NEW DELHI (REUTERS) - India's output of Covid-19 shots for August to December is likely to be lower than the government's public estimate, according to internal projections shared with Reuters by two sources.

Lower-than-expected production could delay India's plans to vaccinate all its adults this year, amid fears the country will face another surge of coronavirus infections in the winter.

India's ongoing second wave, the world's worst since the pandemic began, has overwhelmed its health system.

The government last week publicly estimated that 1.46 billion doses of its three approved shots - AstraZeneca, Sputnik V and Covaxin - would be made between August and December.

Production of the AstraZeneca vaccine, of which the Serum Institute of India (SII) is the biggest maker in the world, is expected to reach 100 million to 110 million doses a month from July and stay at that level for the foreseeable future, a source with knowledge of the matter said.

That would be at least 200 million doses fewer than the government's public forecast of 750 million AstraZeneca doses for the last five months of the year, a 27 per cent shortfall.

An internal government projection for August and September shared by a second source put the monthly number of AstraZeneca doses at 100 million, out of 200 million for all the three approved shots combined.

The sources declined to be named discussing a subject on which Prime Minister Narendra Modi's government has been criticised owing to a shortage of vaccines. Immunisations have fallen sharply since an April peak.

The Ministry of Health and Family Welfare did not respond to queries from Reuters.

SII, which is facing raw material shortages for another vaccine that the government is banking on, declined to comment.

A spokesman for Covaxin-maker Bharat Biotech said that last month, the company raised its annual production capacity to more than 500 million doses, or about 42 million doses a month.

It has been producing about 10 million doses a month.

Dr Reddy's Laboratories, the local distributor for Russia's Sputnik V, did not respond to a request for comment.

Apart from the approved vaccines, the government has also forecast the August to December production of 866 million doses of five yet-to-be-approved vaccines.

Overall, the government has pledged to make 2.67 billion doses available this year.

SII has already halted exports until the end of the year to meet domestic demand, in a major blow to dozens of poor countries relying on it for supplies.

India has so far administered 187 million vaccine doses, the most after China and the United States, but fully immunised only about 3 per cent of its 1.35 billion people.