World Immunization Week, taking place from April 24 to April 30, highlights the critical role of vaccines in public health. Vaccines have saved over 150 million lives from diseases such as measles and polio. However, many individuals experience fever after vaccination, which is a normal response rather than a sign of illness. Dr. Amit Prakash Singh, a Consultant in Internal Medicine at CK Birla Hospital in Delhi, explains that fever occurs as the immune system activates in response to the inactive parts of germs introduced by vaccines. This immune response varies among individuals; younger people often exhibit more robust reactions than older adults. Importantly, the presence of fever does not correlate with the effectiveness of the vaccine. Regardless of visible symptoms, the immune system generates a protective response, ensuring immunity against diseases.