitness, suboptimal eating behaviours and impaired overall growth [3, 4].
Weight changes during infancy play a crucial role in shaping body mass index (BMI) and can have long‐term implications for obesity and related health conditions [5]. While weight changes in adults and older children are affected by multiple factors, including reduced physical activity and increased stress during periods of widespread social isolation [1], infants are primarily dependent on nutritional intake, making the impact on their growth unique. We hypothesised that infants born during the period of widespread social isolation would experience more excessive postnatal weight gain than those born before, reflecting shifts in feeding patterns and caregiving practices. To test this hypothesis, we compared the prevalence of excessive postnatal weight gain between pre‐pandemic and pandemic birth cohorts.
2. Methods
2.1. Study Cohort
The cohort included children enrolled in South Korea's National Health Insurance Service through birth or immigration between 1 January 2014, and 31 December 2021. Participants were those in the National Health Screening Program for Infants and Children (NHSPIC; visits at 4–71 months). Follow‐up weight was obtained at the 9–12‐month NHSPIC screening (routine check‐up closest to 12 months; allowable window, 9–12 months); the mean follow‐up age was 11.9 months (SD 1.3) [6, 7]. We excluded children with missing birth weight, date of birth or prematurity status; those who did not complete the 9–12‐month NHSPIC screening; and children born in 2014 who underwent the 9–12‐month screening in 2014. We then restricted the cohort to children who completed the 9–12‐month NHSPIC screening between 2015 and 2021, yielding 2 062 890 children. Additional exclusions were prematurity, birth weight < 2.5 kg and neonatal intensive care unit (NICU) admission before 6 months to minimise confounding from early birth‐related complications. The final analytic sample comprised 1 809 054 participants (Figure S1). The study period was defined as pre‐pandemic Phase I (January 2015–December 2017), pre‐pandemic Phase II (January 2018–January 2020), and the pandemic phase (February 2020–December 2021). The index date was the 9–12‐month anthropometric assessment, and phase assignment used this date. The pre‐pandemic period was divided into two phases of comparable duration to the pandemic period to ensure a balanced distribution of observation time and to accurately characterise the underlying secular trend prior to the pandemic onset [8]. The authors declare that the research presented in this manuscript adheres to the ethical principles approved by the Institutional Review Board of Hallym University Kangnam Sacred Heart Hospital (IRB No. HKS 2024–07‐022). All procedures involving human participants were conducted in accordance with the ethical standards of the institution and the Declaration of Helsinki (1964), as revised in 2013. Reporting followed the STROBE guidelines (Table S1).
2.2. Classifying Changes in Weight‐for‐Age Z‐Scores
Birth weight and weight‐for‐age Z scores (WAZ) were calculated using the WHO Child Growth Standards [9] and the Korean National Growth Charts [10]; ΔWAZ from birth to 9–12 months quantified postnatal weight gain. Because the focus was infancy (0–12 months), we used WAZ as the primary growth metric. The ΔWAZ‐based definition of rapid weight gain, an increase exceeding 0.67 SD between two time points, has been established in prior literature and adopted as the standard approach in infant weight gain research [11, 12]. This choice aligns with standard growth curves and established rapid weight gain literature [10, 11, 12]. ΔWAZ categories were: decelerated (less than −1.0), slow (−1.0 to less than −0.67), sustained (−0.67 to ≤ 0.67), excessive (> 0.67 to ≤ 1.0), and accelerated (> 1.0). Unless specified, rapid weight gain denotes the combined excessive and accelerated categories [11, 13].
2.3. Outcomes
The primary outcome was the prevalence of accelerated weight gain during pandemic‐related social isolation (ΔWAZ > 1.0). The secondary outcome was rapid weight gain (excessive or accelerated, as above). To assess weight change associated with social isolation, we compared ΔWAZ between the pre‐pandemic period (before February 2020) and the pandemic period (February 2020–December 2021), adjusting for calendar time (months) and social‐isolation status.
