Now that the first COVID-19 vaccines are getting rolled out en mass to the public, many are worried about potential side effects. Actually, these side effects are now the subject of many myths and fear-mongering on social media by all sorts of people with more or less vested interests.

One particularly toxic video recently shared with me by a friend featured various people with “Dr.” in front of their names who, with a very pedantic air and I-know-what-I’m-talking-about certainty in their voice, claimed the coronavirus vaccines can sterilize women, cause devastating COVID disease, or alter DNA. These statements are not just false, they’re gross fabrications!

First of all, every medication on the market will have some sort of clinical side effect associated with it. Official data from clinical trials show that side effects for COVID-19 vaccines are rare and, in the highly unlikely event that they do happen, are very mild and should dissipate within a few days.

“Like with most vaccines, side effects are generally mild and include transient pain at the injection site or mild flu-like symptoms for one or two days. Allergic reactions can occur but they can be managed. Overall the side effects are by far less serious than an infection with SARS-CoV-2, which can kill. There is no reason to be concerned about vaccinating pregnant women,” Hildegund Ertl, Professor at the Vaccine & Immunotherapy Center at the Wistar Institute, told ZME Science.

Here’s exactly what we know so far about coronavirus vaccine side effects.

It’s important to recognize that all vaccines, and all medicine in general, may have side effects

Although vaccines are designed to protect from disease, they can cause side effects, just as any medication can. Throughout your life, it’s very likely that you’ve taken a wide range of vaccines already. The immunization schedule from the CDC lists over 35 vaccines, from Hepatitis B and MMR to polio, that a person in the United States needs to take from birth to 18 years of age. All have some degree of adverse effects, but they’re so rare and mild that they’re deemed safe. Also, their net benefit to society by preventing disease is overwhelmingly higher than the potential downsides.

The coronavirus vaccines are no different.

With any vaccine, you can expect a mild pain during and after injection. After vaccination, some of the most common side effects of the COVID-19 vaccine include:

• Injection site swelling
• Fever
• Chills
• Headache
• Tiredness

These mild side effects can be rather common but severe symptoms are very rare (one in hundreds of thousands).

“Most side effects of vaccines are mild and temporary, such as redness or swelling at the point of injection. Sometimes a vaccination can also be followed by a short-term fever as well as limb pain and headaches that can be treated with antipyretics and NSAIDs [nonsteroidal anti-inflammatory drug],” Maija Kaukonen, Chief Physician of Finnish Medicines Agency Fimea, told YLE.

There are over 60 vaccine candidates currently in development across the world. However, just a handful are currently deployed, including those from Pfizer-BioNTech, Oxford-AstraZeneca, Moderna, Russia’s Sputnik V, and China’s Sinopharm. However, in the United States, just two vaccines (Moderna and Pfizer-BioNTech) have received authorization for emergency use. AstraZeneca has also completed safety trials and passed all three stages and is currently approved in the UK, Mexico, India, and other countries. For these reasons, we will only discuss the side effects of these three vaccines since reliable, official data is available on them.

What are the side effects of the Moderna vaccine (mRNA-1273)?

The Moderna vaccine is injected into the muscle and requires two doses spaced one month apart. Side effects reported for Moderna’s COVID-19 vaccination during clinical trials include:

• Injection site reactions: pain, tenderness, and swelling of the lymph nodes in the same arm of the injection, swelling (hardness), and redness;
• General side effects: fatigue, headache, muscle pain, joint pain, chills, nausea and vomiting, and fever;

There is also a very remote risk that the Moderna vaccine could cause a severe allergic reaction, which would occur within a few minutes to one hour after receiving the dose. This is why vaccine providers should ask that you stay within reach for 30 minutes to one hour for monitoring after vaccination.

Three patients experienced Bell’s Palsy, a sudden, and usually temporary, weakening or paralysis of the facial muscles.

What are the side effects of the Pfizer-BioNTech vaccine?

