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What Are The WORST Covid-19 Vaccine Side Effects?

January 20, 2021

One of the most common questions you may have these days is ‘what are the worst Covid-19 vaccine side effects’.

There are already several approved Covid-19 vaccines available the world over – developed in the UK, USA, China, India and Russia. As with any medicine, one of the important aspects of their use is their side effects, and it’s important, for the Covid-19 vaccines, that you know about them and what to expect.

Estimated reading time: 14 minutes

Side Effects and Drug Benefits – Balancing Act?

First, let us set the stage:

Side Effects are common effects associated with using any drug – they are the unwanted (adverse) effect of taking them. Of course, they can be pretty mild, but they could also become severe and even cause death.

Developing a side effect depends on both the nature of the drug as well as the individual taking them. For different reasons, some of us will get side effects from taking one medicine while the rest of us do not.

So causing side effects is a characteristic of the Covid-19 vaccines. How can we manage any risk of adverse effects, though, given that we also need the benefit of treatment?

Well, think of the risks we take when we fly or even travel by road or sea.

The fact that these risks exist with any of these forms of transport generally makes us work harder towards reducing the risk rather than completely avoiding them.

Common Covid-19 Vaccine Side Effects

With the Covid-19 Vaccine (just like other drugs), there is a wide range of different side effects.

The first three vaccines to gain international approval are:

  • Pfizer BioNtech, Moderna and Oxford AstraZeneca, which we will consider here.

Other Covid-19 vaccines like Russia’s Sputnik V, China’s Sinopharm and India’s (Covaxin and Covishield) are also being used in many countries by the time of publishing this post.

First, let’s look at the more common side effects which most people will experience – generally, they will not be life-threatening; but they can be uncomfortable and distressing to different degrees.

Afterwards, we will consider the worst Covid-19 vaccine side effects.

Pfizer BioNTech and Moderna Vaccine Side Effects

The development of these two vaccines is quite similar, based on mRNA technology – read more about how they work.

Common side effects you may develop could start in the arm where you got the shot and include:

  • pain – aching or heaviness and stiffness
  • swelling around the point where the needle enters your skin
  • redness over or around the needle point

Throughout the rest of your body following your shot with either of these vaccines, you may develop the following:

  • chills and perhaps a fever – mostly low grade, but some have reported temperatures above 38 deg (100.4 F).
  • tiredness
  • A headache – this could be mild or moderate.

These side effects usually start within a day or two of getting the vaccine. They might feel like flu symptoms and might even affect your ability to do daily activities, but they should go away in a few days.

Oxford Astrazeneca Vaccine – Common Side Effects

The information on side effects of the Oxford Covid-19 vaccine provides some detail into how often they could happen:

Very Common (Affecting more than 1 in 10 people)

  • “Tenderness, pain, warmth, redness, itching, swelling or bruising where the injection is given”.
  • “Generally feeling unwell, feeling tired (fatigue), chills or feeling feverish, headache, feeling sick (nausea) joint pain or muscle ache”.

Common (May affect up to 1 in 10 people)

  • “A lump at the injection site, fever, being sick (vomiting), flu-like symptoms, such as high temperature, sore throat, runny nose, cough and chills”.

Uncommon (may affect up to 1 in 100 people)

“Feeling dizzy, decreased appetite, abdominal pain, enlarged lymph nodes, excessive sweating, itchy skin or rash”.

In clinical trials, there were very rare reports of events associated with inflammation of the nervous system, which may cause numbness, pins and needles, and/or loss of feeling. However, it is not confirmed whether these events were due to the vaccine.

In the Oxford AstraZeneca clinical trials, side effects that happen within 7 days of getting the vaccine were common, but they were mostly mild to moderate.

Side effects (such as fever, chills, tiredness, and headache) throughout the body were more common after the second dose of the vaccine.

Most side effects were mild to moderate. However, a small number of people had severe side effects that affected their ability to do daily activities.

Worst Covid-19 Vaccine Side Effects Reported in January 2021

To date, we have received some reports from different countries around the world about severe adverse effects which could be related to receiving the vaccine including:

  • Bell’s Palsy
  • Thrombocytopoenia
  • Anaphylaxis
  • Reports of Death

Bell’s Palsy

There were some incidents of Bell’s Palsy reported during the clinical trials for the Pfizer /Moderna vaccines.

