Congenital hearing loss is estimated to occur in 1–1.5 per 1000 individuals, with congenital cytomegalovirus (CMV) infection being the second most common cause of hearing loss after hereditary deafness [1]. Symptomatic congenital CMV infections are the leading cause of hearing abnormalities.

The 2003 Kimberlin trial recommended the use of an antiviral drug, intravenous ganciclovir (GCV), for the treatment of CMV [2], while the 2015 Kimberlin trial recommended the use of oral valganciclovir (VGCV) at 16 mg/kg for 6 months [3]. We conducted a physician-led clinical trial called VGCV-1, the world's first insurance indication for this administration method, and obtained insurance approval from Japan in April 2023 [4], [5]. VGCV administered within the 2 months of life has been reported to inhibit the progression of hearing impairment and other favorable effects [6]. However, there have been reports that even in cases in which hearing improved immediately after treatment with antiviral drugs, such as VGCV, fluctuations and progression of hearing impairment occur again after 1 year of age and that patients no longer respond to steroids and other therapeutic agents [7], [8]. Additionally, the European Consensus Statement in 2017 recommended that hearing tests be performed every 6 months until 3 years of age and annually thereafter [9].

Therefore, a long-term extension observational study (VGCV-2) was planned to conduct a prolonged hearing assessment following the VGCV-1 trial. Although the treatment and follow-up period for oral VGCV was set at 6 months in the VGCV-1 trial, the VGCV-2 extension study implemented annual follow-up assessments starting 1 year after treatment initiation and continuing for up to a maximum of 6 years. This report presents the results of postnatal VGCV administration for up to 3 years of age.