Causes of a temporary rapid pulse include anxiety, fever, stress and exercise. Tachycardia causes a prolonged, sustained elevated heart rate, and it may cause your heart rate to increase to 400 beats per minute, notes the Heart Rhythm Society.
New flu vaccines are released every year to keep up with rapidly adapting flu viruses. Because flu viruses evolve so quickly, last year's vaccine may not protect you from this year's viruses. After vaccination, your immune system produces antibodies that will protect you from the vaccine viruses.
1 In some years, the influenza viruses used for the vaccine don't match the viruses circulating during the flu season. 2  If this occurs, your flu shot will be less effective, but may still offer some protection. 3  Other illnesses. 4  Many other diseases, such as the common cold, also produce flu-like symptoms.