Childhood vaccination against varicella: persistence of antibody, duration of protection, and vaccine efficacy.
OBJECTIVE
To document the duration of protection afforded by Oka/Merck varicella vaccine over a 7-year period.


METHODS
The subjects were healthy children 1 to 12 years of age originally enrolled in clinical studies to evaluate the primary immune response to varicella vaccine 6 weeks after vaccination. Each was monitored for antibody persistence, breakthrough infection, and household exposure to varicella to produce estimates of vaccine efficacy.


RESULTS
The 6-year cumulative varicella antibody persistence rate was 99.5% (95% CI: 98.9%, 100.0%). The annual breakthrough rate through 7 years ranged from 0.2% to 2.3% per year; the estimated cumulative event rate was 6.5%. Comparison of the observed average annual breakthrough rate with the age-adjusted expected annual incidence rate of varicella in unvaccinated children corresponded to an estimated vaccine efficacy of 93.8% to 94.6%. Eighty vaccinated children were exposed to varicella in the household, resulting in 8 (10%) cases of infection. When compared with the historical attack rate of 86.8% in unvaccinated susceptible persons exposed to varicella in the household, this yields an estimated vaccine efficacy of 88.5% (95% CI: 80.9%, 96.1%). Varicella cases in vaccinated children generally were mild.


CONCLUSIONS
The live attenuated varicella vaccine is highly effective in inducing persistent immunity and long-term protection against breakthrough varicella infection.
Vessey
S J
SJ
Merck Research Laboratories, Merck & Co, Inc, West Point, Pennsylvania 19482, USA.
Chan
C Y
CY
Kuter
B J
BJ
Kaplan
K M
KM
Waters
M
M
Kutzler
D P
DP
Carfagno
P A
PA
Sadoff
J C
JC
Heyse
J F
JF
Matthews
H
H
Li
S
S
Chan
I S
IS
D017677:Age Distribution; D000914:Antibodies, Viral / Q000276:immunology*; D002644:Chickenpox / Q000453:epidemiology / Q000276:immunology* / Q000517:prevention & control; D019433:Chickenpox Vaccine / Q000276:immunology*; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D008297:Male; D012307:Risk Factors; D013997:Time Factors
D000914:Antibodies, Viral; D019433:Chickenpox Vaccine