Clinical characteristics of study infants
During the 9-month study period, 66 out of 234 infants (28.2%) contracted COVID-19, with 63 out of the 66 (95.5%) having a mother with confirmed COVID-19, and a median (IQR) age at diagnosis of 11.3 (10.8-11.9) months. The median ages of the infants at the time their mothers received the initial and subsequent doses of the COVID-19 vaccine were 5.6 and 6.7 months, respectively. The breastfed only/dominant group at the ages of 6-months and 12-months were 69.6% and 30.3%, respectively.
The proportions of COVID-19 in the 6-month breastfed and formula fed dominant infants were 44/163 (27.0%) and 22/71 (31.0%), respectively, p =0.53. The proportions of COVID-19 in the 12-month breastfed and formula fed dominant infants were 21.1% and 31.3%, respectively, p =0.11.
All of the infants who contracted COVID-19 had mild to moderate symptoms. None of them had severe pneumonia nor respiratory failure and all were alive and healthy at the end of the study period.
Comparison of the clinical characteristics of the study infants who contracted and did not contract COVID-19
The proportion of mothers who had a history of COVID-19 during study period was significantly higher in the infants who contracted COVID-19 (95.5%) when compared to infants who did not contract COVID-19 (41.7%). None of the mothers who contracted COVID-19 during the study period had severe illness.
The medians (IQR) of age at beginning of the study (1st January 2022) and average child’s age at the 1st and 2nd maternal immunizations were not significantly different between the infants who developed or did not develop COVID-19 during the study period (Table 1).
The proportions of male gender, breast feeding status at 6 months and 12 months, maternal vaccine regimens with and without mRNA, and mothers vaccinated within 6 months following birth of the study infants were not significantly different between the infants who developed or did not develop COVID-19 (Table 1).
Impact of breastfeeding status and immunization regimens on COVID-19
Although the COVID-19 negative probability curves of breastfed only/dominant infants at both 6 months and 12 months of age were higher compared with the formula-dominant infants, the confidence intervals of the curves overlapped with those of the formula dominant infants (Fig. 1A-B). There were no noticeable differences in the COVID-19 negative probability curves stratified by maternal vaccine regimens (Fig. 1C).
In the multivariate Cox regression models, after adjusting for the covariates, the hazard ratios supported the results of the Kaplan-Meier curves. In all infants, at both 6 and 12 months breastfed only/dominant had a protective effect against COVID-19 that approached statistical significance with the hazard ratio (HR) and 95% confidence interval (CI) of 1.50 (0.90, 2.51), p =0.13 and 1.63 (0.90, 2.51), p =0.09, respectively. Maternal vaccine regimen had no protective effect against COVID-19 with an HR (95% CI) of 0.86 (0.49, 1.52) p =0.61 (Table 2).
The sub-group results of the Cox regression analysis regarding vaccine regimen were omitted, as all 3 infants whose mothers had received one or more mRNA vaccinations within 6 months remained COVID-19 negative throughout the study period. Infants with a mother that received 2 doses within 6 months, breastfed only/dominant at 12 months had a protective effect against COVID-19 with the HR (95%CI) of 3.77 (0.77, 18.43), p =0.053 (Table 2).