Possible side effects of vaccines include sore arm, chills or fatigue, but one you may not know about is swollen lymph nodes under the armpit. Lymph nodes contain immune cells that help fight invaders and are doing their job by building antibodies. However, swollen lymph nodes can present as something serious like cancer on a mammogram. If you haven’t scheduled your annual mammogram yet and plan to get the vaccine, try to schedule your mammogram first. But if you’ve already had the vaccine and have an upcoming mammogram, you don’t necessarily need to reschedule. Many radiology centers have adjusted their normal process to address this head-on.