2.4. Covariates
Covariates included sex, birth weight, exclusive breastfeeding, birth residence, socioeconomic status (SES) (low, intermediate, high), birth season (spring, summer, fall, winter), and perinatal conditions. Breastfeeding status was ascertained from the 4–6‐month survey [14], with exclusive indicating no formula use and non‐exclusive encompassing formula or mixed feeding. SES was derived from National Health Insurance contribution quartiles and categorised as low (≤ 25th percentile), intermediate (25th–75th percentile), or high (≥ 75th percentile) [15]. Perinatal conditions comprised disorders of gestational length and foetal growth (ICD‐10 P05X–P08X), birth trauma (P10X–P15X), infections specific to the perinatal period (P35X–P39X), and congenital malformations, deformations and chromosomal abnormalities (Q00X–Q99X).
2.5. Statistical Analysis
We summarised ΔWAZ annually (mean, standard error [SE], and 98th percentile) from 2015 to 2021 and visualised category distributions (decelerated, slow, sustained, excessive, accelerated) with a heatmap. Using 2018 as the reference, we calculated annual percentage differences to highlight shifts before and during the pandemic (2020–2021).
Interrupted time‐series analysis with monthly intervals evaluated ΔWAZ over time, using the pandemic onset as the interruption point. The segmented regression model was specified as follows:
where Y_t is the monthly proportion of infants with rapid weight gain at time t; T is the elapsed time in months from the start of the study; X_t is a binary interruption indicator coded 0 before and 1 from February 2020 onwards; and T × X_t represents the time elapsed since pandemic onset (0 before February 2020). β1 estimates the pre‐pandemic monthly trend, β2 the immediate level change at pandemic onset, and β3 the change in monthly trend during the pandemic period [8]. The interruption point was defined as February 2020, coinciding with the onset of community transmission and the implementation of social distancing measures in Korea, which did not institute a formal nationwide lockdown [16].
For rapid weight gain, we fit a generalised linear model with a Poisson distribution and log link (robust SEs) to estimate relative risks (RRs), as this modified Poisson regression approach has been shown to provide valid RR estimates for binary outcomes regardless of outcome prevalence [17]. Models were adjusted for sex, birth weight, exclusive breastfeeding, birth residence, SES, birth season and perinatal status. Missing covariate data were handled using multiple imputation [18]. Risks were estimated over 12‐month intervals with 95% confidence intervals (CIs).
To quantify actual weight gain among infants aged 9–12 months during the pandemic versus the pre‐pandemic period, we fit a generalised linear model with ΔWAZ as the outcome, adjusting for the same covariates. Immediate and overall pandemic‐period effects were estimated; immediate change refers to the abrupt level shift at pandemic onset (February 2020) and pandemic change refers to the sustained trend during the pandemic period (February 2020–December 2021), both as interrupted time‐series parameters distinct from the pre‐pandemic phase classifications used to describe the study population. Excess weight gain was inferred from national standard WAZ for Korean infants aged 9–12 months. Analyses used SAS version 9.4 (SAS Institute, Cary, NC, USA) and R version 4.3 (R Foundation for Statistical Computing, Vienna, Austria).
3. Results
3.1. Study Population
The study population included a total of 1 809 054 children (51.2% male) with available information on birth weight and anthropometric measurements recorded at a mean age of 11.78 months (SD of 1.23) on the index date (Table 1). Of these, 874 445 children were observed during the pre‐pandemic Phase I (January 2015–December 2017), 512 441 during the pre‐pandemic Phase II (January 2018–January 2020), and 422 168 during the pandemic period (February 2020–December 2021). Covariate completeness varied across variables and missing data by covariate and study phase are presented in Table S2. Over these three phases, the prevalence of exclusive breastfeeding gradually decreased, SES increased and proportions related to perinatal history showed an upward trend. However, birth weight remained stable at 3.24 kg (SD 0.37), and no differences were observed in birth season.
TABLE 1.
Study population.
