"The rise in the number of outbreaks of vaccine-derived polio has various causes. The COVID-19 pandemic hindered vaccination campaigns and surveillance. Disasters and conflicts, which often occur in countries vulnerable to polio, have done the same. Moreover, some countries decided to delay their outbreak responses and wait for supplies of the nOPV2 vaccine, rather than use the older type 2 vaccine. A modelling study, originally published in 2021, suggested that this delay would increase the risk of outbreaks occurring"

https://www.nature.com/articles/d41586-023-01953-7