Vaccine

Polio

Adverse Events Associated with Childhood Vaccines: Evidence Bearing on Causality (1994)
Polio vaccine history
Wikipedia polio eradication Polio deaths

HPV

https://www.fda.gov/ohrms/dockets/ac/06/briefing/2006-4222B3.pdf gardasil increases risk by 44% with prior exposure.
http://www.huffingtonpost.com/marcia-g-yerman/an-interview-with-dr-dian_b_405472.html

Herd Immunity

route of exposure - two types, 2nd through skin leads to allergy.
part of herd immunity is re-exposure.