The Centers for Disease Control and Prevention said the percentages reporting various side effects in the first seven days after receiving the Pfizer-BioNTech vaccine rose from the first shot to the second shot. Reports of pain rose from 67.7% to 74.8%; fatigue from 28.6% to 50%; headache from 25.6% to 41.9%; chills from 7% to 26.7%; fever from 7.4% to 25.2%; joint pain from 7.1% to 21.2%; and nausea from 7% to 13.9%. So, why do some vaccine recipients experience a strong reaction to the second shot, while others undergo mild or no side effects at all? The first shot primes the immune system, teaching it to recognize the invader, in this case the Spike protein on the new coronavirus. The Spike protein is what allows the virus to attach and penetrate human cells.