Squalene

Squalene is an organic compound. With the formula (C5H8)6, it is a triterpene. It is a colourless oil although impure samples appear yellow. It was originally obtained from shark liver oil (hence its name, as "Squalus" is a genus of sharks). All plants and animals produce squalene as a biochemical intermediate. An estimated 12% of bodily squalene in humans comes from the sebum, squalene has a role in topical skin lubrication and protection. 
Squalene is a precursor for synthesis of all plant and animal sterols, including cholesterol and steroid hormones in the human body.
Squalene is an important ingredient in some vaccine adjuvants: Novartis produces a substance they call MF59, while GlaxoSmithKline produces AS03.
Role in steroid synthesis.
Squalene is the biochemical precursor to steroids. The squalene conversion begins with oxidation (via squalene monooxygenase) of one of its terminal double bonds, resulting in 2,3-Oxidosqualene. It then undergoes an enzyme-catalysed cyclisation to produce lanosterol, which can be elaborated into other steroids such as cholesterol and ergosterol in a multistep process by the removal of three methyl groups, the reduction of one double bond by NADPH and the migration of the other double bond.
Squalene is an ancient molecule. In plants, squalene is the precursor to stigmasterol. In certain fungi, it is the precursor to ergosterol. However, blue-green algae and some bacteria do not manufacture squalene.
Production.
Squalene is biosynthesised by coupling two molecules of farnesyl pyrophosphate. The condensation requires NADPH and the enzyme squalene synthase.
Squalene is prepared commercially from geranylacetone.
Use as an adjuvant in vaccines.
Immunologic adjuvants are substances, administered in conjunction with a vaccine, that stimulate the immune system and increase the response to the vaccine. Squalene is not itself an adjuvant, but it has been used in conjunction with surfactants in certain adjuvant formulations.
An adjuvant using squalene is Seqirus' proprietary MF59, which is added to influenza vaccines to help stimulate the human body's immune response through production of CD4 memory cells. It is the first oil-in-water influenza vaccine adjuvant to be commercialised in combination with a seasonal influenza virus vaccine. It was developed in the 1990s by researchers at Ciba-Geigy and Chiron; both companies were subsequently acquired by Novartis. Novartis was later acquired by CSL Bering and created the company Seqirus. It is present in the form of an emulsion and is added to make the vaccine more immunogenic. However, the mechanism of action remains unknown. MF59 is capable of switching on a number of genes that partially overlap with those activated by other adjuvants. How these changes are triggered is unclear; to date, no receptors responding to MF59 have been identified. One possibility is that MF59 affects the cell behaviour by changing the lipid metabolism, namely by inducing accumulation of neutral lipids within the target cells. An influenza vaccine called FLUAD which used MF59 as an adjuvant was approved for use in the US in people 65 years of age and older, beginning with the 2016-2017 flu season.
A 2009 meta-analysis assessed data from 64 clinical trials of influenza vaccines with the squalene-containing adjuvant MF59 and compared them to the effects of vaccines with no adjuvant. The analysis reported that the adjuvanted vaccines were associated with slightly lower risks of chronic diseases, but that neither type of vaccines altered the rate of autoimmune diseases; the authors concluded that their data "supports the good safety profile associated with MF59-adjuvanted influenza vaccines and suggests there may be a clinical benefit over non-MF59-containing vaccines".
Controversies.
Attempts to link squalene to Gulf War Syndrome have been debunked. 
Shark conservation.
In 2020, conservationists raised concerns about the potential slaughter of sharks to obtain squalene for a COVID-19 vaccine.
Environmental and other concerns over shark hunting have motivated its extraction from other sources. Vegetable sources (primarily vegetable oils) include amaranth seed, rice bran, wheat germ, and olives. Biosynthetic processes using genetically engineered yeast or bacteria is also used.
Safety.
Toxicology studies indicate that in the concentrations used in cosmetics, squalene has low acute toxicity, and is not a significant contact allergen or irritant.
The World Health Organization and the US Department of Defense have both published extensive reports that emphasise that squalene is naturally occurring, even in oils of human fingerprints. The WHO goes further to explain that squalene has been present in over 22 million flu vaccines given to patients in Europe since 1997 and there have never been significant vaccine-related adverse events.