Infant varicella incidence declined after implementation of national vaccine program

Chaves S. Pediatrics. 2011;128:1071-1077.

Varicella incidence among infants decreased nearly 90% after the implementation of a national varicella vaccination program, according to study results published online in Pediatrics.

Clinical, demographic and epidemiologic data were collected prospectively from 1995 to 2008 through a community-based investigation, in which researchers examined disease patterns of infants aged 0 to 5 months and 6 to 11 months. Records were obtained from two active varicella surveillance programs in Los Angeles County and West Philadelphia, comprising more than 600,000 people — approximately 2% of whom were aged younger than 12 months.

After the implementation of the national varicella vaccination program in 1995, the incidence of infant varicella disease decreased 89.7%, decreasing from 15.6 cases/1,000 infants to 1.6 cases/1,000 infants. Infants aged 0 to 5 months exhibited milder clinical disease than those aged 6 to 11 months: at least 50 lesions, 49% vs. 58%; fever (body temperature 38·C), 12% vs. 21%; and varicella-related complications, 6% vs. 14%, respectively.

“These findings that rates of complications are two to three times higher among older infants are likely attributable to decline in passively acquired maternal [varicella-zoster virus] antibodies after the first few months of life,” according to the researchers.

Despite significant reduction in varicella disease, exposure to varicella-zoster virus continued throughout the study. Exposure to another varicella carrier accounted for most of the identified sources of varicella-zoster virus infection among infants, and half of those exposures resulted from unvaccinated household members during the latter part of the study period.

Breakthrough varicella — infection in those previously vaccinated — also contributed to household exposures, as did the increasing percentage of reported herpes zoster exposures during the late period of the study. Between 2004 and 2008, 28% of infant case-patients with varicella for whom the household was reported as the infection source exhibited varicella that resulted from exposure to herpes zoster disease.

“Through reducing varicella disease and exposures, the varicella vaccination program in the United States has provided substantial benefits to infants who are too young to be vaccinated,” the researchers said. “The current two-dose varicella vaccine schedule recommended by the Advisory Committee on Immunization Practices in 2006, as well as herpes zoster vaccine among older age groups, is expected to further reduce the number of varicella cases.”

Disclosure:The researchers report no relevant financial disclosures.

Philip A. Brunell

The authors continue to mine the data from the two surveillence sites established to study the impact of varicella vaccine. In an earlier paper, they showed that infants aged younger than 1 year, for whom vaccine is not recommended, benefited from immunization of older children. In the current paper, they document this effect by demonstrating the decline in cases in this age group. Although many of us believed that the increased severity of those under a year of age was due to the contribution of cases of varicella of the newborn contracted from the infant's mother, this does not appear to be the case as this report records very few cases in the neonatal period. However, this may reflect herd immunity resulting in decreased maternal varicella.

The effect of maternal antibody on varicella during the early months of life has had a curious history. Before vaccines for many childhood diseases, it was clear that varicella was much more common during the early months of life than either measles, mumps, hepatitis or rubella. Some even questioned whether VZ antibody crossed that placenta at all. This was put to rest in 1966 in a paper in which we showed that antibody did indeed cross the placenta and those infants that were born too soon after maternal varicella to benefit from maternal antibody got varicella (Pediatrics. 1966;38:1034-1038). This subsequently led to the recommendation that these infants be passively immunized.

This was supported by a massive study in England and Germany (Lancet. 1989;2:371-3). In the introduction to this paper it is indicated that for several decades before passive immunization became available only one death annually was reported due to neonatal varicella in the U.K. Unfortunatly, the often cited rate of 30% mortality is derived from selected case reports continues to be cited. One should be cautious about citing concurrent illness, eg, diarrhea or otitis media as complications of varicella.

The good news is that varicella vaccine continues to look good but, as cited in a previous report from this group, the effect of vaccine seems to have leveled off.

—Philip A. Brunell, MD

Infectious Diseases in Children Editorial Board

Disclosure: Dr. Brunell reports no relevant financial disclosures.

Follow InfectiousDiseaseNews.com on Twitter.