The chickenpox vaccine was first developed in Japan in the early 1970s and licensed for use in the United States in March 1995. Between those two milestones lies a surprisingly long story of cautious testing, scientific debate, and gradual global adoption that continues today. Some wealthy nations only added the vaccine to their routine childhood schedules within the past few years, and the reasons for that delay reveal real tensions in how countries weigh disease prevention against other population-level concerns.
The Disease Before the Vaccine
Before widespread vaccination, chickenpox was one of those childhood illnesses that almost everyone got. In the United States, roughly four million cases occurred each year, the vast majority in children under ten. Most kids recovered without trouble, but the sheer volume of infections guaranteed a steady stream of complications. Across Europe, modeling estimates put the annual burden at about 5.5 million cases, with roughly 18,000 to 23,500 hospitalizations and around 80 deaths each year.1PubMed Central. Estimation of the burden of varicella in Europe before the introduction of universal childhood immunization In the U.S. during the first half of the 1990s, about 145 people died annually from varicella-related causes.2PubMed. Decline in mortality due to varicella after implementation of varicella vaccination in the United States
The patients who fared worst were predictable: newborns, pregnant women, adults who had somehow escaped childhood infection, and anyone with a weakened immune system. But previously healthy children could also end up in the hospital. In one pre-vaccination study from Ankara, the most common complications in otherwise healthy kids were skin and soft-tissue infections, and the chickenpox-related mortality rate for children was about 3 per million.3The Turkish Journal of Pediatrics. Chickenpox complications, incidence and financial burden in previously healthy children and those with an underlying disease in Ankara in the pre-vaccination period The numbers were small on a per-child basis, but multiplied across millions of annual infections, the toll added up.
How the Vaccine Was Created
The story starts with an understanding of the virus itself. In the mid-twentieth century, Thomas Huckle Weller isolated varicella-zoster virus (VZV) from patients with both chickenpox and shingles, providing early evidence that the same virus was responsible for both diseases.4Seminars in Pediatric Infectious Diseases. Thomas Huckle Weller, MD: Nobel Laureate and research pioneer in poliomyelitis, varicella-zoster virus, cytomegalovirus, rubella, and other infectious diseases That discovery set the stage for vaccine development, because it meant a vaccine against chickenpox would also be, in a sense, a vaccine against the virus that causes shingles later in life.
The actual vaccine was the work of Dr. Michiaki Takahashi at Osaka University. In the early 1970s, he isolated VZV from the fluid of blisters on an otherwise healthy three-year-old boy with a typical case of chickenpox. The boy’s family name was Oka, and the viral strain took that name.5PubMed. Development of varicella vaccine in Japan and future prospects Takahashi then weakened the virus by growing it through multiple rounds of cell cultures, passing it through human embryonic lung cells and then guinea pig embryo cells, until the virus was alive but too feeble to cause full-blown disease.6The Journal of Infectious Diseases. Development of Varicella Vaccine That Oka strain remains the basis for every chickenpox vaccine used in the world today.
When Countries Started Using It
Japan and South Korea were the first to license and use the vaccine in the 1980s, initially focusing on high-risk children such as those with leukemia. The United States took longer. The FDA approved Merck’s version, marketed as Varivax, in March 1995, and the CDC’s immunization advisory committee recommended it for all children between 12 and 18 months of age, with catch-up vaccination for unvaccinated older children up to age 13.7PubMed. Varivax (Merck & Co) Australia, Canada, Germany, and several other countries followed over the next decade or so.
But adoption was far from universal. Many developed nations held off, and some prominent examples resisted for a long time. The United Kingdom, for instance, did not recommend the varicella vaccine for routine childhood use until November 2023.8PubMed. Parental perceptions of chickenpox and the varicella vaccine: A qualitative systematic review The reasons for such delays were not about doubting the vaccine’s safety or efficacy, but about a more complicated epidemiological question involving shingles, which we will get to shortly.
The Shift to Two Doses
When the U.S. first rolled out the vaccine, the recommendation was a single dose. It worked well for most children, but over the following years doctors noticed “breakthrough” cases: vaccinated kids who still caught chickenpox, though usually a much milder version. Studies confirmed that one dose was not enough for long-lasting, reliable protection, and that a second dose given at least four to six weeks after the first offered significantly better immunity.9PubMed Central. Varicella: Efficacy of Two-Dose Vaccination in Childhood
In 2006, the U.S. updated its recommendation to a two-dose schedule, with the first shot at 12 to 15 months and the second between ages four and six. Most countries that include the vaccine in their programs now follow a two-dose approach. There is also a combination vaccine (MMRV) that bundles measles, mumps, rubella, and varicella into a single shot. It is convenient, but data from a large Canadian cohort showed the risk of febrile seizures in the week or so after vaccination was about twice as high with MMRV compared with giving the MMR and varicella shots separately, though the absolute risk was still small: roughly 3.5 extra seizures per 10,000 doses.10PubMed Central. Risk of febrile seizures after first dose of measles-mumps-rubella-varicella vaccine: a population-based cohort study Because of this, some guidelines suggest giving MMR and varicella as separate injections for the first dose, particularly in children under two, and reserving the combination vaccine for the second dose.
