title: A long-term prospective study of varicella vaccine in healthy children.
abstract: BACKGROUND
  Studies in Japan and the United States have shown that varicella vaccine is both safe and efficacious. In 1984, we undertook a 10-year prospective study using a research lot of Oka/Merck varicella vaccine to assess antibody persistence and breakthrough chickenpox rates. In 1987, we began a similar prospective study with lots made in production facilities that ended after 6 years because many children were given a second dose. The purpose of this study is to report humoral antibody persistence and breakthrough chickenpox rates after 6 to 10 years of prospective follow-up.
  
  
  METHODS
  One hundred forty-three seronegative children received a research lot (950 plaque-forming units/dose) with 97.9% seroconversion by an assay for fluorescent antibody to membrane antigen (FAMA). One hundred thirty-eight children received production lots (1145 to 3265 plaque-forming units/dose) with 93.5% seroconversion. Yearly chickenpox exposure surveys were completed by phone, and children were seen by a study nurse whenever chickenpox was suspected. A subset in each group had serum collected every 2 years and tested for FAMA antibody.
  
  
  RESULTS
  In the research group there have been 25 cases of chickenpox in 137 seroconverters in a period of more than 10 years (yearly rate of 1.7%). In the production lot group there have been 22 cases of chickenpox in 129 seroconverters in a 6-year period (yearly rate of 2.8%). In the research group the median titer rose from 1:16 to 1:64 between 1 and 10 years. In the production group, the median titer did not change between 1, 2, and 4 years. Median antibody titers were compared between the research and production groups at 1, 2, and 4 years and did not differ. The rate of development of modified chickenpox has not increased with time since vaccination, and neither has the case severity. Children with FAMA titers </=8 at 6-weeks' postvaccination were four times more likely to develop chickenpox than those with titers >/=64.
  
  
  CONCLUSIONS
  1) Modified chickenpox has occurred in approximately 2% to 3% of vaccinees per year, regardless of the vaccine lot given. 2) FAMA titers have risen between 1 and 10 years in research lot recipients and remained the same in production lot recipients. 3) The likelihood of modified chickenpox developing is inversely related to the 6-week postvaccination FAMA titer.
authors.affiliation: Department of Pediatrics, Case Western Reserve School of Medicine at MetroHealth Medical Center, Cleveland Ohio 44109, USA.
authors.forename: C E
authors.initials: CE
authors.lastname: Johnson
authors.forename: T
authors.initials: T
authors.lastname: Stancin
authors.forename: D
authors.initials: D
authors.lastname: Fattlar
authors.forename: L P
authors.initials: LP
authors.lastname: Rome
authors.forename: M L
authors.initials: ML
authors.lastname: Kumar
mesh_terms: D000293:Adolescent; D000914:Antibodies, Viral / Q000097:blood*; D002644:Chickenpox / Q000453:epidemiology / Q000276:immunology* / Q000517:prevention & control; D019433:Chickenpox Vaccine / Q000276:immunology*; D002648:Child; D002675:Child, Preschool; D005455:Fluorescent Antibody Technique; D014645:Herpesvirus 3, Human / Q000276:immunology; D006801:Humans; D007223:Infant; D011446:Prospective Studies
chemical_list: D000914:Antibodies, Viral; D019433:Chickenpox Vaccine