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How effective is Chickenpox vaccine?
In 1996, when the CDC’s Advisory Committee on Immunization Practices (ACIP) released its written report recommending the routine use of Merck’s newly licensed live chickenpox virus vaccine, Varivax, in young children 12 months of age and older, the vaccine was estimated to be between 70 and 90 percent effective at preventing chickenpox infections and 95 percent effective at preventing severe illness, for at least 7 to 10 years.
As vaccination rates increased, the number of chickenpox infections decreased; however, by 2001, rates began climbing again. Previously vaccinated children were developing chickenpox infections and frequently found to be responsible for classroom outbreaks. Outbreaks typically impacted younger elementary school children and in certain classrooms, up to 40 percent of vaccinated students developed the infection.
In 2002, a study published in the New England Journal of Medicine reported the chickenpox vaccine to be 44 percent effective against chickenpox infection and 86 percent effective against moderate or severe infection. Additional studies also reported on the failure of a single dose of chickenpox vaccine to prevent outbreaks, especially among school children.
In June 2006, ACIP voted to recommend that a booster dose of chickenpox vaccine be administered to all children, prior to school entry, between 4-6 years of age. Children under 12 who had previously received 1 dose of chickenpox vaccine were recommended to receive a second dose, with a minimal interval of at least 3 months. All persons over 12 years of age who had previously received one dose of chickenpox vaccine were also advised to receive a second dose, with a minimum interval of at least 4 weeks.
Exposure to natural chickenpox infection has been reported to delay or even eliminated the risk of shingles infection, an illness which is often more severe and potentially debilitating than chickenpox. European countries, such as the United Kingdom (UK) and the Netherlands, have chosen not to routinely recommend the chickenpox vaccine due to concerns that the reduction or elimination of chickenpox illness would result in a significant increase in shingles infections.
By the early 2000s, as chickenpox vaccine uptake increased, the incidence of shingles also rose significantly. One study found a 141 percent increase in the number of shingles cases over a 5 year period (1999-2003), while another found that shingles rates among children and adolescents increased by 63 percent between 2000 and 2006.
In 2006, when ACIP voted to recommend a booster dose of chickenpox vaccine, the CDC reported that the widespread use of the chickenpox vaccine had resulted in a significant decrease in chickenpox-associated hospitalizations and deaths. The CDC, however, chose not to report on the significant rise in rates of shingles which had occurred following the introduction of the chickenpox vaccine. Even prior to the introduction of the chickenpox vaccine, shingles related complications, hospitalizations, and deaths were reported to be 4-5 times higher than chickenpox morbidity and mortality.
A 2008 study reported that while chickenpox-associated hospitalizations and costs had decreased in the post-vaccine era, those decreases were significantly less than the increase in hospitalizations and costs associated with the rise in shingles cases.
The increase rate of shingles prompted Merck to introduce Zostavax, a live virus shingles vaccine in 2006, for use in adults 60 and older. In 2017, Shingrix, a recombinant, adjuvanted shingles vaccine manufactured by GlaxoSmithKline, received FDA approval for use in adults 50 years and older.
Since the two- dose chickenpox vaccine recommendation in 2006, chickenpox illness continues to be reported among vaccinated children, including those with a history of receiving 2 doses of the vaccine.
In 2006, an outbreak of chickenpox in Arkansas involving 84 students found that 25 (30 percent) students had received 2 doses of chickenpox vaccine and 53 (63 percent) had previously been vaccinated with a single vaccine dose. In 2011, two schools in Texas experienced an outbreak of chickenpox. In one school, one dose of chickenpox vaccine was found to be 80.9 percent effective, while two doses of the vaccine was noted to be 94.7 percent effective. At the second Texas school, vaccine effectiveness for one dose was 80.1 percent while 2-dose effectiveness was only at 84.2 percent.
In 2015, 9,455 cases of chickenpox were reported to the National Notifiable Disease Surveillance System (NNDSS) by the 40 reporting states. Of the 53 percent (4,982) of reported cases where vaccination status was known, 58 percent (2,900) of infections occurred among persons with a history of receiving at least one dose of chickenpox vaccine. Of those persons reporting a history of receiving at least one dose of the vaccine and who had information on the number of doses they received (2,207), 57 percent (1,261) reported receiving two doses.
A 2017 published study of approximately 10,000 Air Force recruits found that previously vaccinated recruits were 24 percent less likely to have the minimally required vaccine acquired antibody levels considered protective against chickenpox when compared with recruits reporting a history of natural chickenpox infection. Study authors concluded that antibody levels in young adults previously vaccinated with the chickenpox vaccine could not be considered acceptable to maintain herd immunity. Further, they also warned that “If vaccination in accordance with the current US VZV vaccination schedule is inadequate to maintain herd immunity, young adults not previously exposed to wild-type VZV may be at increased risk for varicella outbreaks.”
In addition to reports of vaccine failure, published studies have also found that the live virus chickenpox vaccine can cause vaccine strain chickenpox infection in both healthy and immune compromised recipients or among close contacts of a vaccinated individual.
A case report published in 2000 describes the case of two healthy brothers who were administered the chickenpox vaccine. Five months later, one of the boys developed shingles and then several weeks later the other boy developed a mild case of chickenpox. The chickenpox vaccine was found to be responsible for causing the vaccine strain of chickenpox that developed from exposure to vaccine strain shingles. The study authors cautioned that vaccinated individuals who develop shingles must still be considered contagious and noted that the risk of chickenpox transmission from a vaccinated person who develops shingles is unknown.
A similar case was reported in Japan, where a healthy 3 year old girl developed shingles two years after receiving the chickenpox vaccine and her healthy brother developed vaccine strain chickenpox with a rash and fever shortly thereafter.
Chickenpox vaccine may also fail to protect immunocompromised individuals, including those with leukemia, and vaccinated persons with a history of asthma may also be at a higher risk of chickenpox illness.
IMPORTANT NOTE: NVIC encourages you to become fully informed about Chickenpox and the Chickenpox vaccine by reading all sections in the Table of Contents , which contain many links and resources such as the manufacturer product information inserts, and to speak with one or more trusted health care professionals before making a vaccination decision for yourself or your child. This information is for educational purposes only and is not intended as medical advice.