WHAT COULD BE THE POSSIBLE CAUSE OF THE ALLERGIC REACTIONS CAUSED BY COVID - 19 VACCINE?

Many of you out there reading my blog, might be wondering what I was wondering - "Now that a vaccine has come, yet it causes severe allergic reactions, etc." So, this post is especially for all those curious minds !

Severe allergic reactions in many people who received the COVID - 19 vaccine produced by Pfizer and BioNTech maybe due to a compound in the packaging of the mRNA that forms the vaccine's main ingredient. A similar mRNA vaccine developed by Moderna, which was authorised for emergency use in US, also contains the compound, polyethylene glycol (PEG).

PEG has never been used before in an approved vaccine, but it is found in many drugs that occasionally trigger anaphylaxis - a potentially life - threatening reaction that can cause a fast heartbeat, a plunging blood pressure, rashes and shortness of breath. Some immunologists believe that a small number of people previously exposed to PEG may have high levels of antibodies against PEG, putting them at risk of an anaphylactic reaction to the vaccine. Some others are still skeptical of the link.

Anaphylactic reactions can occur with any vaccine, but are usually extremely rare. Since the Pfizer and Moderna mRNA vaccines use a new platform, the reactions call for a careful scrutiny. Pfizer's and Moderna's clinical trials of the vaccines, did not find serious adverse events caused by the vaccine, even though both the studies excluded people with a history of allergies to components of the COVID - 19 vaccines. People with previous allergic reactions to drugs or food, were not excluded.

Both the vaccines contain mRNA wrapped in lipid nanoparticles that help carry it to human cells, but also act as a vaccine ingredient that boosts the immune response. The lipid nanoparticles are chemically attached to PEG molecules that cover the outside of the particles, thereby increasing their stability and lifespan. 

However, the mechanism behind PEG - conjugated anaphylaxis is relatively unknown because it does not involve Immunoglobulin E (IgE), the type of antibody that causes classic allergic reactions. Instead, PEG triggers two other classes of antibodies - Immunoglobulin M (IgM) and Immunoglobulin G (IgG). IgM and IgG are involved in a branch of the body's innate immunity called the complement system. 

Complement Activation - Related Pseudo Allergy (CARPA) is a non specific immune response to nanoparticle - based medicines, often PEGylated that are mistakenly recognised by the immune system as viruses. CARPA explains severe anaphylactoid reactions which few PEGylated drugs are occasionally known to cause, including the cancer blockbuster - Doxil.

To understand the risk, it is crucial to unravel the mechanisms underlying the immune reactions and find out how often they are likely to occur. Anaphylactic reactions produce biomarkers that remain in the blood only for a few hours. 

If PEG does turn out to be the culprit, the question is, what can be done? Screening millions of people for anti - PEG antibodies before they are vaccinated is not feasible. Instead, as per my knowledge, I feel it is recommended to not give the COVID - 19 vaccines to anyone with a history of severe allergic reaction to any component of the vaccine. For people who have had a severe allergic reaction to any other vaccine or injectable medication, the pros and cons of vaccination should be carefully taken into consideration, and people who might be at a high risk of an anaphylactic reaction should stay at the vaccination site for 30 minutes after their shot, so that they can be treated, if necessary. At least, anaphylaxis is something that happens quickly, so it is something which you can be very much alerted to, prepared to recognise and treat early.