Bell’s Palsy is the sudden development of weakness to one half of the face where the affected side goes slack, so the eyelid and mouth don’t close properly.

There are problems when eating; or stiffness and numbness to that side of the face. The condition may also affect taste and hearing.

We do not know exactly what causes Bell’s Palsy, but it is not a stroke – and the rest of the body under the head is NOT affected.

The current thinking is that if you develop Bell’s Palsy, it may have happened after a recent viral illness or other stressful condition.

Whatever makes it happen leads to reversible damage of the big nerve in the brain, which affects the facial muscles, skin, part of the tongue and ear.

It is reversible, so there is NO brain damage and can be treated quite easily with steroids.

8 people out of 74,000 developed Bell’s Palsy among the trials for both the Pfizer and Moderna vaccines (one of eight people was in the control group and did not receive the vaccine.

However, after investigating, the US Food and Drug Administration (FDA) does not consider the condition happened any more frequently in the trials than what we would expect in the general population.

They did not conclude these cases were caused by the Covid-19 vaccines, and therefore, persons who have previously had Bell’s Palsy may receive an mRNA COVID-19 vaccine.


Florida based – healthy GP dies 16 days after Pfizer vaccine” – News Headline. According to Pfizer – It is a highly unusual clinical case of severe thrombocytopenia, a condition that decreases the body’s ability to clot blood and stop internal bleeding. He was a healthy obstetrician aged 56 years who received the Pfizer jab on 18th Dec 2020. The CBC (complete blood count) that was done at his arrival showed his platelet count to be zero. (A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood). He was admitted in the ICU (intensive care unit) with a diagnosis of acute ITP (idiopathic thrombocytopoenic purpura) caused by a reaction to the COVID vaccine.” (Information based on a Facebook post by the widow).

Sadly, the passing of Dr Gregory Michael from possible complications following vaccination with the Pfizer vaccine has raised concern about its effect on the blood cells.

We understand he developed the first symptoms three days after the vaccine – dark non-fading bruises on the skin, which indicate internal bleeding. Despite treatment in the hospital, he developed a stroke linked to the problem with his blood cells and died on the 4th of January 2021.

Thrombocytopoenia is a disorder of blood cells known as platelets. In this condition, there is a lack of platelets, the cells whose main responsibility is to help blood clot. (Other examples of a blood condition are Anaemia which could develop from a lack of red blood cells).

The condition can be caused by cancer, anaemia, heavy drinking, viruses, some genetic conditions, toxic chemicals and medications such as diuretics and the rarely used antibiotic chloramphenicol.

In extremely rare cases, the measles, mumps and rubella vaccine has been linked to thrombocytopenia in young children, according to a 2003 study.

While investigations are still ongoing by regulatory authorities, the development of thrombocytopoenia has not been definitely linked to the Pfizer vaccine.


The worst Covid-19 vaccine side effect we’ve seen so far is called Anaphylaxis. There have been a number of reports of people developing the condition after getting their shot.

Anaphylaxis is a potentially life-threatening condition – a severe allergic reaction that can happen in any individual caused by things like medication, food, animals etc.

You may have heard of people having severe allergies to peanuts, shellfish, bee stings and even hair dye – well, this is the same process as what happens with the vaccines.

Can We Predict Who Will Get an Allergy Reaction?

Not always. If someone has had a severe allergic reaction in the past, they can have another severe response if exposed to the trigger or some other allergen – which is why they always carry an adrenaline injection or Epipen.

Anaphylaxis is an overreaction of the body’s immune system, which the National Health Service describes as severe and sometimes life-threatening.

Symptoms of Anaphylaxis develop VERY quickly, and in the cases reported within less than 15-30mins: breathing difficulty, feeling lightheaded or dizzy, rapid breathing and heart rate, swelling of tissues like the eyes, tongue, and tissues in the throat, collapse and – if untreated – death.

Stats – How Many Developed Anaphylaxis after Vaccination?

Cases of people developing anaphylaxis have been reported in both US and Britain – and likely in other places.

