Amid Row Over Quarantine In UK, PM Modi Bats For Mutual Recognition Of Vaccine Certificates

The UK released an updated travel advisory, adding four listed vaccines, for travellers to the country. However, people travelling from India still have to quarantine as the UK does not recognize the CoWIN certificate. Prime Minister Narendra Modi has now called for mutual recognition of vaccine certifications for ease of international travel. Why has the UK approved Covishield but only from 17 countries? Also what is the difference in covid vaccines manufactured in different parts of the world?