| Weight gain metric, n (%) | Total | Pre‐pandemic a | Pandemic phase | |
|---|---|---|---|---|
| Phase I | Phase II | |||
| No. of observations | 1 809 054 | 874 445 | 512 441 | 422 168 |
| Sex | ||||
| Male | 927 138 (51.2) | 448 409 (51.3) | 262 589 (51.2) | 216 140 (51.2) |
| Female | 881 916 (48.8) | 426 036 (48.7) | 249 852 (48.8) | 206 028 (48.8) |
| Index date, month, mean (SD) | 11.78 (1.23) | 11.61 (1.16) | 11.78 (1.08) | 12.14 (1.46) |
| Birth weight, kg | 3.24 (0.37) | 3.25 (0.37) | 3.24 (0.36) | 3.23 (0.36) |
| Breast milk feeding b | ||||
| Yes | 404 663 (22.4) | 243 648 (27.9) | 101 025 (19.7) | 59 990 (14.2) |
| No | 1 172 453 (64.8) | 519 058 (59.4) | 359 336 (70.1) | 294 059 (69.7) |
| Missing | 231 938 (12.8) | 111 739 (12.8) | 52 080 (10.2) | 68 119 (16.1) |
| Birth residence c | ||||
| Seoul | 333 006 (18.4) | 165 607 (18.9) | 92 643 (18.1) | 74 756 (17.7) |
| Metropolitan | 445 391 (24.6) | 219 738 (25.1) | 125 695 (24.5) | 99 958 (23.7) |
| City | 902 828 (49.9) | 427 825 (48.9) | 258 297 (50.4) | 216 706 (51.3) |
| Rural | 124 278 (6.9) | 58 066 (6.6) | 35 511 (6.9) | 30 701 (7.3) |
| Missing | 3551 (0.2) | 3209 (0.4) | 295 (0.1) | 47 (< 0.1) |
| Socioeconomic status d | ||||
| Low (< 25P) | 436 996 (24.2) | 235 615 (26.9) | 117 655 (23.0) | 83 726 (19.8) |
| Intermediate (25P–75P) | 874 352 (48.3) | 437 589 (50.0) | 248 364 (48.5) | 188 399 (44.6) |
| High (< 75P) | 437 005 (24.2) | 174 136 (19.9) | 129 320 (25.2) | 133 549 (31.6) |
| Missing | 60 701 (3.4) | 27 105 (3.1) | 17 102 (3.3) | 16 494 (3.9) |
| Birth season | ||||
| Spring (March–May) | 485 838 (26.9) | 235 445 (26.9) | 134 509 (26.2) | 115 884 (27.4) |
| Summer (June–August) | 452 682 (25.0) | 220 678 (25.2) | 122 853 (24.0) | 109 151 (25.9) |
| Fall (September–November) | 434 764 (24.0) | 215 039 (24.6) | 117 269 (22.9) | 102 456 (24.3) |
| Winter (December–February) | 435 770 (24.1) | 203 283 (23.2) | 137 810 (26.9) | 94 677 (22.4) |
| Perinatal condition e | ||||
| Any | ||||
| Yes | 545 109 (30.1) | 244 463 (28.0) | 158 174 (30.9) | 142 472 (33.7) |
| No | 1 263 945 (69.9) | 629 982 (72.0) | 354 267 (69.1) | 279 696 (66.3) |
| Disorders related to gestational age and foetal growth (P05X–P08X) | ||||
| Yes | 33 652 (1.9) | 153 77 (1.8) | 9883 (1.9) | 8392 (2.0) |
| No | 1 775 402 (98.1) | 859 068 (98.2) | 502 558 (98.1) | 413 776 (98.0) |
| Birth injuries to the central nervous system (P11) | ||||
| Yes | 116 (0) | 67 (0) | 27 (0) | 22 (0) |
| No | 1 808 938 (100) | 874 378 (100) | 512 414 (100) | 422 146 (100) |
| Perinatal cardiovascular and respiratory disorder (P20–P29X) | ||||
| Yes | 134 518 (7.4) | 58 148 (6.6) | 38 730 (7.6) | 37 640 (8.9) |
| No | 1 674 536 (92.6) | 816 297 (93.4) | 473 711 (92.4) | 384 528 (91.1) |
| Nontraumatic intracranial haemorrhage in newborns (P52) | ||||
| Yes | 556 (0) | 255 (0) | 161 (0) | 140 (0) |
| No | 1 808 498 (100) | 874 190 (100) | 512 280 (100) | 422 028 (100) |
| Newborn convulsions and cerebral disturbances (P90–P91) | ||||
| Yes | 2563 (0.1) | 969 (0.1) | 674 (0.1) | 920 (0.2) |
| No | 1 806 491 (99.9) | 873 476 (99.9) | 511 767 (99.9) | 421 248 (99.8) |
| Congenital malformations (Q00X–Q89X) | ||||
| Yes | 421 551 (23.3) | 189 238 (21.6) | 122 644 (23.9) | 109 669 (26.0) |
| No | 1 387 503 (76.7) | 685 207 (78.4) | 389 797 (76.1) | 312 499 (74.0) |
| Chromosomal abnormalities (Q90X–Q99X) | ||||
| Yes | 2929 (0.2) | 1563 (0.2) | 815 (0.2) | 551 (0.1) |
| No | 1 806 125 (99.8) | 872 882 (99.8) | 511 626 (99.8) | 421 617 (99.9) |
The study period was divided into three distinct phases: pre‐pandemic Phase I (January 2015–December 2017), pre‐pandemic Phase II (January 2018–January 2020), and the pandemic phase (February 2020–December 2021), encompassing the entire timeline of interest. The index date was defined as the date when a child's anthropometric measurements were collected between 9 and 12 months of age. Children were classified in study phases based on this index date.
