Not all mattress covers will protect against bed bugs.  You must make sure that the encasements are not only “escape-proof” but also “bite-proof”.  So, avoid low-cost mattress covers altogether.  Select a mattress encasement specifically designed for bed bug protection with special seams and zippers.  The zipper end stop is especially vulnerable to escaping bed bugs.  Also, check to make sure that the mattress encasements are in good condition, i.e. do not have any rips or holes.  Zipper end stops that have a velcro cover for instance is made to prvent bed bugs from escaping through it.  I would suggest to ensure that any claims for protection against bed bugs is backed by scientific research.