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Covaxin's Covid-19 jab, developed and manufactured by Hyderabad-based Bharat Biotech, generates antibodies that can effectively neutralise the Alpha (B117) and Delta (B1617) variants of SARS-CoV-2, the virus causing Covid-19, the US National Institutes of Health has said citing results from two studies of blood serum from people who received the shots.

Covaxin comprises a disabled form of SARS-CoV-2 that cannot replicate but still stimulates the immune system to make antibodies against the virus.

The NIH said an adjuvant, developed by the agency, has also contributed to the success of the highly efficacious Covaxin Covid-19 vaccine, which roughly 25 million people have received to date in India and elsewhere.

Adjuvants are substances formulated as part of a vaccine to boost immune responses and enhance a vaccine's effectiveness.

"Ending a global pandemic requires a global response," said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), part of NIH.

"I am pleased that a novel vaccine adjuvant developed in the United States with NIAID support is part of an efficacious Covid-19 vaccine available to people in India," he added.

The adjuvant used in Covaxin -- Alhydroxiquim-II -- was discovered and tested in the laboratory by the biotech company ViroVax LLC of Lawrence, Kansas with support exclusively from the NIAID Adjuvant Development Programme.

The adjuvant comprises a small molecule attached in a unique way to Alhydrogel, a substance frequently called alum that is the most commonly used adjuvant in vaccines for people.

Alhydroxiquim-II travels to lymph nodes, where the small molecule detaches from alum and activates two cellular receptors. These receptors, TLR7 and TLR8, play a vital role in the immune response to viruses. Alhydroxiquim-II is the first adjuvant in an authorised vaccine against an infectious disease to activate TLR7 and TLR8.

Molecules that activate TLR receptors stimulate the immune system powerfully, but the side effects of Alhydroxiquim-II are mild. This is because, after Covaxin is injected, the adjuvant travels directly to nearby lymph nodes, which contain white blood cells that play an essential role in identifying pathogens and fighting infection.

Results from a Phase 2 trial of Covaxin indicate that it is safe and well tolerated, while unpublished interim results from the Phase 3 trial indicate that the vaccine has 78 per cent efficacy against symptomatic disease, 100 per cent efficacy against severe Covid-19, including hospitalisation, and 70 per cent efficacy against asymptomatic infection with SARS-CoV-2.

Bharat Biotech signed a licensing agreement with ViroVax to use Alhydroxiquim-II in their candidate vaccines in 2019. This license was expanded during the Covid-19 pandemic to include Covaxin, which has received Emergency Use Authorization in India and more than a dozen other countries.

Bharat Biotech developed Covaxin in collaboration with the Indian Council of Medical Research-National Institute of Virology. Bharat Biotech expects to produce an estimated 700 million doses of Covaxin by the end of 2021.