New Delhi: India is the largest buyer of COVID-19 vaccines in the world with 1.6 billion doses, according to a global analysis, a number some scientists say could cover 800 million people, or 60 per cent of its population, and will be 'enough to develop herd immunity'.

India has purchased 500 million doses of the Oxford University-AstraZeneca vaccine candidate, one billion from the US company Novavax and 100 million doses of the Sputnik V candidate from Russia's Gamaleya Research Institute, according to the US-based Duke University Global Health Innovation Center.

The Launch and Scale Speedometer' analysis, which is updated every two weeks, shows India has confirmed 1.6 billion doses of three vaccines as of November 30 while the US and the EU have purchased doses of six candidates.

According to the analysis, India is the top COVID-19 vaccine buyer followed by the European Union which has confirmed 1.58 billion doses and the US, the worst affected country so far, that has managed to purchase just over a billion doses.

As countries strategize to secure more shots to protect their populations against the novel coronavirus infection, Duke University has released a new global assessment of purchasing agreements for COVID-19 vaccines, even before any candidates are in the market.

Countries with manufacturing capacity, such as India and Brazil, have been successful in negotiating large advance market commitments with leading vaccine candidates as part of the manufacturing agreements, the Duke researchers noted in their analysis.

"The data is based on what is in the public domain and after speaking to government officials, especially in India," explained noted virologist Shahid Jameel.

In November, Union Health Minister Harsh Vardhan said 400-500 million doses of COVID-19 vaccines were estimated to be made available for 250 to 300 million (25 crore to 30 crore) people in India by July-August 2021.

Jameel told PTI that all the vaccines pre-booked' by India are manufactured by Indian companies Oxford-AstraZeneca and Novavax by Serum Institute of India (SII) in Pune and Sputnik V by Dr Reddy's Lab in Hyderabad.

India's domestic vaccine candidate by Bharat Biotech also entered Phase 3 clinical trials this week.

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Bharat Biotech and Zydus-Cadila would also add about 400 million doses annually. In brief, the numbers appear reasonable over 2021 and 2022, said Jameel, director of the Trivedi School of Biosciences, Ashoka University.

We can expect the first 250 million to be vaccinated in 2021; the remaining in the following years. The problem will not be vaccine doses, but the ability to deliver them, Jameel explained.

He noted that 1.6 billion doses would cover 800 million people or 60 per cent of India's population. This, the virologist said, should be enough to develop herd immunity, a form of indirect protection from infectious disease that occurs when a sufficient percentage of a population has become immune to an infection.

The first 500 million doses, the virologist said, are likely to go to 250 million people in the group that includes frontline workers, healthcare workers, sanitation, emergency services, and security services.

This group also includes the elderly above 65 and patients with comorbidities.

Immunologist Satyajit Rath said there is currently no particularly reliable estimate of the percentage of community coverage needed to be vaccinated in order to reach the point of 'herd immunity'. The reliability of models being used for these estimations is far from clear, he stressed.

Since all the vaccine candidates that are being discussed as purchased have two-dose vaccine regimens, the current number for India would vaccinate, at best, 80 crore (800 million) people. This, of course, does not account for logistical losses, which can be substantial, Rath, from the National Institute of Immunology (NII) in New Delhi, told PTI.

Also, this seems to imply that COVID-19 vaccination is a one-time exercise. But it is not clear how long the vaccine-mediated protection will last, and therefore it is not clear when repeat vaccination will be needed, and how that will be delivered, he added.

Flagging some uncertainties about the COVID dose purchases, Rath said he is curious about about the "confirmed" purchase by India.

Have these been paid for or at least committed to at a given price, are these acquisitions through the COVAX programme or independent of it, and most of all, of course, what is the delivery schedule agreed to in this acquisition, etc.

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Rath also noted that it is difficult to predict how many will be immunised, and when.

The next set of uncertainties I have is whether there are credible and well-worked out plans in India to match these delivery schedules with downstream transport, uptake and actual vaccination, plans for multi-stage transport, multi-point cold storage, provision of injection accessories... , the recording-keeping needed and most of all, the skilled personnel to administer the injections.

In Rath's view, a certain sector of healthcare workers will likely be immunised in the next few months, as well as a certain proportion of 'first responder' people at high occupational risk of infection. Beyond that, we are simply whistling in the wind at this stage, he said.

The Duke analysis also found that Canada and the UK have each struck deals for over 350 million doses from seven developers.

The analysis doesn't include Russia and China, both of which have their own vaccine programmes for their citizens.

The researchers noted that aflurry of nearly 200 COVID-19 vaccine candidates are moving forward through the development and clinical trials processes at unprecedented speed.

More than 10 candidates are already in Phase 3 large-scale trials and several have received emergency or limited authorisation.

The number of advance market commitments (AMCs) made by countries and multilateral partnerships eager to reserve vaccine supply, even before any candidates are on the market is unprecedented.

High-income countries currently hold a confirmed 3.8billion doses, upper middle-income countries hold 829 million doses, and lower middle-income countries hold more than 1.7 billion doses, according to the report.

The researchers said they have not been able to find evidence of any direct deals made by low-income countries, suggesting that such countries will be entirely reliant on the 20 per cent population coverage from COVAX.

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COVAX is a global initiative led by the WHO, the Coalition for Epidemic Preparedness Innovations (CEPI) and international vaccine alliance organization Gavi that aims to bring governments and vaccine manufacturers together to ensure all countries have access to COVID-19 vaccines once they become available.

Since none of these vaccines are approved, Duke researchers said some of the purchased candidates may prove to be unsuccessful, adding countries that can afford to are purchasing a portfolio of vaccines, in hopes that one or more will get through the regulatory process.