Squalene, a natural oil made in the livers of sharks, is set to be used in a number of Covid-19 vaccine candidates.
The ingredient is known as an adjuvant and helps increase the effectiveness of a vaccine by prompting a stronger immune response.
It's currently used in the flu vaccines made by British company GlaxoSmithCline.
The company has said it would make one billion doses of the substances for use in a potential Covid-19 vaccine, according to Sky News.
It takes around 3,000 sharks to produce one tonne of squalene.
Shark Allies, a California-based conservation group, has said that around 250,000 sharks will need to be killed to harvest enough squalene to provide the world's population with one dose of a coronavirus vaccine containing the oil.
Researchers have said that any vaccine might need two doses to provide adequate immunisation, pushing that number up to half a million sharks.
Stefanie Brendl, founder and executive director of Shark Allies, said: "Harvesting something from a wild animal is never going to be sustainable, especially if it's a top predator that doesn't reproduce in huge numbers.
"There's so many unknowns of how big and how long this pandemic might go on, and then how many versions of it we have to go through, that if we continue using sharks, the numbers of sharks taken for this product could be really high, year after year after year."
To avoid the major hit to shark populations, scientists are trailing and testing a synthetic alternative to squalene made from fermented sugar cane.
Conservationists estimate that around three million sharks are killed every year for squalene - which is used in machine oil and cosmetics as well as medicine.
Shark Allies has launched a petition urging the US and the Food and Drug Administration (FDA) to use existing sustainable options for a vaccine.
The petition reads in part: "Using sharks in covid-19 vaccines is short-sighted, unpredictable, and unsustainable.
"There are better alternatives. The industry must listen."
Featured Image Credit: PA