Most people have a more significant reaction after receiving the second dose of the COVID-19 vaccine. This is because your immune system recognizes the COVID-19 spike protein as an invader it has seen before. It's like your immune system is making a note to keep an eye on this one since it keeps coming back. The fever, chills, muscle aches and any other unpleasant side effects are all part of your immune system responding appropriately to the threat.
Three weeks after these three Nebraska Medicine health care workers got the first dose of the COVID-19 vaccine, they were ready for their second shot. We asked them how they felt this go around.
Respiratory therapist Komi Gbedjeha 
My experience with the second dose of the vaccine was different from the first one. Mainly, I had a headache, which was easily managed with Tylenol and rest. The soreness in the arm where I received the shot was gone after 48 hours. For the second dose, plan at least a day off ahead and have some OTC pain meds available.
COVID-19 unit nurse manager Brandi Johansen, MSN, RN-BC 
I felt just fine right after. Later on in the day, I did get a slight headache and felt fatigued. The day after, I had a headache, was fatigued and had mild body aches. Two days after, I felt fine in the morning, but later on in the day, I had nausea and vomiting and a headache. No fever or chills.
I did take ibuprofen, and it did help with the headache and body aches. Overall, I am very thankful for getting the vaccine! I knew that I would probably get side effects, but this was my body's response to building up antibodies and protection, and for that, I am extremely grateful!
Critical care anesthesiologist Daniel Johnson, MD
The day after my second shot, I had mild chills, muscle aches (especially in my low back) and fatigue. But no fever. My heart rate was slightly up again. I took one ibuprofen. The site of injection wasn't swollen or sore at all, and I did not have a fever.
Two days later, I had mild fatigue for the first half of the day. I felt normal the second half of the day.