After Pfizer Vaccine

Most cases of anaphylaxis happened after receiving the Pfizer vaccine, but there are reports of a Boston doctor with shellfish allergy developing a severe allergic reaction after receiving the Moderna vaccine.

If and when other Covid-19 vaccines are authorized for use, health authorities will be watching closely to see whether anaphylaxis is linked to all Covid-19 vaccines. Or do they merely affect the Pfizer and Moderna vaccines made from messenger RNA (mRNA).

In a study released January 6 by the U.S. Centers for Disease Control and Prevention (CDC), researchers revealed that the risk of anaphylaxis—a severe, potentially life-threatening allergic reaction—from the vaccine is extremely low.

Based on data from people who have received the first of the two recommended doses for the Pfizer vaccine, only about one in every 90,000 people, on average, will experience this adverse reaction. Apparently, that’s less than 3 per cent of the lifetime risk of dying from choking on food.

Technically, the chance of severe allergic reactions from the Pfizer-BioNTech vaccine is about 8.5 times higher than the risk from the seasonal flu vaccine (which has a rate of about one in 769,000).

But CONTEXT is key.

This is because experts point out it’s still a tiny number: According to the CDC study, 1.89 million people in the U.S. received the first dose of the vaccine between December 14 to December 23, and more than 99.998 percent of them did not experience anaphylaxis.

Of the 21 cases of severe anaphylaxis associated with the Pfizer-BioNTech vaccine, 19 happened in women, and half were in people between the ages of 40 and 60. (The other half were between 27 and 40 years old.)

  • Seventeen of the cases (81%) were in people with a history of allergic reactions to food, medications, other types of vaccines, or bee and wasp stings.
  • Seven (33.3%) had a past history of anaphylaxis.

In 18 of the 21 documented cases, patients started showing symptoms of anaphylaxis within 30 minutes of getting the dose.

Nearly all were treated immediately with epinephrine, a common treatment for anaphylaxis and the active ingredient in an Epi-Pen.

Only four patients required hospitalization. At least 20 of the 21 patients had either fully recovered by December 23 2020 or had been discharged from the hospital by then.

After Oxford AstraZeneca Vaccine

No cases of anaphylaxis have yet been reported (by publish time) with the Oxford Astrazeneca vaccine.

This removes the need for a 15-minute wait post-vaccinations, which is a medical recommendation following the Pfizer BionTech shot.

By the 7th of January 2021, the UK medicine’s regulator UK MHRA (Medicines and Healthcare Products Regulatory Agency), said this advice was based on the fact that no reports of anaphylactic adverse reactions have so far been associated with the Oxford vaccine.

Patients who receive the Pfizer/BioNTech vaccine must still wait for observation for 15 minutes after receiving their jab.

In an update to the standard operating procedures for vaccination, NHS England also says: ‘For the Oxford/AstraZeneca, there is not a requirement for 15 minutes observation unless this is indicated after clinical assessment.’

Death Following Covid-19 Vaccines – What We Know

“Six People died during Pfizer vaccine last phase trials”. These claims are partly false.

Again context is important wh reading ‘shocking’ headlines like the one above.

Six people did die during the Pfizer-BioNTech vaccine trials, but only two of them were given the vaccine.

The other four who died were given a safe placebo solution of salt and water, and their deaths were unrelated to the vaccine.

In addition, the investigation into the two deaths did not show the vaccine was the cause of death.

They had died of problems linked with heart disease, and investigators felt these were natural events as could happen if they were not in the trial.

Covid-19 Vaccine Allergic Reactions

An immediate allergic reaction means a reaction happening within 4 hours of getting the vaccine. This includes symptoms such as hives, swelling, or wheezing (problems with breathing).

What causes you to react to the Covid-19 vaccine?

We think the answer is in the content of the vaccines.

In the segment below, we consider the different ingredients inside each of the vaccines and learn which competent scientists feel may be responsible for some of the allergies.

Causes of Allergic Reaction in Pfizer Vaccine

One dose (0.3ml) of the Pfizer-BioNTech COVID-19 Vaccine includes the following ingredients: mRNA, lipids ((4-hydroxybutyl)azanediyl) bis(hexane- 6,1-diyl)bis(2-hexyldecanoate), 2 [(polyethyleneglycol)-2000]-N,N- ditetradecylacetamide, 1,2-Distearoyl-sn- glycero-3-phosphocholine, and cholesterol), potassium chloride, monobasic potassium phosphate, sodium chloride, dibasic sodium phosphate dihydrate, and sucrose.

