Varicella, commonly known as chickenpox, was once a nearly universal childhood illness characterized by a distinct, itchy rash and fever. Before the advent of a preventive measure, this highly contagious virus infected millions annually, leading to thousands of hospitalizations and over one hundred deaths each year from severe complications. The disease was generally considered a harmless rite of passage, but it carried the risk of serious outcomes like bacterial superinfections, pneumonia, and neurological damage. The development and subsequent widespread adoption of a vaccine fundamentally transformed the public health landscape, shifting chickenpox from a common threat to a largely preventable disease.
The Research That Led to the Vaccine
The foundational work for the modern chickenpox vaccine began in the early 1970s with Dr. Michiaki Takahashi, a Japanese virologist. Dr. Takahashi was motivated to create a vaccine after his own son contracted a severe case of chickenpox, demonstrating the disease’s potential for serious illness. His research focused on creating a live-attenuated vaccine, which uses a weakened form of the virus to stimulate immunity without causing the full-blown disease.
He successfully isolated the varicella-zoster virus from a child and, through serial passages in cell cultures, created the virus strain that would become the global standard. This weakened strain was named the Oka strain, after the family from whom the original sample was taken. Initial trials, starting in 1974, demonstrated that the Oka strain vaccine was safe and effective, first in high-risk pediatric patients and then in healthy children.
Regulatory Licensing and Initial Availability
The vaccine candidate, based on the Oka strain, was eventually brought to the United States for clinical trials and regulatory review. This marked the transition from a successful research project to a commercially viable pharmaceutical product available to the public. The U.S. Food and Drug Administration (FDA) granted its licensure in March 1995.
The vaccine was initially released under the brand name Varivax, manufactured by Merck & Co. This approval made it the first vaccine licensed in the country to prevent chickenpox. Upon its release, a single-dose regimen was approved for healthy individuals aged 12 months and older. It was immediately available for purchase by healthcare providers.
Integrating the Vaccine into Routine Childhood Schedules
While the vaccine was licensed in 1995, its widespread public health impact began when it was formally incorporated into routine immunization recommendations. The Advisory Committee on Immunization Practices (ACIP) recommended routine vaccination for all healthy children starting in 1996. This recommendation was a pivotal step, establishing the vaccine as a standard part of pediatric care.
The initial strategy involved a single dose for children between 12 and 18 months of age, with catch-up vaccination for older susceptible children. However, observation showed that while the single dose was highly effective, cases of “breakthrough” varicella still occurred, and outbreaks continued in highly vaccinated populations. In response to this data, the ACIP revised its policy in 2006 to recommend a routine two-dose schedule. The two doses, typically given at 12–15 months and 4–6 years of age, substantially improved the vaccine’s effectiveness and were designed to prevent outbreaks by boosting immunity before children entered school.
Measuring the Decline of Varicella
The implementation of the routine vaccination program led to a rapid change in the epidemiology of varicella. In the years following the vaccine’s introduction in 1995, the number of chickenpox cases declined by more than 97% overall.
Before the vaccination program, the disease caused between 100 and 150 deaths and an estimated 10,500 to 13,500 hospitalizations annually. In the post-vaccine era, these severe outcomes have been nearly eliminated, with a 94% reduction in hospitalizations and a 99% reduction in deaths among children and adolescents. The decline in severe disease also led to the near eradication of varicella-related complications, such as Reye’s Syndrome, a condition often linked to aspirin use during viral infections.