Authorities aim to generate herd immunity from COVID-19 in Metro Manila and 6 surrounding provinces by November this year, an official leading the coronavirus vaccination strategy said on Tuesday.

Herd immunity is the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection, according to the World Health Organization.

The Philippines could experience a shortfall in vaccine supply because it is sourcing 30 million shots from India, said vaccine "czar" Carlito Galvez Jr.

The world's largest vaccine manufacturer, India is struggling with a second wave of infections that pushed its tally of COVID-10 cases past 20 million, swamped the health system, and drained supplies of medical oxygen vital for survival for those infected.

This is why the Philippines eyes herd immunity in areas with the "highest health and economic impact," said Galvez, who is also chief implementer of the National Task Force Against COVID-19.

"Iyon ang ginagawa nating strategy. Kasi, kung nakikita natin na realistically, we will have a shortfall of supply, we need to strategize that we will get the same effect," he said in a press briefing.

"Ang target namin... we can have herd immunity in NCR, plus 6 provinces around NCR, by November," added the official.

(That is the strategy we are adopting. Because if we see that realistically, we will have a shortfall of supply, we need to strategize that we will get the same effect. Our target is that we can have herd immunity in NCR, plus 6 provinces around NCR, by November.)

Metro Manila alone is home to about a tenth of the country's 108 million people. The region accounts for about a third of the country's gross domestic product.

The Philippines has so far secured 4.040 million COVID-19 shots, of which at least 1.9 million have been administered as of May 2.