Weight Trajectories of Vulnerable Infants Before and During the COVID-19 Era

Background Infants born preterm, with low birth weight, or requiring neonatal intensive care may be especially vulnerable to growth disruptions during events like the COVID-19 pandemic. We evaluated the pandemic's impact on their growth. Methods We conducted a retrospective cohort study using an interrupted time-series design to assess changes in weight gain among 11,663 vulnerable infants born in South Korea between January 2015 and December 2021. Infants were grouped into two pre-pandemic cohorts (2015–2017 and 2018–January 2020) and a pandemic cohort (February 2020–December 2021). Weight gain from birth to 12 months (corrected age) was assessed using changes in weight-for-age z-scores (ΔWAZ), categorized into four ranges: ΔWAZ < 0; 0 ≤ ΔWAZ ≤ +0.67; +0.67 < ΔWAZ ≤ +1.0; ΔWAZ > +1.0. Segmented regression was used to evaluate level and trend changes at the pandemic onset, adjusting for demographic and perinatal factors. Results The mean ΔWAZ was +0.31 (SD 0.89). Across all birth weight groups, ΔWAZ increased from the pre-pandemic to the pandemic period, with a noticeable inflection at the onset of the COVID-19 pandemic. In the normal birth weight group (≥2.5 kg), mean ΔWAZ rose from +0.66 to +0.78 and then to +0.83. Extremely low birth weight infants (<1.0 kg) showed improvement, from –0.89 to –0.82 and –0.59. There was a significant immediate increase in accelerated weight gain (IRR 1.18, 95% CI: 1.11 to 1.26, p < 0.001), most pronounced among infants with higher gestational age, though this was not sustained. Conclusion Despite initial concerns, vulnerable infants did not experience growth faltering during the pandemic. Instead, modest increases in weight gain were observed, with variations across different subgroups.