How the Vaccine Changed the Numbers
The impact in the U.S. was dramatic. In the years right after widespread vaccination began, varicella-related deaths dropped sharply. Age-adjusted mortality fell by about two-thirds between the early 1990s and the period from 1999 through 2001, and the steepest decline was in children ages one to four, where deaths dropped by 92 percent.2PubMed. Decline in mortality due to varicella after implementation of varicella vaccination in the United States Hospitalization rates and overall case counts plummeted as well. Similar patterns played out in other countries that adopted universal childhood vaccination, with accumulating evidence showing large reductions in disease incidence and hospitalizations.11PubMed Central. Global impact of varicella vaccination programs
In adults, the vaccine works but somewhat less impressively. Trials showed protective efficacy of about 65 percent against infection after household exposure. That sounds modest, but when vaccinated adults did get breakthrough chickenpox, the illness was invariably mild, making the vaccine’s effectiveness against severe disease essentially 100 percent.12PubMed. Prospects for use of a varicella vaccine in adults
The Shingles Question
This is the issue that made some countries, most notably the UK, hesitate for years. The logic goes like this: after you recover from chickenpox, the virus never truly leaves your body. It hides in nerve cells and can reactivate decades later as shingles. One thing that appears to keep shingles at bay is periodic re-exposure to chickenpox in the community. When you are around a child with active chickenpox, your immune system gets a natural “booster” that helps suppress the dormant virus. One influential study estimated that this exogenous boosting from exposure to chickenpox conferred about 20 years of protection against shingles.13Vaccine. Exposure to varicella boosts immunity to herpes-zoster: implications for mass vaccination against chickenpox
The concern was straightforward: if mass childhood vaccination eliminates chickenpox from circulation, adults and older people lose those natural boosts, and shingles rates could climb. It was a plausible worry, and a systematic review found that the majority of studies supported the boosting hypothesis as a biological phenomenon.14PLOS ONE. Herpes Zoster Risk Reduction through Exposure to Chickenpox Patients: A Systematic Multidisciplinary Review
But here is where the real-world data gets interesting. When researchers looked at what actually happened to shingles rates in countries that rolled out mass chickenpox vaccination, the predicted surge largely did not show up in a clear or consistent way. A systematic review and meta-analysis examining the population-level impact found that herpes zoster incidence was already rising before vaccination programs began, and that after implementation, there was no conclusive evidence of an additional increase in shingles in unvaccinated age groups attributable to the vaccination program itself.15Clinical Infectious Diseases. Systematic Review and Meta-analysis of Chickenpox Vaccination and Risk of Herpes Zoster: A Quantitative View on the “Exogenous Boosting Hypothesis” The researchers noted that the findings did not refute the boosting hypothesis altogether, but they questioned whether the effect is large enough to matter at the population level. The availability of dedicated shingles vaccines for older adults has also changed the calculation, since you no longer need circulating chickenpox to protect against shingles if you can vaccinate against it directly.
Safety Track Record
The chickenpox vaccine has been in use for three decades now, and the safety data is extensive. U.S. surveillance covering roughly 133 million doses of the standalone varicella vaccine distributed between 2006 and 2020 found that about 97 percent of reported adverse events were non-serious: injection-site reactions, rashes, and fevers. Serious events were reported at a rate of about 1.3 per 100,000 doses, and no new or unexpected safety signal emerged over that period.16PubMed Central. Safety Surveillance of Varicella Vaccines in the Vaccine Adverse Event Reporting System, United States, 2006-2020 Chinese surveillance data from Zhejiang Province paints a similar picture, with the vast majority of adverse events being mild and self-resolving.17PubMed Central. Surveillance of Adverse Events Following Varicella Vaccine Immunization in Zhejiang Province, China, from 2020 to 2022
A large population-based study of nearly 1.2 million vaccinated children specifically looked for serious conditions like immune-related blood disorders, meningitis, encephalitis, and stroke, and found no increased risk from the varicella vaccine given alone. One caveat: when the varicella vaccine was given at the same time as MMR, there was a modestly higher rate of a blood-clotting disorder called ITP, which reinforces the rationale some guidelines use for separating the two shots on the first dose.18Vaccine. Assessment of pre-specified adverse events following varicella vaccine: A population-based self-controlled risk interval study
One wrinkle worth knowing about: the weakened vaccine strain can, like wild chickenpox virus, settle into nerve cells and occasionally reactivate. Research confirmed that the Oka vaccine strain persists in the body and can reactivate as antibody levels decline after vaccination.19Nature Medicine. Varicella vaccination: Evidence for frequent reactivation of the vaccine strain in healthy children When this happens, the resulting illness is almost always milder than natural shingles, but it means vaccinated individuals are not completely free from the virus for life.
