The hard-hit United States passed a hopeful milestone of giving at least one Covid-19 vaccine dose to half its adults, as India locked down its capital New Delhi for a week from last night to try and control a raging outbreak.

There was good news from Australia and New Zealand as well as the two neighbours opened a travel bubble that would allow people to travel without a mandatory quarantine.

Infections are skyrocketing in India, however, with hospitals running out of beds and the government forced to reimpose economically painful restrictions again.

Delhi Chief Minister Arvind Kejriwal said the capital's "health system is at a tipping point".

"If we don't impose a lockdown now, we will be looking at a bigger disaster."

Kejriwal said businesses would be shut and movement around the city of 20 million limited to essential services.

The Delhi lockdown came after the vast nation of 1.3 billion people reported a record high of 273,810 infections yesterday -- the fifth consecutive day of more than 200,000 cases.

The restrictions followed similar measures in other parts of India, including in the western state of Maharashtra, home to financial capital Mumbai, reports AFP.

India said yesterday it will open its vaccination drive to all adults from May 1.

Former Indian prime minister Manmohan Singh has tested positive for coronavirus and was admitted to the All India Institute of Medical Sciences (AIIMS) in New Delhi.

The UK government said yesterday Prime Minister Boris Johnson's visit to India has been cancelled over the Covid-19 surge there.

The surge has overwhelmed the healthcare infrastructure in many parts of India, and authorities are scrambling to free up hospital beds and secure additional supplies of oxygen and treatment drugs.

India has the world's second-highest caseload with more than 15 million known infections.

Many Indians are pillorying Prime Minister Narendra Modi over his response to the scary surge in cases, sickened by him addressing tens of thousands of people at state election rallies and letting Hindu devotees congregate for a festival, reports Reuters.

Tags like #ResignModi and #SuperSpreaderModi have trended on Twitter in the past two days, as bodies piled up in mortuaries and crematoriums, and desperate cries for hospital beds, medical oxygen and coronavirus tests flooded social media.

US IN 'PRECARIOUS POSITION'

The coronavirus has killed more than 3 million people, devastating the world economy and upending daily life since emerging in China in late 2019.

The United States remains the hardest-hit nation, with more deaths and known infections than anywhere else, but it passed a major vaccine milestone on Sunday with roughly 130 Americans -- half its adult population -- receiving at least one dose.

The country is a world leader in vaccinations, and all its over-18s were eligible for a shot from yesterday.

But its top pandemic advisor Anthony Fauci warned Sunday that the United States remains in a "precarious position".

"We're having a seven-day average of over 60,000 new infections per day. That's a place you don't want to be," he said.

"We also have to make sure that people don't throw caution to the wind and declare victory prematurely. That's not the time to do that."

Neighbouring Canada illustrated the threat of a fresh coronavirus wave, as authorities scrambled to funnel additional health staff and equipment into virus-hit Ontario to battle a surge in infections.

Ontario is Canada's most populous province, and record cases are threatening to overwhelm its healthcare system.