COVID-19 Vaccine Quick Question: Why are the second dose side effects likely worse than the first dose?

If you have received either a Pfizer or Moderna COVID-19 vaccine, you may have felt some side effects. Whether that was a sore arm, a fever, fatigue, headaches, or one of the other common side effects, this is your body giving you a signal that the vaccination is working! However, the second dose of the two currently available COVID-19 vaccines in the U.S. has gained a reputation for causing worse side effects, but why is that?

Simply put, the first dose of the vaccine taught your body to identify the mRNA component found within the vaccine as an unwelcome invader. After the second dose is received, your body is already familiar with the pathogen from its previous encounter and your immune system knows that it must ramp up its response to get rid of it. You may feel your body’s immune response in the form of side effects (e.g., chills, fever, muscle aches, headache, etc.). 

  • You can think of it this way: when you first encounter a problem, you are working to understand the problem. However, when you’ve dealt with the same problem again, you immediately know what to do to solve it and you can jump right into action with even more determination than when you first encountered the issue.

It should be known that not everyone will feel side effects after their first or even second dose of a vaccine and if you are one of those people, you should not worry. Everyone’s immune systems are different and respond differently to vaccinations!

Those who do experience side effects, usually only feel them for a very short period of time (a day to a few days). Remember that these side effects from the vaccine are very minimal compared to the potential long-term effects of COVID-19. So, do not be scared of the side effects – you are only receiving a confirmation that the vaccine is working!

If you’re experiencing side effects for more than several days, be sure to: 

  1. Report your symptoms to the Vaccine Adverse Event Reporting System (VAERS)
  2. Sign up for and report your symptoms to V-Safe, the CDC’s after vaccination health checker
  3. Contact your doctor

It is also a good idea to know the difference between COVID-19 symptoms from an exposure to the virus and COVID-19 vaccine side effects. Respiratory issues are unlikely to be related to a vaccine, so if you are experiencing respiratory symptoms (e.g., shortness of breath, difficulty breathing) make an appointment to get tested. 


If you are pregnant or breastfeeding, the choice to get vaccinated should be decided with your doctor or health care provider’s input. Each circumstance may differ depending on health status, risk of COVID-19 exposure, what stage of pregnancy you are in and other factors that should be considered with your health care professional. 

Comments are closed.