It makes sense why the location of the injection would make a difference, said Dr. William Schaffner, a professor in the Division of Infectious Diseases at Vanderbilt University Medical Center in Nashville and former medical director of the nonprofit National Foundation for Infectious Diseases, quoted CBS news. Schaffner was not involved in the study.

The cells that provide the immune response are in local lymph nodes, he said. If the immune cells in those lymph nodes are re-stimulated in the same place, there is a greater immunological response, the study said. According to the American Cancer Society, lymph nodes are across the body in places including the neck, chest, abdomen and armpits.