TL;dr : The same thing.
India reached 3.6% fully vaccinated against C19 on about June 19th.
India's cases per million metric fell 83% from its May 8th peak to June 18th (from 283.5 to 48.07).
Vaccines had nothing to do with the drop.
The US reached 3.6% fully vaccinated against C19 way back on Feb 12th.
US peak cases per million occurred about a month before that, on about Jan 8th, at 758.56.
On Feb 12th cases per million had fallen to 293.61, or by 61%.
Vaccines had nothing to do with it.
Virus gonna virus.
Click any chart to enlarge.
