Study finds common adverse events between Flu and COVID-19 vaccines

The ongoing coronavirus disease 2019 (COVID-19) has proven to be the most significant health crisis over the past century, causing over 4.5 million deaths worldwide.

Study: Comparison of adverse events between COVID-19 and Flu vaccines. Image Credit: insta_photos/ ShutterstockStudy: Comparison of adverse events between COVID-19 and Flu vaccines. Image Credit: insta_photos/ Shutterstock

The Food and Drug Administration (FDA) issued three Emergency Use Authorizations (EUA) of COVID-19 vaccines since December 11, 2020; the first for the Pfizer-BioNTech COVID-19 mRNA vaccine (BNT162b2), then the Moderna COVID-19 mRNA vaccine (mRNA-1273), and finally the Janssen COVID-19 attenuated viral vector vaccine (Ad26CoV2S).

Despite prompt approvals and supplies, COVID-19 vaccine hesitancy exists across the United States (US). Major causes of vaccine hesitancy are – safety and concerns of vaccine-related side effects; trust in information propagated by public health experts; lack of long-term evidence; and concerns regarding the novelty of available vaccines.

Meanwhile, flu vaccines have been tried and tested for over 75 years. A study published on the preprint server medRxiv* compared the top 10 common adverse events of COVID-19 vaccines – Pfizer, Moderna, and Janssen, and flu vaccine’s adverse effects. Here, the relative risk and associated 95% confidence intervals were calculated. Adverse events from each report were coded in the VAERS using Medical Dictionary for Regulatory Activities (MedDRA).

A preprint version of the study is available on the medRxiv* server while the article undergoes peer review.

The study

The current study was an observational study wherein VAERS data from January 1, 2020, to August 20, 2021, were used for descriptive analysis. Individuals 12 years or older with an indication of either COVID-19 or Flu vaccination were recruited. Data pre-processing and descriptive analyses were conducted using R version 4.0.2.

The findings revealed that during the study period, Moderna rendered the highest number of reported adverse events (121,581), followed by Pfizer (100,752), Janssen (26,911), and Unknown COVID vaccine (456). While, during this period, 4,554 adverse events were reported with the Flu vaccine. Most of the events were reported in females.

Additionally, most reports were in the age group of 31-64 years. However, there was missing information regarding the type of medical attention received across the five cohorts.

Overall, one-third of patients reported having recovered from vaccine-associated adverse effects. At the same time, another third reported having no recovery. The mortality rate was comparable across cohorts.

It was also found that among the 12-15-year-olds, Pfizer had the largest number of reported adverse events—the most frequent being central neuropathy in 24%, after the first dose, and chest pain after the second in 31%.  The most frequently reported adverse events for Moderna were – injection site complication (1%); fever (1%); and nonspecific musculoskeletal pain (1%) for the first dose, and headache (16%); nosebleed (16%); and weakness (16%) for the second dose. Meanwhile, the most frequently reported adverse event for the Janssen vaccine was headache (4%) for the first dose.

For the Unknown COVID vaccine, a single adverse event record was chest pain. The most frequently reported adverse event for the Flu vaccine was central neuropathy – in 23%.

In the age group of 16-30 years, the highest number of adverse events were reported for Moderna – the most frequent were injection site complications in 22% for the first dose and fever in 25% for the second dose. Even in the 31-64 years age group, Moderna had the highest reported adverse events. The same was true for the +65-year age group.


The results specified the common adverse events of COVID-19 vaccines: central neuropathy, fever, headache, injection site pain, nonspecific musculoskeletal pain, and chest pain. Although there were common adverse events shared by COVID-19 and Flu vaccine users,  such events were more likely to occur in COVID-19 vaccine users.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
  • Pradhan, P. et al. (2021) "Comparison of adverse events between COVID-19 and Flu vaccines". medRxiv. doi: 10.1101/2021.09.22.21263711.
Nidhi Saha

Written by

Nidhi Saha

I am a medical content writer and editor. My interests lie in public health awareness and medical communication. I have worked as a clinical dentist and as a consultant research writer in an Indian medical publishing house. It is my constant endeavor is to update knowledge on newer treatment modalities relating to various medical fields. I have also aided in proofreading and publication of manuscripts in accredited medical journals. I like to sketch, read and listen to music in my leisure time.


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