The shaded box above shows the contents of Pfizer’s Covid-19 vaccine. Unlike the common allergens we find from other vaccine products, it doesn’t contain egg, yeast, latex, or the SARS-Co-V2 itself (live or killed).

Instead, it contains synthetic (man-made) mRNA, fats like cholesterol, salts (sodium chloride or potassium chloride) and polyethyleneglycol (PEG).

It is this last component that may be responsible for allergic reactions. In their research, scientists observe allergic reactions to polyethylene glycol (PEG) and polysorbate in other conditions.

Polysorbate is not an ingredient in either mRNA COVID-19 vaccine, but it is closely related to PEG, which is in the vaccines. People who are allergic to PEG or polysorbate should not get an mRNA COVID-19 vaccine.

Allergic Reaction with Moderna Vaccine

The Moderna COVID-19 Vaccine contains the following ingredients: messenger Ribonucleic Acid (mRNA), lipids (SM-102, Polyethylene Glycol [PEG] 2000, Dimyristoyl Glycerol [DMG], cholesterol, and 1,2-distearoyl-sn- glycerol-3-phosphocholine [DSP C]), tromethamine, tromethamine hydrochloride, acetic acid, sodium acetate, and sucrose.

Similar to the Pfizer vaccine, Moderna’s Covid-19 vaccine does not contain eggs, yeast cells, latex, live or killed /virus.

Instead, it has a mixture of synthetic mRNA with fat cells, acid, salts, sugar and the chemical called PEG (Polyethylene glycol).

Oxford AstraZeneca Vaccine Allergy Reactions

One dose (0.5 ml) of the Oxford vaccine contains: Chimpanzee adenovirus vector encoding the SARS CoV 2 Spike glycoprotein (produced in genetically modified human embryonic kidney (HEK) 293 cells), L-histidine, L-histidine hydrochloride monohydrate, magnesium chloride hexahydrate, polysorbate 80, ethanol, sucrose, sodium chloride, disodium edetate dihydrate water for injections.

The vaccine does not contain egg or yeast cells and latex. It contains genetically modified organisms (GMOs) with weak Chimpanzee Adenovirus, Polysorbate 80, which is similar to PEG (Polyethylene glycol).

What is Polyethylene glycol (PEG), and How Can It Affect You?

You have probably noticed that many cosmetics and personal care products you use have different types of PEGs among the ingredients.

PEGs are compounds containing a mixture of different chemicals.

Those used in cosmetics help as emollients (which help soften and lubricate the skin), emulsifiers (which help water-based and oil-based ingredients mix properly), and as vehicles that help deliver other ingredients deeper into the skin.

In other words, they can help other materials penetrate the skin more effectively.

PEGs and PEG derivatives are generally regulated as safe for use in cosmetics, with the condition that impurities (which could cause cancer) are removed before they are mixed in cosmetic formulations.

Caution with Previous Allergies

Specifically, the CDC cautions that COVID-19 vaccines that use messenger RNA (mRNA)— including the Pfizer BioNTech and Moderna vaccines—also contain polyethylene glycol to shield the mRNA, so people with allergies to the compound shouldn’t get the vaccine.

Likewise, people allergic to polysorbates— which chemically resemble polyethylene glycol—(Oxford Astra Zeneca) shouldn’t receive the injection.


We still expect more revelations about the Covid-19 vaccines as more people receive their shots; and as more vaccines come into use.

Are these the worst Covid-19 vaccine side effects you’ve heard about?

If you had your Covid-19 vaccine and developed a side effect, let us know your experience in the comments below.

More Reading


Editing by AskAwayHealth Team


All AskAwayHealth articles are written by practising  Medical Practitioners on a wide range of healthcare conditions to provide evidence-based guidance and to help promote quality healthcare. 

The advice in our material is not meant to replace the management of your specific condition by a qualified healthcare practitioner.
To discuss your condition, please contact a health practitioner or reach us directly

Image Credits: Canva

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