Mechanisms Behind Vaccine Side Effects: The Science That Causes This Arm Pain

After receiving a vaccine, we are likely to experience side effects. These can range from a simple sore arm to swollen lymph nodes and even a fever. The side effects to expect depend on the exact vaccine, with each type and variant having its own list of common side effects. Each person’s immune system will also react differently, making it difficult to say exactly what to expect after receiving the vaccination.

What we can do is take a closer look at the underlying mechanisms that cause these side effects, to try to understand why they occur and how best to treat them. Most relevant here for the initial response is the body’s innate immune system, with dendritic cells usually among the first to come into contact with the vaccine and present the antigen to the body’s adaptive immune system.

The key to redness, swelling, and fever are substances produced by the body that include various cytokines as well as prostaglandins, producing the symptoms seen with inflammation and injury.

Fight the intruder

The first difference between vaccines is how the antigen is introduced into the body. Traditional vaccines use either whole weakened viruses or parts of viruses, which are therefore the antigen or contain the expected antigen as part of their structure. Viral vector and mRNA (lipid vector) vaccines use a vector that transfers RNA into cells in the body, where the cell’s ribosomes then make copies of the antigen. As a foreign protein, this is then presented on the surface of the cell as an antigen of lymphocytes.

If antibodies already exist against this specific antigen, the lymphocytes (B and T cells) of the body’s adaptive immune system will immediately act on this antigen, and therefore the (suspected) intruder (virus, bacteria or toxin) that is attached to it. the antigen. If there are no antibodies, the process is similar, but takes a little longer for the immune response to build up.

Meanwhile, as the adaptive immune system increases, dendritic and other cells of the innate immune system that possess pattern recognition receptors (PRRs) for pathogen-associated molecular models (PAMPs) like macrophages and mast cells also release a number of cytokines, including inflammatory cytokines which cause vasodilation. The resulting influx of blood causes the skin to redden (to blush) and increased heat (hot).

As the blood vessels dilate, other mediators work to make the walls of these vessels more permeable, causing an increase in the amount of fluid outside the blood vessels (intracellular space). This exudation of fluid causes swelling (tumor) which decreases the likelihood of pathogens coming into contact with body tissue, while allowing macrophages and other leukocytes (white blood cells) to move freely so that they can hunt down pathogens.

In addition to the vasodilation process, some of the cytokines released cause hyperalgesia (pain), mainly increased sensitivity to pain. All of these are part of the acute inflammation process and are responsible for the “sore arm” often experienced after vaccination.

Vaccine fever

Among the symptoms commonly experienced after receiving certain types of vaccines, fever (accompanied by a headache) is not as common as pain in the arm, but it is still frequently reported. If we look at the results of the CDC study for the Pfizer-BioNTech mRNA vaccine for COVID-19 where people self-reported their symptoms, we can see headaches as a frequently reported symptom, however due to the prevalence in the placebo group of this symptom and a strong causal link appears to be absent.

There is a clearer link between vaccines and fevers, because one of the substances released during the inflammatory process is an eicosanoid called prostaglandin. This compound is involved in vasodilation, as well as in the inhibition of platelet aggregation (anti-coagulation) and especially in this case strongly affecting the thermoregulatory center of the body in the hypothalamus.

Simply put, the inflammatory response of the body’s innate and adaptive immune system results in a positive input to the body’s thermoregulatory system. The result of this may differ from a simple feeling of thermal discomfort, to a full-blown fever where the body temperature can exceed its normal levels by a significant amount.

None of this is to worry about as long as the fever does not rise above 40 ° C (104 ° F), which, as results from the previously linked CDC vaccine study show, is extremely rare. It is safe to use an NSAID like ibuprofen to relieve any fever as well as any pain symptoms that may occur after vaccination. Since there is no real pathogen present in the vaccine, the only important factor is that the adaptive immune system has the opportunity to learn the new antigen, which means that there is no immunological benefit to it. develop a fever, not to mention pain around the injection site.

Heart inflammation

Myocarditis (inflammatory cardiomyopathy) is an inflammation of the heart muscle that is most often caused by a viral infection. In an average year, more than one million people worldwide will develop myocarditis, and more than 300,000 patients will die from it. The cause of myocarditis is usually the infiltration of the heart muscle by pro-inflammatory white blood cells in response to an infection. In the case of the SARS-CoV-2 virus, this pathogen binds directly to ACE2 receptors on the surface of heart tissue, allowing it to infect these cells.

In the case of SARS-CoV-2 vaccines, myocarditis is a known side effect, although it is rare. An article by Biykem et al. speculates that its cause is molecular mimicry between the SARS-CoV-2 virus spike protein and autoantigens, including α-myosin. This is a protein also known as MYH6, which is significantly expressed in both the atria and the ventricles of the heart.

If misidentified by the adaptive immune system, it could lead to the heart muscle being targeted by leukocytes. Whether or not this is the actual mechanism responsible, David K. Shay et al. report that compared to the natural onset of myocarditis, its occurrence in cases related to COVID-19 vaccination is extremely rare. Even so, without understanding the exact mechanism involved, any link between specific cases of myocarditis and a SARS-CoV-2 vaccination remains based on circumstantial evidence.

Side effects of placebo

As noted in the context of the Pfizer-BioNTech study results published by the CDC, there were also many side effects reported in the placebo group, including prominent symptoms such as fever, diarrhea, and vomiting. This highlights a big problem not only with placebos during medical trials, but also as a corollary when people are given the real drug.

Fueled in part by media reports and things one may have read on social media or overheard by friends and colleagues, the mere fact of having been vaccinated can lead to symptoms such as nausea and pain. . When you are already looking for something going on, you are much more likely to find it. The exaggerated negative effects of a treatment or vaccine are generally called the “nocebo effect”, when the expectation of negative effects of the treatment (even if it is a placebo) can cause these effects in the patient. patient.

The nocebo effect (or phenomenon) is increasingly the subject of studies, due to its effects on medical treatments and patient care, as reported by Winfried Häuser et al. in 2012. We will particularly note here the importance of communicating with a patient about his expectations and adjusting them accordingly. In the context of a global vaccination campaign such as the one currently underway with the SARS-CoV-2 pandemic, one can also consider the impact of (social) media in terms of mass psychogenic illness (“hysteria of mass “).

While clear communication to set expectations is attempted by healthcare professionals and others, there is enough misinformation through other channels that some people are convinced of the suspected negative effects of these vaccines. The risk here is that even in those who are not necessarily against vaccination, they may still subconsciously project a subset of the negative effects they have heard about on their own bodies, causing a nocebo phenomenon.

Take a cooling pill (sugar)

As vaccines have become more refined and we understand better the effects and symptoms (desired and unwanted) that they cause in the body, they have become common enough that, generally, we hardly know how they are. protect us. After our childhood vaccines, we most often need vaccines when we need a tetanus booster, or when we travel to a foreign country that requires proof of certain vaccines, or just to protect ourselves against it. specific diseases that are endemic there.

What the world is currently experiencing as a result of the SARS-CoV-2 pandemic is (so far) truly unique, in that it is the first pandemic we are vaccinating against globally. . Compared to the leisurely pace of the decades-long smallpox eradication program, the current goal is to get the whole world immunized within a few years. Due to the simultaneous vaccination of so many individuals on such a massive scale, even rare side effects will occur more often on the basis of pure cases.

Even so, for the overwhelming majority of people vaccinated today, tomorrow and in the months and years to come, arm pain and perhaps feelings of nervousness will be the most they will ever experience from this campaign. mass vaccination. These are just natural signs of your body’s immune system doing exactly what it is supposed to do.