Doctors have been warning people that first COVID-19 immunization can have a kick to it. And now, people are starting to report that second dose can cause more side-effects than the first dose. It's not an unexpected finding. Moderna and Pfizer both said in their submissions to the US Food and Drug Administration that there was a noticeable difference in the reactions to the doses when they were testing their vaccines in volunteers. Researchers like UArizona Ecology & Evolutionary Biology department head and BIO5 associate director Dr. Michael Worbey, discuss how the body's immune response contributes to the reactions seen in some of the population.