Breast milk feeding was categorised as “yes” for exclusive breastfeeding, while “no” included formula feeding or mixed feeding (a combination of breast milk and formula).
Metropolitan areas were defined as five metropolitan cities (Busan, Incheon, Gwangju, Daejeon and Ulsan), urban areas as cities and rural areas as non‐city areas.
Categorised by household income as low, intermediate or high.
Perinatal conditions included disorders related to length of gestation and foetal growth (ICD‐10: P05X–P08X), birth trauma (ICD‐10: P10X–P15X), infections specific to the perinatal period (ICD‐10: P35X–P39X), and congenital malformations, deformations and chromosomal abnormalities (ICD‐10: Q00X–Q99X).
3.2. Trends in Weight‐for‐Age Z‐Scores Changes and the Proportion of Excessive Weight Gain
The mean ΔWAZ remained relatively stable from 2015 to early 2020, ranging between 0.32 and 0.34. However, a significant increase was observed starting in February 2020, coinciding with the onset of the pandemic. The mean Z‐score rose from 0.34 (pre‐pandemic, 2019–January 2020) to 0.45 in 2020 (February–December) and further increased to 0.47 in 2021. The 98th percentile ΔWAZ, which represents extreme weight gain, also exhibited an increasing trend. It remained relatively stable before the pandemic, ranging from 4.67% to 4.92%, but increased to 5.26% in 2020 and further to 5.56% in 2021 (Figure S2).
3.3. Trends in Weight Gain Categories From 2015 to 2021
Figure 1 presents a heatmap analysis of annual changes in five weight gain categories (decelerated, slow, sustained, excessive and accelerated) from 2015 to 2021, using 2018 as the reference year. The results indicate a notable shift in weight gain distribution, particularly after the onset of the pandemic. Before 2018, decelerated and slow weight gain fluctuated, peaking in 2016 and 2017, before returning to baseline in 2018. However, these categories declined sharply from 2019 onward, reaching −18.46% and −11.94% in 2020, and further decreasing to −19.27% and −13.02% in 2021, respectively. Similarly, sustained weight gain, which remained stable before 2018, declined by −5.09% in 2020 and −6.41% in 2021. Conversely, after a pre‐2018 decline, the prevalence of excessive postnatal weight gain increased during the pandemic, rising by 2.01% in 2020 and 1.33% in 2021. The most pronounced change was observed in accelerated weight gain, which surged by 10.56% in 2020 and 12.93% in 2021, reflecting a significant rise in excessive weight gain among infants. The SD of ΔWAZ increased from approximately 0.94 in the pre‐pandemic period (2015–2019) to 1.70 in 2020 and 1.76 in 2021.
FIGURE 1.
Heatmap showing changes in weight Z‐scores before and after 2018. This heatmap illustrates the percentage change in weight metrics among infants, using 2018 as the reference point. The y‐axis represents different weight change patterns, including decelerated, slow, sustained, rapid and accelerated growth, while the x‐axis shows the years from 2015 to 2021. Each cell indicates the percentage difference in weight‐for‐age metrics compared with 2018. Positive values in darker blue represent an increase in weight trends, whereas negative values in lighter blue indicate a decline.
