The B.1.617 variant contains two key mutations to the outer "spike" portion of the virus that attaches to human cells, said senior Indian virologist Shahid Jameel
Vietnam has placed orders for more vaccines from several suppliers and received around 2.9 million doses from its purchases and via the international COVAX scheme.
Vietnam has registered 6,396 coronavirus cases so far, half of which were recorded in the latest outbreak.
The European Medicines Agency's endorsement comes weeks after it began evaluating extending use of the vaccine, developed with Germany's BioNTech, to include 12- to 15-year-olds. It is already being used in the European Union for those aged 16 and older.
Inoculating children and young people is considered a critical step toward reaching "herd immunity" and taming the pandemic.
Muhyiddin said the stricter lockdown from June 1 to 14 was for all social and economic sectors, and that only essential services would remain in operation.
"With the latest rise in daily cases showing a drastically upward trend, hospital capacity across the country to treat COVID-19 patients are becoming limited," Muhyiddin said in a statement.
Britain also cut its order for the single-dose shot, also known as J&J unit Janssen's vaccine, amid issues with the company's supply chain and reports of rare blood clots.
J&J's vaccine is already approved in the United States and European Union, where reports of rare blood clots are being reviewed.
"I'm at the end of my tether, I had to see a psychiatrist because I can't stay shut inside," complained pensioner Nadia Mariella, 73, after receiving her vaccine in Buenos Aires.