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.