Similarly, the interaction between the dynamics of transmission and protective immunity have been critical in the evaluation of strategies for switching from oral polio vaccine (OPV) to inactivated polio vaccine (IPV)Tebbens 2014.  OPV produces a stronger protective immune response, including secretory IgA at the mucosal surface, and is shed into the environment leading to passive environmental vaccination Hird 2012.  However, the live-attenuated virus used in OPV can, rarely, result in disease symptoms and reversion to wild-type virulence Hird 2012.  IPV does not present a risk for the emergence of vaccine-derived polioviruses, but at the cost of weaker immunity.  The risks of OPV outweigh those of IPV once disease rates drop sufficiently, though the specific rates at which this transition occurs depend on the local transmission and demographic context. Dynamic models are useful to identify this phenomenon in the first place, and combined with local surveillance and demographic observations can be of practical use to identify and evaluate locally tailored strategies for switching to IPV.