The herd immunity level is “far away” for the Indian population and it can only be achieved through immunisation by vaccines, a top health ministry official said on Thursday and added that reaching this threshold – believed to be crucial for the Covid-19 outbreak to end – in any other manner will exact too high a cost.

The official also hinted that India is looking at a multilateral, WHO-led mechanism to secure Covid vaccines and the country is yet to begin direct discussions with any of the developers that are leading the development race.

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“For a large country like India, herd immunity cannot be a strategic choice or a strategic option… It will come at too high a cost and can be done only through immunisations via vaccination,” said Rajesh Bhushan, the officer on special duty, health ministry, during a briefing on Thursday.

The march of an infectious disease such as Covid-19 can be halted only when there is a large enough proportion of the population that is immune to it – a threshold known as herd immunity. Besides a vaccine, which is yet to be approved, the only way people become immune is if they have had the disease and recovered.

“Government has not signed an agreement with any vaccine manufacturing company. There are multiple stakeholders within and outside government and ministry of health has started actively engaging with such stakeholders. It’s too premature to comment on supply and distribution of vaccine at this stage but whenever it comes it has to be deployed on a much larger scale than the existing vaccines,” said Bhushan.

Bhushan also said the question on who would get the vaccine first was still under deliberation at the health ministry.

World over, there are roughly 25 vaccines in human trial phases – including two from India. Three vaccine candidates – one each from US, UK and China – are largely regarded as the frontrunners since they are now at advanced stages of experiments among people.

Several countries have entered into commercial deals with the UK candidate’s developer AstraZeneca and the US candidate’s Moderna.

“India is in touch with global multilateral organisations such as GAVI, CEPI, WHO etc. to see how it can be made accessible for people in India,” said Bhushan. GAVI (Gavi-The Vaccine Alliance) and CEPI (Coalition for Epidemic Preparedness Innovations) have partnered with WHO and several of the vaccine developers to help pool resources for the production, acquisition of distribution of any potential Covid-19 vaccine, with fair access being one of the factors that they will keep in mind.

The two Indian vaccine candidates are Bharat Biotech’s Covaxin and Zydus Cadila’s ZyCov-D, both in phase I/II human trials for determining its safety and dosage.

About 141 vaccine candidates globally are in the pre-clinical stage, which means these are into research stages or in pre-clinical trials where animal experiments are taking place to generate toxicity data.

The virus strain isolated for Bharat Biotech’s Covaxin was isolated at the Indian Council of Medical Research-National Institute of Virology (ICMR-NIV) in Pune, and transferred to the company. It received central drugs controller’s approval for early human trials on June 29.

Zydus Cadila’s vaccine candidate is called ZyCov-D, a DNA plasmid vaccine that received drug controller’s approval for early human trials on July 2. The candidate was developed indigenously at the company’s Vaccine Technology Centre in Ahmedabad, Gujarat, and uses DNA to train the body to recognise parts of Sars-Cov-2 and build an immune response.

Experts said vaccine is the most cost-effective way to achieve immunity against an infectious disease.

“The vaccine will be ultimate to check the disease spread but we don’t know when an effective vaccine will be available for use even though all our efforts are being directed towards making it happen as soon as possible. A good vaccine is the most cost-effective way of preventing a disease,” said Dr Amita Jain, head, microbiology department, KGMU, Lucknow.

Till the time a vaccine comes, Covid-19 appropriate behaviour such as wearing a mask, maintaining physical distancing, and maintaining hand hygiene, etc, will work the best, Bhushan said at the briefing.

As the Covid-19 outbreak turned into a pandemic, epidemiologists around the world said countries would need to use a combination of suppression (not letting the virus spread) and mitigation (focussing on helping infected people and letting the virus spread in a limited manner to allow immunity to build up) strategies till a vaccine is developed.

For India, the second part has been deemed as not feasible since the country has fewer hospitals per person than developed countries – which raises the possibility of health services being overwhelmed.