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Arrival of COVID-19 vaccines warrants the need for staying informed on side effects.

March 29, 2021

Cornell Experts Dispel Fears of COVID Vaccine Side Effects

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Although ramped up COVID-19 vaccine distribution marks the beginning of the end of the pandemic, it is accompanied by rising concerns over the possibility of unpleasant side effects following vaccination. But these side effects are expected — and normal.

After getting the COVID-19 vaccine, many people have reported feeling “flu-like symptoms” such as headache, fatigue and swelling or redness at the site of the injection. But these are just signs that the immune system is doing its job.

“These side effects are the typical immune response. It’s natural, it’s normal, it’s supposed to happen,” said Dr. Cynthia Leifer, microbiology and immunology. “It’s basically our immune system thinking we are infected with whatever the microorganism is and mounting a response to that.” 

In most vaccinated people, these responses tend to not be too severe and usually fade within a day or two.

For the Moderna and Pfizer-BioNTech mRNA vaccines, clinical trials and anecdotal reports show that flu-like symptoms tend to be more severe after the second dose. However, this response is expected. Since the body already has some infection-fighting antibodies from the first vaccine dose, the second dose can prompt a stronger immune response, leading to longer-lasting side effects.

“When someone gets a booster vaccine the side effects can be more intense than with the first dose,” Leifer said. “This is because there are already educated immune cells circulating throughout the body, and more are recruited and activated when the second dose is administered.”

Others worry about the risk of having an allergic reaction — or anaphylactic shock — in response to the vaccine. 

The Centers for Disease Control and Prevention places the likelihood of having an anaphylactic shock at one per million vaccine recipients, making it rare. Experts recommend those who have had allergic reactions in the past talk to their doctor before getting the COVID vaccine. 

“Severe adverse reactions are extraordinarily rare, and they are often due to problems with the individual’s immune response in general, [such as] an allergy to one component of the vaccine,” Leifer said.

According to Leifer, anaphylactic shock is rare because doctors ask patients screener questions about allergies to components of the vaccine and past allergic reactions. This reduces the risk of a person getting vaccinated if they could potentially have a serious allergic reaction.

Leifer added that the COVID-19 vaccines leave out common allergens that are ingredients in some other vaccines, such as eggs — flu vaccines, for example, often contain trace amounts of chicken egg due to their manufacture — so that even people with past reactions to vaccines can likely take the COVID-19 vaccine and not experience an allergic response. 

In even rarer instances, a handful of Pfizer and Moderna COVID-19 vaccine recipients developed a blood disorder called immune thrombocytopenia. 

According to Dr. James Bussel, a hematologist and professor emeritus at Weill Cornell Medicine, immune thrombocytopenia is the result of the body producing antibodies — proteins typically produced to fight foreign substances — against its own platelets. 

However, Bussel emphasized that the risks of severe consequences from COVID-19 infection far outweighs the risks of vaccination, even for young and healthy people. 

“COVID-19 infection is a very severe infection, and even if you’re … a young, healthy adult that appears to have a low risk of anything serious happening, it’s a low risk,” Bussel said.  “It’s not a zero risk.” 

Platelets are the cells responsible for preventing leaks from blood vessels, Bussel said. Since  immune thrombocytopenia causes the immune system to attack the body’s own platelets and platelet-producing cells, the condition results in a stark drop in the blood’s platelet levels, which can cause severe bleeding. 

Although the direct causes of immune thrombocytopenia are unclear, Bussel said it’s possible the mRNA COVID-19 vaccine could trigger the reaction in exceedingly rare individuals who have a predisposition for the disorder.

Bussel added that his observations are specific to Moderna and Pfizer, as limited data is available on the incidence of immune thrombocytopenia following immunization with the AstraZeneca and Johnson & Johnson vaccines.

Bussel said the COVID-19 vaccine is not the only known vaccine to possibly trigger immune thrombocytopenia — the disorder can be a side effect of other viral vaccines as well, such as influenza, measles and rubella.

Additionally, Bussel explained that researchers cannot confirm a causal relationship between the disorder and the COVID-19 vaccine because the incidence of immune thrombocytopenia is low.

“If you have X number of cases of [immune thrombocytopenia] occurring every day in the United States and you get some people who have it right after the vaccine, you don’t know if it’s just that was their day or [if] the vaccine had anything to do with it,” Bussel said.

Aside from investigating the cause of some serious side effects, researchers have also questioned why there is an apparent sex difference in the number of side effects reported.

Higher rates of vaccine side effects have been reported by women than men, The New York Times reported. According to this article, women have reported nearly 80 percent of all side effect reports following vaccination, even though women only account for roughly 60 percent of those who have been vaccinated. 

This could be due to hormones, genes and dosing of the shots, although some have attributed these differences to the fact that women might be more likely to report their side effects than men. 

Prof. Avery August, microbiology and immunology, said researchers do not know why these apparent sex differences in side effects exist, but there have been well-documented instances when women have different responses to vaccines compared to men, with sometimes stronger or weaker reactions.  

Even so, August said side effects are not considered to have an extreme sex difference and should not be a reason to avoid the vaccine. 

“I don’t think the side effects should be a deterrent because the alternative is being unprotected,” August said. “The risks of having severe COVID-19 is more severe than the malaise, [which is just] not feeling so well for 12 to 24 hours [after being vaccinated]. I think the tradeoff is well worth it.” 

Ultimately, August and Leifer both encouraged people to get whatever vaccine is available to them. 

“Everyone should know you should get the vaccine unless you have a medical reason not to,” Leifer said. “Take whatever vaccine is available to you. They are all highly effective, and vaccines save lives. We have lost over half a million lives in the U.S., and we don’t need to lose many more.”