ReviewMetaanalysis of vaccine effectiveness in varicella outbreaks
Introduction
In the US, a universal varicella immunization program for children has been introduced in 1995 and an immunization rate of 73% in children 19–35 months of age had been reached by 2002 [1]. This immunization program [2] initially included one dose to be administered between 12 and 18 months of age with attenuated virus from the OKA-strain (Varivax®, Merck and Company, Westpoint, PA). Unexpectedly, several varicella outbreaks in nursing homes, kindergartens and schools have been observed [3], raising concerns about robustness of VE. These concerns were supported by immunological data, that showed incomplete seroconversion after one dose in >5% of vaccinees (range from 0 to 18%, depending on the test used, see [4]) and also pointed to potential waning immunity. The potential role of waning immunity over time since immunization is subject of controversial discussions [5], [6], [7], [8]. However, a second dose of varicella immunization to be given at 4 to 6 years of age has recently been recommended in the US [9]. In Europe, the vaccine is licensed in at least 13 countries [10]. In seven of these varicella vaccination for all children is currently considered. Varicella vaccination for all children was first introduced in Germany. At present a second dose is only recommended for the Varicella-MMR combined vaccine. In order to provide an overall assessment of the VE for one dose of immunization with the live-attenuated OKA strain we performed a metaanalysis of outbreaks published between 1 January 1995 and 31 December 2006. By means of subgroup analysis we attempted to assess the time course of waning immunity.
Section snippets
Identification of studies
PubMed and Embase were searched applying the terms ‘varicella OR chickenpox’ and ‘outbreak’ and ‘vaccination OR vaccine’. Publication time limits from 1995 (year of approval of Varivax® by the US food and drug administration) until 31 December 2006 were chosen.
Inclusion criteria
We considered any reports of outbreaks in at least partially vaccinated general populations available in English or German language. Outbreaks in restricted subgroups, e.g. immunocompromized children were not considered. To be included, a
Identified studies
The applied search strategy yielded 100 potentially relevant publications in Medline and Embase which were reduced to 82 by eliminating double hits from both sources. After reviewing title and abstract 63 publications were mostly comments or discussions on different topics such as vaccine coverage or recommendations, and outbreaks in special settings (e.g. immunocompromized children). Of the remaining 19 publications, five dealt with public health responses to outbreaks, vaccine coverage, were
Discussion
To our knowledge this is the first metaanalysis on varicella VE for one dose of attenuated OKA strain virus administered at an age of least 12 months in outbreak settings. Overall a VE of 72.5% was calculated. Our findings additionally pointed to waning immunity after immunization following an either linear or exponential course.
Outbreak situations offer a good opportunity to evaluate the effect of immunization under high infectious pressure in the field where it is most needed. The fact that
Acknowledgement
O. Bayer is supported by LMUinnovativ research priority project MCHealth (sub-project II).
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