Statistical analysis
Sample size was calculated using the binomial proportion test. The rate of vertical transmission was estimated to be 7% when defined by RT-PCR9. Assuming that using serology tests increases the rate to 10% versus 0% in non-infected population, 71 women were required (80% power, 5% one-sided alpha).
Categorical variables were analyzed using the chi-squared test or Fisher’s exact test. The correlation between maternal and neonatal IgG antibody levels was assessed by the Pearson coefficient. The locally scatter plot smoothing (LOESS) non-parametric regression model was utilized to compare the mean drop in antibody levels over time from COVID-19 diagnosis and delivery. Antibodies levels were normalized by dividing each value by the largest value. Anti-spike antibodies were also multiplied by 3 in order to fit the scale. Antibodies levels below the threshold for positive results according to the kit instructions were set to be zero.
Statistical analyses were carried out with SAS version 9.4 (SAS Institute, Cary, NC, USA). Significance was set at a p value of <0.05. Data were analyzed by the authors E.Y and Z.N.