Lead
Not long ago, chickenpox was discussed as something every child would catch sooner or later. That assumption changed fundamentally when Japan added varicella to its routine immunization schedule in October 2014. The reported case count, which had been roughly one million per year, dropped sharply. A generation of children is now acquiring immunity through vaccination rather than infection.
The shift from "a disease everyone gets" to "a disease most children are protected against" carries implications for how parents think about the schedule — and why two doses matter.
Before and After the 2014 Schedule Change
Before the routine varicella program launched in Japan in October 2014, approximately one million cases were estimated annually. Surveillance data following the program's introduction show a substantial reduction in case numbers, reflecting population-level herd protection [6].
The American experience offers a longer arc of evidence. Following vaccine introduction in the United States in 1995, hospitalizations had already fallen by 75–88% by 2000–2001 [2]. The public health impact of varicella vaccination is clearly demonstrated in the data.
Why Two Doses Are Necessary
A single dose of varicella vaccine is approximately 70–85% effective at preventing disease, while two doses raise effectiveness to 92–95%, according to ACIP: Advisory Committee on Immunization Practices, the US expert body that sets the national childhood vaccination schedule guidance [1].
After a single dose, exposure to varicella virus can cause "breakthrough varicella" — a milder form of infection with fewer lesions, occurring in a vaccinated person whose immunity is incomplete. Breakthrough cases typically have fewer lesions and less fever, which can cause the infection to go undetected, but the child can still transmit the virus to others [1]. Two doses reduce an individual's risk of disease and also reduce transmission chains across a population.
Japan's routine schedule places the first dose at 12–15 months of age and the second dose at least three months later [6]. Confirming that both are recorded in the boshi techo (Maternal and Child Health Handbook) is the practical first step.
Who Is at Greatest Risk of Severe Disease?
In immunocompetent children, varicella usually follows a self-limiting course. The following groups carry elevated risk of severe disease [1]:
- Neonates and infants under two weeks (particularly if maternal antibody transfer was limited)
- Children with immunodeficiency (during leukemia treatment, on prolonged corticosteroid therapy, etc.)
- Unvaccinated adults (adult infection tends to be more severe than in childhood)
- Pregnant women (high risk of pneumonia complications, and potential fetal effects)
Vaccinating the siblings of an immunocompromised child provides indirect protection to the child who cannot receive the vaccine.
The Relationship with Shingles
Once a person has had varicella — including through vaccination — the virus establishes latent infection: a dormant viral state in which the virus persists in nerve cells without causing symptoms in nerve ganglia: clusters of nerve cell bodies outside the brain and spinal cord, where varicella hides indefinitely and can reactivate as herpes zoster (shingles) years or decades later, typically when immunity declines with age.
Whether the risk of shingles is higher after natural infection or after vaccination remains an area of ongoing study without a clear consensus [5]. But there is no well-established evidence that childhood vaccination significantly increases the risk of shingles, and the evidence points against any meaningful increase.
What Parents Can Do
- Check the boshi techo to confirm both the first and second dose dates. If a dose is missing, consult your pediatrician.
- Completing the schedule before daycare or preschool entry reduces the risk of outbreaks in group settings.
- If a sibling develops chickenpox, an unvaccinated or single-dose child has the option of emergency post-exposure vaccination within 72 hours — contact your pediatrician promptly.
Summary
Varicella has become a preventable disease since routine immunization was introduced. A single dose does not always prevent illness; two doses matter both for individual protection and for maintaining herd immunity. Following the recommended schedule and confirming both doses in the vaccination record is the realistic approach to this infection.
References
- Marin M, Güris D, Chaves SS, Schmid S, Seward JF; Advisory Committee on Immunization Practices. Prevention of varicella. MMWR Recomm Rep. 2007;56(RR-4):1–40. PMID: 17585291.
- Seward JF, Watson BM, Peterson CL, et al. Varicella disease after introduction of varicella vaccine in the United States, 1995–2000. JAMA. 2002;287(5):606–611. doi:10.1001/jama.287.5.606. PMID: 11829699.
- National Institute of Infectious Diseases (Japan). Varicella surveillance data. IASR. 2024.
- Weinmann S, Chun C, Schmid DS, et al. Incidence and clinical characteristics of herpes zoster among children in the varicella vaccine era, 2005–2009. J Infect Dis. 2013;208(11):1859–1868. doi:10.1093/infdis/jit405. PMID: 23908483.
- Ministry of Health, Labour and Welfare (Japan). Overview of varicella vaccine routine immunization. 2014.