You might want to look at the data on the prevalence of B12 deficiency and serious neurological disease in India.   It is estimated by medical and nutritional researchers based on large-scale surveys that about 60% of vegetarians in India and 90% of vegans in suffer from health problems related to B12 deficiency.   Similar estimates for vegans and vegetarians in developed countries vary depending on the approach taken to test for and treat B12 deficiency.