Mice vaccinated against malaria can give the parasite an environment to acquire immunity and become more virulent.
Links:
[1] http://admin.indiaenvironmentportal.org.in/feature-article/malaria-vaccines-could-make-malaria-worse
[2] http://admin.indiaenvironmentportal.org.in/category/author/debora-mackenzie
[3] http://admin.indiaenvironmentportal.org.in/category/journal/new-scientist
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/malaria
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/drugs
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vaccination
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/medical-research
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/finance