How Long Does Immunity Last?
Immunity from the vaccine is durable but not permanent in all recipients. In adults, antibodies to VZV were detectable in about 80 percent of vaccinated individuals after one year and about 70 percent from two to six years out.12PubMed. Prospects for use of a varicella vaccine in adults A retrospective study of healthcare workers found that an estimated half had lost detectable antibodies by about nine years after vaccination.20PubMed Central. Long-term persistence of antibodies against varicella in fully immunized healthcare workers: an Italian retrospective cohort study Studies in children with leukemia showed that about 30 percent became seronegative within five years, though the vaccine still provided meaningful clinical protection.21PubMed. Persistence of immunity to varicella in children with leukemia immunized with live attenuated varicella vaccine
It is important to note that losing detectable antibodies does not necessarily mean losing all protection. Immune memory involves more than just circulating antibodies, and many people whose blood tests show low antibody levels can still mount a protective response when exposed to the virus. The two-dose schedule was partly designed to address waning immunity by building a stronger initial immune response.
Using the Vaccine After Exposure
One of the more useful features of the chickenpox vaccine is that it can work as emergency post-exposure prophylaxis. If an unvaccinated person is exposed to someone with active chickenpox, getting the vaccine quickly can prevent or soften the disease. A meta-analysis found that when the vaccine was given within three days of exposure, it prevented about 80 percent of chickenpox cases. After three days, effectiveness dropped to around 50 percent. Two doses given as post-exposure prophylaxis performed better than one, with about 60 percent efficacy compared to 43 percent for a single dose.22PubMed Central. Effectiveness of varicella vaccine as post-exposure prophylaxis: a meta-analysis This makes the vaccine a practical tool during outbreaks in schools or households, not just a scheduled childhood immunization.
The Economics of Vaccination
Cost-effectiveness has been studied extensively, and the numbers tend to favor universal vaccination. An Italian modeling study projected that without vaccination, the country would face about 35 million varicella cases over 50 years, with societal costs of around €23 billion. The most effective vaccination strategy modeled cut cases by about two-thirds and reduced deaths by 30 percent, while saving roughly €13 billion in societal costs.23PubMed Central. The Cost-Effectiveness of Universal Varicella Vaccination in Italy: A Model-Based Assessment of Vaccination Strategies A more recent English model similarly found universal varicella vaccination cost-effective, with vaccine price and treatment costs being the main variables that could change the conclusion.24PubMed. Modelling the impact and cost effectiveness of universal varicella vaccination in England
These analyses help explain why the UK, after years of deliberation, finally moved toward adding the vaccine to its schedule. The combination of falling vaccine prices, strong real-world evidence from other countries, and updated shingles vaccine availability tipped the balance.
Why Some Parents Still Have Doubts
Despite the data, parental hesitancy around the chickenpox vaccine is real and distinct from general vaccine skepticism. A qualitative systematic review identified five recurring themes in parental attitudes: a perception that chickenpox is a mild illness not worth vaccinating against, concerns about vaccine safety and efficacy, a belief that natural immunity gained from infection is superior, the influence of social and economic factors on decision-making, and a strong sense that vaccination is a personal parental choice.8PubMed. Parental perceptions of chickenpox and the varicella vaccine: A qualitative systematic review The “it’s just chickenpox” attitude is particularly stubborn. For the vast majority of children, chickenpox is indeed mild and self-limiting, which makes the case for vaccination feel less urgent to parents than, say, the measles vaccine. Public health messaging has to contend with the fact that the strongest argument for the vaccine involves relatively rare but serious complications, which most individual families never witness.
Cold Chain and Practical Hurdles
One reason the chickenpox vaccine was slower to reach lower-income countries has to do with its physical fragility. As a live vaccine, it requires continuous refrigeration from the factory to the clinic. Maintaining this cold chain is a general challenge for many vaccines, but live attenuated vaccines tend to be more sensitive to temperature fluctuations than inactivated ones. In regions where reliable refrigeration infrastructure is scarce, distributing the chickenpox vaccine is harder and more expensive than distributing some other routine childhood vaccines. Formulation strategies and cold-chain logistics remain active areas of work for expanding global access.