Varicella is a highly contagious acute infection caused by varicella-zoster virus. Although the disease is usually mild in children, serious complications can occur, particularly in infants, adolescents, adults, pregnant women, and immunocompromised individuals. Importantly, varicella is vaccine-preventable. The World Health Organization recommends universal varicella vaccination in countries where the disease represents an important public health burden and sustainable vaccine coverage of at least 80% can be achieved. A single dose reduces severe disease and mortality, whereas a 2-dose schedule provides greater effectiveness and is recommended when the programmatic goal is to further reduce cases and outbreaks [1].

Mohamed et al. [2] report important findings from Bahrain, where varicella vaccine was introduced into the routine immunization schedule in May 2015 as 2 doses administered at 12 months and 3 years of age. Based on national surveillance data, the annual incidence of varicella declined from 251.8 per 100,000 in 2015 to 28.5 per 100,000 in 2024, representing an 88.7% reduction. The decline was particularly pronounced in the target age groups. Among children aged 0–4 and 5–9 years, incidence decreased from 1,150.2 and 1,156.9 per 100,000, respectively, in 2014 to 116.8 and 61.1 per 100,000, respectively, in 2024. The usual April peak also became less prominent. More than 98% of the 16,598 cases were managed in primary care, and all 12 hospitalized patients were unvaccinated. The screening method yielded vaccine effectiveness (VE) estimates ranging from 97.7% to 100%. However, this method provides only a crude estimate of VE. These findings demonstrate a substantial decline in reported varicella incidence following the introduction of a routine 2-dose vaccination program, although the additional benefit attributable specifically to the second dose cannot be determined from these surveillance data.

The situation in Korea is different. Varicella has been a nationally notifiable disease since 2005, the same year that a single dose of varicella vaccine at 12–15 months of age was included in the National Immunization Program. Vaccination coverage has exceeded 95% since the 2010s. Fig. 1 shows the age-specific number of varicella cases reported to the Korea Disease Control and Prevention Agency (KDCA) from 2006 to 2025 [3]. In the early years of mandatory reporting, reported cases differed considerably from estimates based on National Health Insurance Service reimbursement data; although this gap has narrowed substantially, some discrepancy may remain and warrants continued monitoring [4]. Moreover, only the number of reported cases is available, not incidence rates in the pediatric population, which makes precise comparison with earlier periods difficult. With this caveat in mind, reported cases declined markedly during the coronavirus disease 2019 pandemic in 2020–2022, likely reflecting public health measures such as social distancing and personal hygiene practices, and have not returned to pre-2020 levels. Nevertheless, breakthrough infections continue to occur, particularly among children aged 5–9 and 10–14 years. In a national cohort of 421,070 children born in 2011, 55,940 (13.3%) were diagnosed with varicella. One-dose VE was 86.1% (95% confidence interval [CI], 81.4%–89.5%) during the first year after vaccination but declined to 49.9% (95% CI, 43.3%–55.7%) at 6 years after vaccination, corresponding to an annual decrease of 7.2% [5]. Waning protection has also been observed in the United States, where the annual rate of breakthrough varicella increased from 1.6 per 1,000 person-years within 1 year after vaccination to 9.0 at 5 years and 58.2 at 9 years. Disease severity also increased with time since vaccination [6]. In recent years, interest in administering a second dose has grown among pediatricians in Korea, and some children receive a voluntary, privately funded second dose at 4–6 years of age.

Experience from countries that transitioned from a 1-dose to a 2-dose schedule provides further context. In Germany, where a second dose was added in 2009, nationwide surveillance showed VE of 86.6% after 1 dose and 97.3% after 2 doses, with an incremental effectiveness of 84.6% [7]. In the Munich area, pediatric varicella cases declined in a stepwise manner, with a further marked decrease after the 2-dose recommendation was introduced [8]. In the United States, the addition of a second dose at 4–6 years of age in 2006 was followed by a further 84.6% reduction in incidence between 2005–2006 and 2013–2014. The largest declines occurred among children aged 5–9 years (89.3%) and 10–14 years (84.8%) [9].

In summary, a single dose of varicella vaccine provides substantial initial protection, but its effectiveness wanes over time, and the potential benefit of a second dose is receiving increasing attention in Korea. The findings from Bahrain, together with the experience of Germany and the United States, support consideration of a routine second dose of varicella vaccine in Korea. Such a policy should be informed by Korean epidemiological data, VE, cost-effectiveness, and programmatic feasibility, together with continued surveillance of varicella incidence and breakthrough infections.