Inner Nature: Vaccine Formulations

By Vidya Rajan, Columnist, The Times

In early October, I was listening to Weekend Edition on National Public Radio when an interview with the Executive Director of Shark Allies, Stefanie Brendl, came on the air [1]. Her concern was urgent: a world-wide coronavirus vaccine that could contain the ingredient squalene, isolated from from shark livers, could lead to over a half-million sharks being harvested just for this purpose.

This, in a world where shark numbers have been shrinking for the last 70 years [2]. 2.5 million sharks are already killed each year for use in the cosmetics industry, and about 100 million sharks are killed for shark fin soup [3], often by the cruel practice of “finning” where the dorsal fin is sliced off living sharks, and the mutilated live shark thrown back into the ocean. A recent survey conducted out of Texas A&M found that six countries’ reefs had no signs of sharks at all: Dominican Republic, the French West Indies, Kenya, Vietnam, the Windward Dutch Antilles, and Qatar [2]. This is alarming, because sharks are a top predator, and a decline in their numbers forebodes a decline in reef health, and by extension, marine health. In this ecosystem, Shark Allies has a small ask: that other sources of squalene, such as plants, could be used instead.

I was overwhelmed by this interview. The thought of sharks being killed for vaccines had just not occurred to me although I was aware of the use of squalene in vaccines and in research. I guess I had just not stopped to think from where it was sourced. Squalene is a precursor of steroids and is produced by a variety of plants, fungi, and animals, including humans. It is also made by recombinant methods using bacteria and yeast. The commercial sources I examined for squalene did not indicate the source, but it appears the largest source is shark livers: the word squalene is derived from a genus name for sharks, Squalus. Once a vaccine’s formulation is established, it is very difficult and time-consuming to make changes, and clinical trials have to be repeated with the new ingredient in a lengthy and expensive process. If sharks matter to you, it would be appropriate to contact your representative to urge the FDA to push for non-shark sources of squalene now, before it’s too late to change components.

I thought that this might also be a good time to examine the other components of a vaccine formulation and the roles that each component plays in eliciting the required immune response. According to The New York Times Vaccine Tracker (https://www.nytimes.com/interactive/2020/science/coronavirus-vaccine-tracker.html), as of 15 October 2020, there are 30 vaccines in Phase I trials, 15 in Phase II trials, 11 in Phase III trials, and 6 vaccines that have received limited approval.

The key ingredient in a vaccine is the antigen, against which the immune response is directed. There are several ways of obtaining the antigen from the pathogen (https://www.vaccines.gov/basics/types). Live attenuated (weakened) or inactivated whole pathogens have been a mainstay of the vaccine industry, are still popular because of their simplicity, and trigger the most effective immune response. MMR, rotavirus, chickenpox, and yellow fever vaccines are examples of attenuated pathogen-based vaccines. Inactivated vaccines are produced by using radiation, chemicals, or heat to denature the pathogen prior to use as an antigen. They are less effective than live vaccines in eliciting an immune response and may require the use of boosters. Polio, hepatitis A, rabies, and influenza vaccines are examples of inactivated vaccines. Subunit antigen vaccines contain only a portion of the pathogen, such species-specific proteins or sugars produced in a recombinant system. These antigens can be linked to other viruses or virus-like-particles that can present them to the immune system, or can be formulated for delivery directly. They also possess the advantage of having no chance of reverting to virulence because only a part of the pathogen is used. Toxoid vaccines are made from toxins that some dangerous bacteria produce, and are inactivated prior to use, such as tetanus toxoid, or diphtheria toxoid. DNA and RNA vaccines present a portion of the genetic material of the pathogen that is taken up in the body, translated to protein, and an immune response elicited. There are inactivated, viral-presented, mRNA, DNA, and protein-based vaccines against coronavirus in clinical trials. Hopefully all, or several, types will be effective in stimulating an immune response in these trials currently being conducted.

Although the antigen is the key ingredient of a vaccine, several other ingredients are also required for stability and increasing the body’s immune response [4, 5]. Stabilizers prevent the vaccine from temperature and pH-related effects which could cause clumping or hydrolysis of the antigen. Stabilizers include magnesium salts or sugar formulations.

To stimulate the immune response, additives known as adjuvants are used [6]. According to the CDC [6], some common adjuvants are aluminum salts, a squalene oil-water emulsion called MF59, short stretches of DNA containing cytosine-phosphoguanine units called CpG 1018, a mycobacterial extract called Freund’s adjuvant, an extract of bacteria called monophosphoryl lipid A (MPL) mixed with QS-21, an extract of the Chilean soapbark tree.

This is such a random and exotic clutch of adjuvants that I feel a little context is needed.

After all, why would a tree extract trigger an immune reaction? The answer is that our immune systems have been evolving in the presence of pathogens for millions of years, and so a kind of recognition occurs when molecules called microbe-associated molecular patterns, or MAMPs, come into contact with receptors called the Toll-like receptors (TLRs) or pattern recognition receptors (PRRs). MAMPs are very variable, and include common bacterial molecules like lipopolysaccharides, lipid A, flagellin, and DNA. This shortcut to recognition of pathogens alerts the immune system that a pathogen has entered the body, even before an infection can occur. TLRs and PRRs then activate the immune system through a variety of ways such as controlling the inflammation pathway, inducing fever, or antigen presentation to the adaptive immune system resulting in the formation of pathogen-targeted B-cells and T-cells. Thus, the job of the adjuvant is to replicate the function of MAMPs. Besides using known microbial structures, adjuvant identification is very much a trial and error discovery process. Somehow, someone discovered the Chilean soapbark tree was effective. This finding is not as random as it seems. The word “soap” in the tree’s common name indicates that the tree produces some sort of saponification agent, because soaps are actually made from fats. Saponins contain a lipid structure that is recognized by the immune system, and therefore this is not all that different from squalene, also a lipid.

Antibiotics are added to vaccine formulations to prevent the growth of bacteria. Aseptic production is an important consideration in the production of vaccines, so this is an added safety measure; sometimes these antibiotics are used to prevent bacterial infection of the tissues cultures in which viruses are in the produced for vaccines. Neomycin is the only antibiotic that I could find named, and it is present at very low concentrations (<25µg) in the mumps, measles, rubella vaccine (MMR) and inactivated polio vaccine (IPV).

Preservatives are added to prevent bacterial and fungal growth. The best known are thimerosol (ethyl mercury) and formaldehyde. Thimerosol has come under particular scrutiny for its presence in MMR vaccine and was falsely tied to autism [7]. This was highly damaging for the public health good of childhood vaccinations, and, as a precautionary measure, thimerosol has been discontinued since 2001 for use in recommended childhood vaccines in the United States and Europe, although it is present in some other non-routine vaccines and treatments for snake and spider bites. Another preservative is formaldehyde which is sometimes used to inactivate the pathogen, but this component is found in miniscule quantities that do not pose health concerns.

Lastly, many vaccines require refrigeration as part of their life cycle. A push to develop orally-delivered vaccines, particularly for pathogens that enter via the mucosal route, seeks to move towards formulations that can be eaten or taken as a pill. The advantage of these formulations is that they would avoid the cold chain, and also have less reliance on the use of sterile needles and trained professionals for administering the vaccines. No more “shots” or “sticks”, and no more tears.

We can be safe with a vaccine. The question is: Will sharks be safe from us?