Like Moderna’s vaccine, the Pfizer-BioNTech product is also an mRNA vaccine that is injected intramuscularly. Two doses spaced roughly four weeks apart are also required to boost protection.

The most commonly reported side effects, which typically lasted several days, were pain at the injection site, tiredness, headache, muscle pain, chills, joint pain, and fever. These side effects were reported after taking either of the two doses.

 The FDA reported that four patients who received the vaccine experienced Bell’s Palsy.

This vaccine may also cause a severe allergic reaction, but this happens exceedingly rarely. Out of almost 2 million Pfrizer-BioNTech vaccine doses administered, there have only been 21 cases of severe allergic reactions and no fatalities, a CDC report shows.

The average anaphylaxis rate for the COVID-19 vaccine is 11.1 cases per one million doses administered. Typically, the rate of adverse reactions for the flu vaccine is about one per million, and it may be possible for these mRNA vaccines to also settle closer to one per one million as more and more people become vaccinated.

What are the side effects of the Oxford-AstraZeneca vaccine?

Unlike Moderna’s and Pfizer’s vaccines, AstraZeneca and Oxford University used a more conventional design for their vaccine, known as ChAdOx1 nCoV-19. The vaccine uses a modified adenovirus—the virus that causes the common cold—that contains genetic material from the SARS-CoV-2 virus.

This kind of technology has been employed in vaccine development for decades. Recombinant vaccines, such as ChAdOx1 nCoV-19 are generally safe to use in a large population of people—even those with chronic health problems or people who are immunocompromised. Similar vaccines include pneumococcal vaccines and vaccines for meningococcal disease.

This vaccine had its fair share of controversy. On September 6, 2020, AstraZeneca put a halt on its phase 3 trial due to safety concerns about a concerning illness in a participant in the U.K. The patient involved in the study had been reportedly suffering from neurological symptoms associated with a rare spinal inflammatory disorder called transverse myelitis, however, it’s not clear whether the vaccine had anything to do with the man’s condition. The trial recommenced shortly after getting the green light from safety watchdogs.

Results for the vaccine’s phase 3 clinical trial, involving nearly 24,000 people over 18 years old across the UK, Brazil, and South Africa, were published in early December.

Side effects of COVID-19 vaccines have included arm pain, swelling, and redness where the vaccine was injected.

What’s the difference between side effects and adverse reactions?

A side effect is a documented therapeutic effect that occurs when treatment goes beyond the desired effect, or a problem occurs in addition to the desired therapeutic effect. For example, Percocet’s (oxycodone-acetaminophen) most common side effects are somnolence, fatigue, and difficulty concentrating. 

Side effects like fever, chills, and fatigue after a vaccination indicate that your immune system is responding to the vaccine. As for bruising, swelling and sensitivity at the injection site, the body would respond in a similar way after a needle punctured the skin for any other purpose, so the vaccine’s contents have nothing to do with them.

An adverse event is an undocumented therapeutic event that is either unforeseen or a dangerous reaction to the medication. These types of reactions are not usually studied during drug development for the simple fact that they are unique to the dose, patient, and possible interaction. For this reason, adverse events are completely unpredictable and occur far less often than side effects.

An example of an adverse event to a COVID-19 vaccine is an allergic reaction. As of right now, the CDC recommends caution for patients who have a history of anaphylactic reactions to any vaccination but suggests that they can still be vaccinated as long as they take precautions. Patients are now being monitored for 15-30 minutes after receiving the vaccine to watch for signs of anaphylaxis.

Important safety information

In order to minimize any risks associated with vaccination, tell your provider all your medical conditions. These may include:

• any allergies
• signs of fever
• a bleeding disorder or the use of a blood thinner
• whether you’re immunocompromised or are on a medicine that affects your immune system
• pregnancy or planning to become pregnant
• breastfeeding
• have received another COVID‑19 vaccine

This article is part of a series by the ZME Science editorial staff meant to inform and educate the public concerning the coronavirus vaccines. This is not medical advice. You should consult your doctor before making a health decision based on information you read online.

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