3.4. Interrupted Time Series Analysis of Weight‐for‐Age Z‐Score Changes
Table 2 presents the results of the interrupted time series analysis, evaluating the impact of the pandemic on ΔWAZ. For accelerated weight gain, the intercept RR was 0.677 (95% CI: 0.664–0.684), indicating the baseline trend before the pandemic. The annualised change RR was 0.968 (95% CI: 0.965–0.970), suggesting a gradual decline in accelerated weight gain before the pandemic. Against this improving background trend, with the onset of pandemic‐related social isolation, there was a significant immediate increase in accelerated weight gain (RR: 1.173, 95% CI: 1.154–1.193), followed by a sustained pandemic‐period increase (RR: 1.093, 95% CI: 1.063–1.125). A broadly consistent pattern was observed for rapid weight gain (ΔWAZ > 0.67): the intercept RR was 1.242 (95% CI: 1.226–1.257), with a gradual pre‐pandemic decline (annualised RR: 0.970, 95% CI: 0.968–0.973). In contrast, a significant immediate increase was observed (RR: 1.173, 95% CI: 1.154–1.192), with a sustained pandemic‐period increase (RR: 1.040, 95% CI: 1.025–1.055).
TABLE 2.
Interrupted time series results.
| Change in weight Z score | Parameter a | RR (95% CI) |
|---|---|---|
| Accelerated weight gain (change in ΔWAZ > 1) | Intercept | 0.677 (0.664–0.684) |
| Annualised change | 0.968 (0.965–0.970) | |
| Immediate change | 1.173 (1.154–1.193) | |
| Pandemic change | 1.093 (1.063–1.125) | |
| Rapid weight gain (change in ΔWAZ > 0.67) | Intercept | 1.242 (1.226–1.257) |
| Annualised change | 0.970 (0.968–0.973) | |
| Immediate change | 1.173 (1.154–1.192) | |
| Pandemic change | 1.040 (1.025–1.055) |
Abbreviation: RR, relative risk.
Intercept RR represents the baseline level of the outcome; Annualised change represents the estimated yearly change in RR before the onset of the pandemic. Immediate change indicates the sudden shift in RR at the onset of the pandemic (February 2020), reflecting an abrupt change in rapid weight gain patterns. Pandemic change represents the change in RR trend following the onset of the pandemic, capturing the sustained effects on rapid weight gain during the pandemic period (February 2020–December 2021). Note that immediate change and pandemic change are interrupted time‐series parameters, distinct from the study phase classifications (pre‐pandemic Phase I, pre‐pandemic Phase II and pandemic phase) used to describe the study population.
3.5. Impact of the COVID‐19 Pandemic on Actual Weight Gain
To assess the effect of the pandemic on weight gain in infants aged 9–12 months, an interrupted regression analysis was performed using ΔWAZs as a continuous outcome. During the immediate pandemic period, the estimated ΔWAZ was 0.146 (95% CI, 0.137–0.155). The pandemic‐period change estimate was 0.201 (95% CI, 0.176–0.226), reflecting the overall increase in ΔWAZs. To estimate actual weight gain, we analysed data from healthy children aged 9–12 months between 2015 and 2019, whose mean weight at the 9–12‐month NHSPIC screening was 9.92 kg (SD, 1.22). Over the 23‐month pandemic period, infants born during the pandemic experienced an excess weight gain of 244 g (95% CI, 214–275 g) compared with pre‐pandemic cohorts.
3.6. Stratified Analysis of Accelerated Weight Gain and Pandemic‐Associated Trends
We calculated the stratified RRs in order to measure ΔWAZ by demographic indicators (Figure 2) and the actual weight gain. The stratified RRs and increases in weight indicated significant associations between accelerated weight gain and the pandemic across various demographic factors. These results were consistent across sex, SES and perinatal status. Notably, the RR of immediate change in infants who were exclusively breastfed showed a statistically significant difference; however, this significance was not observed in the pandemic period. Both immediate change and pandemic RRs remained significant regardless of perinatal status, but the RR of immediate change was relatively higher in participants without a perinatal history. This difference in significance may be due to body weight changes being measured up to 1 year of age, a period when perinatal status and breastfeeding can influence health outcomes. Pandemic‐associated RRs for accelerated weight gain across all subgroups, including immediate and post‐pandemic change estimates, are presented in Table S3.
FIGURE 2.
