What do we know about mRNA vaccine side effects?

This article was published on
April 13, 2021

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There is no evidence to suggest that mRNA COVID-19 vaccines or non-mRNA COVID-19 vaccines would result in death, neuro-cognitive issues, debilitating/long-term inflammation, or infertility. 

There is no evidence to suggest that mRNA COVID-19 vaccines or non-mRNA COVID-19 vaccines would result in death, neuro-cognitive issues, debilitating/long-term inflammation, or infertility. 


What our experts say

Research shows that mRNA vaccines are safe and effective. Nonetheless, false and misleading online claims are sowing confusion about mRNA side effects. COVID-19 mRNA vaccines have not caused any deaths in clinical trials, and there is no theoretical reason to believe that mRNA vaccines would cause deaths or severe harm to people.

False claims about mRNA mortality risks point to the Vaccine Adverse Event Reporting System data (VAERS) data. The VAERS system is an unverified reporting system. It does not determine if a vaccine caused the events that are reported. The U.S. Food and Drug Administration, which administers VAERS, and the U.S. CDC warn that “reports submitted to VAERS often lack details and sometimes contain errors.” While it is common for new vaccine side effects to be reported after clinical trials due to the larger numbers of people receiving the inoculations, any fatal incidents would be noted in the VAERS system and immediately examined.

There is also no evidence that COVID-19 vaccines cause lethal cytokine storms—which are an overproduction of proteins called cytokines that can result in damaged lung tissue, respiratory problems, and death. This severe immune reaction occurs when the body quickly releases cytokines into the bloodstream.

There is also no evidence to suggest that the COVID-19 mRNA vaccines would lead to neurocognitive issues. A neurocognitive issue or disorder is a general term that means a person is experiencing decreased mental function due to a medical disease other than a psychiatric illness. When you immunize millions of people, some of them are bound to later get diagnosed with diseases and conditions, including neurocognitive disorders. That doesn't mean vaccines caused those diseases and conditions.

It should be noted that one neurological illness called functional neurological disorder (FND) can likely result from getting vaccinated. However, those cases are very rare and not associated with the vaccine itself, but rather the circumstances surrounding the vaccination. FND is a medical challenge with how the brain and body send and receive signals to each other. It can be triggered by physical and emotionally charged events.

Common side effects of mRNA vaccines include pain, redness, and swelling, which leads to misconceptions that the mRNA vaccines can cause significant inflammation. However, research from clinical trials of the Moderna and Pfizer-BioNTech vaccines has not linked the vaccines with any long-term inflammation or swelling. 

In one of the first studies to look at the impact of mRNA COVID-19 vaccines on people with pre-existing chronic inflammatory diseases (such as rheumatoid arthritis and inflammatory bowel disease), researchers found patients responded well with relatively minimal side effects.

While there have been some case reports of flare-ups of the autoimmune disease rheumatoid arthritis following Moderna and Pfizer-BioNTech COVID-19 vaccinations, there have not been enough cases to definitely say that the vaccines are causing those flare-ups. Side effects will continue to be monitored in the population as the vaccine rollouts proceed, but there is no existing evidence or theoretical reason to suggest that inflammation from mRNA COVID-19 vaccines is a concern. 

Similarly, there is no evidence to suggest that the COVID-19 mRNA vaccines cause infertility or miscarriage. A key origin of this myth was a 2021 letter sent to the European Medicines Agency by two European anti-vaccination propagandists who falsely claimed that the “vaccine contains a spike protein called syncytin-1 [that is] vital for the human placenta in women.” Syncytin-1 is a vital protein for the human placenta, and destroying this protein would disrupt its formation, which could lead to infertility or miscarriage. However, the COVID-19 mRNA vaccines do not contain syncytin-1 or message the body to generate antibodies to it. 

Other claims of COVID-19 mRNA vaccines causing infertility stem from the vaccines’ newness in the population. While these specific vaccines are new, mRNA technology and existing safety data provide reassurance about the safety of COVID-19 mRNA vaccines during pregnancy. While fertility was not studied in vaccine clinical trials, no decrease in fertility has been reported among trial participants, or among the millions who have received the mRNA vaccines in the public. In addition, no signs of infertility appeared in animal studies. 

What can lead to adverse pregnancy outcomes is COVID-19: evidence shows that infection earlier in pregnancy increases the risk of complications, ranging from increased need for ventilation to fetal death.

Context and background

Side effects and adverse events of all COVID-19 vaccines are closely monitored at a large scale. That data tells us much more than individual events do. In order to understand if there’s a link between a vaccine and a medical outcome, we need to understand side effects and health conditions in a large population of vaccinated people. Even then, if these results are not observed from clinical trials with highly credible methodologies, such as the ongoing trial phases of Moderna and BioNTech, we can not consider those associations causal.


  1. SPI-M-O: Summary of further modelling of easing restrictions – Roadmap Step 2 (UK Government)
  2. Video spreads false information about COVID-19 vaccines (Associated Press)
  3. The "urban myth" of the association between neurological disorders and vaccinations (Journal of Preventive Medicine and Hygiene)
  4. No Excess Risk for Neurologic Events Observed to Date from COVID-19 Vaccines (Neurology Today)
  5. Helping the Public Understand Adverse Events Associated With COVID-19 Vaccinations: Lessons Learned From Functional Neurological Disorder (Journal of the American Association Neurology)
  6. Vaccines and Functional Neurological Disorder: A Complex Story (Neuroscience News)
  7. Immunogenicity and safety of anti-SARS-CoV-2 mRNA vaccines in patients with chronic inflammatory conditions and immunosuppressive therapy in a monocentric cohort (BMJ Journals)
  8. Possible Side Effects After Getting a COVID-19 Vaccine (U.S. Centers for Disease Control and Prevention)
  9. Addressing fertility questions and concerns with the COVID-19 vaccine (The University of Alabama at Birmingham)
  10. Shattering the infertility myth: What we know about Covid-19 vaccines and pregnancy (STAT News)
  11. COVID-19 vaccines and pregnancy: What you need to know if you're pregnant, trying to get pregnant, or breastfeeding (The University of Chicago Medicine)
  12. Cytokine Storm (U.S. National Cancer Institute)

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