This study from 2018 establishes a strong correlation between the introduction of a new version of a polio vaccine in 2005 in India and a very high number of Non-Polio Flaccid Acute Paralysis (NPAFP) cases:
Analysis of data over 10 years (from 2000 to 2010) showed that the NPAFP rate increased nationally during this time . The NPAFP rate in 2010 was 12/100,000, which was some way away from the expected number of 2/100,000. It has been reported that in 2005 there was a sharp increase in the national NPAFP rate, which coincided with the introduction of a high-potency monovalent vaccine that contained 5 times the number of Type 1 viruses, compared to that contained in the previously used vaccine . The NPAFP rate, which was 3.11/100,000 in 2004, more than doubled (to 6.43/100,000) in 2005.
While the number of NPAFP cases started dropping along with the number of doses of vaccine administered, the authors can't establish a causal link and don't question the necessity of the vaccination campaign, but recommend to keep lowering the number of doses now that the disease has pretty much been eradicated.
This previous study from 2013 actually goes further and says the correlation is strong enough to suspect a causal link
The incidence of NPAFP was strongly associated with the number of OPV doses delivered to the area. A dose–response relationship with cumulative doses over the years was also observed, which strengthens the hypothetical relationship between polio vaccine and NPAFP. The fall in the NPAFP rate in Bihar and UP for the first time in 2012, with a decrease in the number of OPV doses delivered, is evidence of a causative association between OPV doses and the NPAFP rate.
Based on the number of polio cases in India at the time (less than 1,000 a year, figure 2 of this other study) and the excess number of NPAFP cases (490,000 over 12 years - table 2 of the first study), can we safely say that the "high potency" version of the vaccine introduced in 2005 ended up having dire detrimental effects? If so, were there legitimate reasons for the change of vaccine? If not, what could be a plausible cause of the unusually high number of NPAFP cases?