Associations Between the changes in weight Z‐scores using population stratified identifiers. RR, relative risk. This figure presents stratified analysis of RRs with 95% confidence intervals (CIs) for rapid weight gain before and during the pandemic. The analysis examines whether ΔWAZ > 1.0 follows similar risk patterns across subgroups. Regardless of subgroup classification—sex, breastfeeding status (BMF), birth residence, socioeconomic status (SES), or perinatal status—RRs consistently indicate an increased risk during the pandemic period. Immediate change and Pandemic change refer to interrupted time‐series parameters, not to the pre‐pandemic study phases. *The RR of immediate change in infants who were exclusively breastfed showed a statistically significant difference; however, this significance disappeared in the pandemic change.
4. Discussion
This study identifies a significant increase in weight gain among infants aged 9–12 months during the period of social isolation in the pandemic era. Despite stable birth weights, ΔWAZ rose immediately and remained elevated, with infants born during the pandemic gaining more weight than pre‐pandemic cohorts. Notably, mean ΔWAZ was already above zero prior to the pandemic, consistent with previously reported tendencies of Korean infants to exceed WHO growth standard reference values during the first year of life [19]. The simultaneous decline in decelerated and rise in accelerated weight gain, together with the near‐doubling of the SD of ΔWAZ from approximately 0.94 in the pre‐pandemic period to 1.76 in 2021, indicates that the pandemic was associated with both a rightward shift and a substantial widening of the weight gain distribution, suggesting heterogeneous impacts of social isolation across infants. The pandemic‐associated increase in weight gain persisted across sex, SES and perinatal conditions, with higher risk in males and non‐exclusively breastfed infants. These findings highlight the importance of surveillance and early prevention strategies in infancy.
Our findings align with prior work showing pandemic‐era weight gain across age groups. Global disruptions strained public health systems, with downstream effects on lifestyle and weight status, particularly in youths and adults [1]. In Korea, unfavourable trends were reported among adolescents [20]. In younger children, large‐scale data demonstrated sharp increases in excessive weight gain; for example, Vogel et al. [21] observed marked acceleration among 4–12‐year‐olds. U.S. cohorts similarly reported weight gains far exceeding normative expectations among children with overweight or obesity [22, 23], with social disparities such as race and ethnicity contributing. Most studies focused on preschoolers and school‐aged children, but our results extend these patterns to the first year of life, a critical growth window, corroborating findings in infants 4–6 months old [24]. Growth in infancy is strongly linked to later obesity and metabolic syndrome, emphasising the importance of early patterns for long‐term health [2]. Our nationally representative analysis identifies distinct shifts in infant growth during the pandemic era, which may inform targeted interventions to prevent excessive gain early in life.
While mechanisms were not directly tested, several pathways are plausible in the context of 9–12 month old infants, who are entirely dependent on caregivers for nutrition and physical activity. At this age, infants are transitioning to complementary foods and pandemic‐related disruptions may have altered the timing and quality of this transition. Prior studies in Korean infants have shown that delayed introduction of complementary foods is associated with adverse growth outcomes, and that early introduction before 4 months is linked to higher BMI in later childhood [11, 14]. Physical activity in this age group is largely caregiver‐mediated; infants depend on adults to facilitate tummy time, floor play and exploratory movement. Social restrictions likely reduced opportunities for such activities and increased sedentary time [25], contributing to positive energy balance even among pre‐ambulatory infants [26]. Caregiver stress is another plausible contributor: the pandemic was associated with heightened parental stress and mental health challenges [25], which can influence feeding responsiveness and household routines. These findings are consistent with prior reports of greater weight gain in infants born during the pandemic as early as 4–6 months of age [24], suggesting that pandemic‐related growth disruptions begin even earlier in infancy than our study window captures.
Dietary patterns also shifted during this period, with food insecurity rising, access to fresh foods declining (especially in lower‐income groups), and consumption of shelf‐stable, energy‐dense items increasing in high‐income settings [27], patterns associated with paediatric weight gain [26]. In parallel, declines in exclusive breastfeeding may have removed a protective factor against rapid gain, as breastfeeding supports optimal nutrition and metabolic regulation and lower exclusivity has been linked to faster weight accrual [24, 28, 29]. Caregiver stress is another plausible contributor. The pandemic was associated with heightened parental stress and mental health challenges, which can influence feeding practices and household routines; pre‐existing parental obesity and family history are also linked to childhood weight gain [4]. These family‐level stressors may have interacted with environmental constraints to amplify risk in early infancy.
