On 19 September 2019, the Philippines declared an outbreak of polio. Two cases have been reported to date, both caused by vaccine-derived poliovirus type 2 (VDPV2). Environmental samples taken from sewage in Manila on 13 August and a waterway in Davao on 22 August have also tested positive for VDPV2.
Note: The notion of "herd immunity" originally applied in the context of natural immunity which is acquired after contracting the disease in early childhood. If roughly two-thirds of the population have natural immunity, then herd immunity will be provided to the rest of the population. And as natural immunity decreases below this level, others will catch the disease until herd immunity is reached again. This notion of natural herd immunity was applied to vaccination, which is artificial and unnatural. However, vaccine caused outbreaks of disease have been shown to occur in 100% vaccinated populations.