Cytomegalovirus vertical transmission rate according to IgG avidity value and valacyclovir treatment of maternal primary infection in the first trimester of pregnancy

Background CMV serology in the first trimester (T1) of pregnancy relies on IgG and IgM testing, followed by IgG avidity measurement when both IgG and IgM are positive. Valacyclovir is proposed to prevent vertical transmission in maternal primary infection (MPI) during T1. Our objective was to assess the risk of transmission with and without valacyclovir, based on IgG avidity values and MPI timing. Methods Cases (2012-2024) with positive or equivocal IgM, intermediate or low IgG avidity (Vidas and/or Liaison) in T1 and a CMV PCR performed in amniotic fluid were retrieved from the laboratory software. The MPI date was estimated using a logarithmic model based on Vidas avidity results. Results 561 women (58% untreated, 42% treated) were included. In cases with low avidity and no treatment, transmission was 34% and 31% with Vidas and Liaison assays respectively. Intermediate avidities and no treatment were also associated with high transmission rates (14% and 22%). Vidas avidity values ≥0.60 or ≥0.50 in samples collected before 12 weeks were associated with 0% and 5% transmission rates. Without treatment, transmission occurred in 26% (9/34) of women with a Liaison avidity value between 0.250 and 0.350. Valacyclovir significantly reduced transmission to 2%, 6%, and 16% in preconceptional, periconceptional, and T1 periods respectively (p=0.038, 0.008 and <0.001). Conclusions Our results suggests that the Liaison threshold for high avidity should be higher than ≥0.250. Except for Vidas avidity values ≥0.60 before 12 weeks, treatment is beneficial for women with positive IgM and low/intermediate avidity, regardless of MPI timing.