To our knowledge, this is the first study to evaluate population‐based changes in infant weight during the period of social isolation in the pandemic era using nationwide data. Earlier investigations centred on older children and adolescents [30, 31] or infants up to 6 months [24]. By examining infants at 9–12 months, we provide evidence that pandemic‐related disruptions also affected growth in late infancy, complementing prior reports and strengthening external validity through a large, representative sample. We employed ΔWAZ as the primary measure of postnatal weight gain, consistent with the established literature on rapid weight gain in infancy [12, 13]. Future studies modelling weight status at follow‐up while adjusting for birth weight as a covariate would offer complementary insights, particularly in disentangling the independent contribution of birth weight from postnatal weight gain, consistent with approaches used in prior infant growth research [2].
This study has limitations. We could not fully account for socioeconomic and regional disparities that may modify growth responses. Additionally, parental anthropometric data, including maternal and paternal BMI, were not available in this dataset, precluding assessment of the contribution of parental obesity to infant weight gain trajectories. Household arrangements and caregiving changes were not measured, limiting mechanistic inference. Individual‐level length data were not available in this dataset, precluding calculation of weight‐for‐length or BMI‐for‐age Z‐scores; future studies incorporating linked anthropometric data would allow more comprehensive characterisation of infant growth. Nonetheless, infants with major health abnormalities were excluded by design, and ΔWAZs are validated indicators of adiposity in infancy, supporting interpretability of ΔWAZ trends. Our data largely reflect the early pandemic phase; future work should track trajectories as restrictions ease and routines normalise to determine whether elevated ΔWAZ persists or attenuates.
In conclusion, infants in their first year of life experienced accelerated weight gain during the pandemic era, mirroring trends seen at older ages [1, 20, 21, 22, 23]. These findings underscore the need for clinical and public‐health measures to support families during crises, promoting healthy feeding, opportunities for movement and play and caregiver mental well‐being to mitigate long‐term risks associated with early‐life weight gain [2, 24, 28, 29].
Author Contributions
Eunkyo Ha: writing – original draft; visualistion. Ju Hee Kim: investigation. Jeewon Shin: research participation. Boeun Han: formal analysis; software; data curation. Min Seo Kim: formal analysis; data curation; visualisation. Seonkyeong Rhie: project administration; funding acquisition. Seung‐Hun You: statistical consultation and review. Man Yong Han: conceptualisation; formal analysis; methodology; validation; funding acquisition; supervision; writing – review and editing. All authors reviewed and approved the final manuscript.
Funding
This research was supported by a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (grant number: HR22C1605).
Conflicts of Interest
The authors declare no conflicts of interest.
Supporting information
Table S1: STROBE statement—Checklist of items that should be included in reports of cohort studies.
Table S2: Missing data summary and pattern.
Table S3: Pandemic‐associated changes in accelerated weight gain (ΔWAZ > 1.0) by subgroup.
Figure S1: Flow of participants in the study.
Figure S2: Trends in weight‐for‐age Z‐scores and the proportion of extreme weight gain in children aged 9–12 months (2015–2021).
Acknowledgements
The authors have nothing to report.
Data Availability Statement
This study was based on the National Health Claims Database established by the National Health Insurance Service (NHIS) of the Republic of Korea. Applications for using NHIS data are reviewed by the Inquiry Committee of Research Support. If the application is approved, raw data is provided to the applicant for a fee. Investigators who wish to reproduce our results or replicate the procedure can use the database, which is open for research purposes (https://nhiss.nhis.or.kr/).
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Table S1: STROBE statement—Checklist of items that should be included in reports of cohort studies.
Table S2: Missing data summary and pattern.
Table S3: Pandemic‐associated changes in accelerated weight gain (ΔWAZ > 1.0) by subgroup.
Figure S1: Flow of participants in the study.
Figure S2: Trends in weight‐for‐age Z‐scores and the proportion of extreme weight gain in children aged 9–12 months (2015–2021).
Data Availability Statement
This study was based on the National Health Claims Database established by the National Health Insurance Service (NHIS) of the Republic of Korea. Applications for using NHIS data are reviewed by the Inquiry Committee of Research Support. If the application is approved, raw data is provided to the applicant for a fee. Investigators who wish to reproduce our results or replicate the procedure can use the database, which is open for research purposes (https://nhiss.nhis.or.kr/).