A long-term prospective study of varicella vaccine in healthy children

The varicella vaccine appears to be both safe and effective in preventing chicken pox. Different lots were tested in two groups of children with an over 93% positive antibody response rate in both. In group one, 137 with a positive response were followed for 10 years. In the second, 129 showed positive responses and were followed for 6 years. Only 2% to 3% of children responding to the vaccine developed chicken pox. Compared to the estimated yearly chicken pox rate in unvaccinated children, these children showed up to an 81% reduction in yearly disease rates.
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 chickenpax 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 (PA MA). 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 liters 8 at 6-weeks' postvaccination were four times more likely to develop chickenpox than those with titers [is greater than or equal to]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. Pediatrics 1997;100:761-766; varicella vaccine, chickenpox, varicella zoster.
ABBREVIATIONS. FAMA, fluorescent antibody to membrane antigen; pfu, plaque-forming units; ELISA, enzyme-linked immunosorbent assay; VZV, varicella zoster virus; PV, postvaccination. In March 1995, the twenty-one-year development and testing phase for [...]