Indian opposition leader Rahul Gandhi has called for a nationwide lockdown as the country's tally of coronavirus infections surged past 20 million, becoming the second nation after the United States to pass the grim milestone.

India's deadly second wave of infections, the world's biggest surge in coronavirus infections, has seen it take just over four months to add 10m, versus more than 10 months for its first 10m. Currently, the country has 3.45m active cases.

Today, India reported 357,229 new cases over the last 24 hours, while deaths rose 3,449 for a toll of 222,408, health ministry data showed.

Medical experts say actual numbers in India could be five to ten times higher than those reported.

"The only way to stop the spread of Corona now is a full lockdown... GOI's inaction is killing many innocent people," Congress MP Gandhi said on Twitter, referring to the Government of India.

Prime Minister Narendra Modi's government is reluctant to impose a national lockdown due to the economic fall out, yet several states have imposed various social restrictions.

The surge in cases of the highly infectious variant of Covid-19 seen in India has swamped the health system, drained supplies of medical oxygen vital for survival for those infected, and seen patients dying in ambulances and carparks outside hospitals.

Rows of funeral pyres in parks and carparks cremate the overflow of corpses.

India has postponed exams for trainee doctors and nurses in a desperate effort to fight the infections sweeping across the world's second-most populous country.

Mr Modi has been criticised for not moving sooner to limit the latest wave of infections and for letting millions of largely unmasked people attend religious festivals and crowded political rallies during March and April.

The surge in Covid-19 in India has coincided with a dramatic drop in vaccinations, due to problems with supplies and delivery.

Despite being the world's biggest producer of vaccines, India does not have enough for itself.

Public forecasts by its only two current vaccine producers show their total monthly output of 70-80 million doses would increase only in two months or more, although the number of people eligible for vaccines has doubled to an estimated 800 million since 1 May.

Just 9.5% of the population of 1.35 billion has received at least a single dose.

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Brazil struggles to deliver Covid vaccine second doses

Shortages of Covid-19 vaccines have forced several large Brazilian cities to suspend administering second doses, officials and media reports said, the latest breakdown in the hard-hit country's troubled immunisation drive.

Seven of Brazil's 26 state capitals have stopped administering the booster shot of Chinese-developed CoronaVac because of a lack of supply, according to news site G1.

And Rio de Janeiro, Brazil's second city, announced at the weekend it was halting second doses of the vaccine for ten days, before revising its immunisation calendar to ensure booster shots would be administered by age group - albeit at a slower rate.

The city of 6.7 million people resumed second doses yesterday, starting with over-70-year-olds.

However, those aged under 60, including health workers, could have to wait two to 12 days beyond the recommended 28-day interval between doses under the city's new immunisation schedule.

Other capitals that have suspended second doses include major cities such as Belo Horizonte and Porto Alegre, population 2.5 million and 1.5 million, respectively.

More than half Brazil's state capitals do not currently have enough vaccines to ensure everyone who has received a first dose will get the second on time, according to newspaper Folha de Sao Paulo.

Brazil, where Covid-19 has claimed more than 400,000 lives - second only to the United States - is struggling to vaccinate its 212 million people.

It has been using two vaccines, both of which require two doses: CoronaVac and the Oxford/AstraZeneca vaccine.

Last week, it received its first million doses of the two-dose Pfizer vaccine, which it began distributing yesterday.

Far-right President Jair Bolsonaro faces criticism for downplaying the pandemic and defying expert advice on containing it.