Reason for withdrawal from publication
This review was prepared before Dr Wang joined Aventis Pasteur.
This review was withdrawn from The Cochrane Library issue 3, 2006.
The electronic searches were last conducated in 1999. The authors have withdrawn from any further updating of this review.
This review is currently being updated by a new team of review athors.
The protocol is expected to be ready for publication in The Cochrane Library by April 2007.