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.
How long your own flu shot lasts depends on how powerfully your own immune system reacts to it, said Dr. John J. Treanor, a vaccine expert at the University of Rochester medical school. In a young, healthy person, protection might last for years, even as circulating flu strains change. Conversely, he said, there is some debate about whether older people or those with compromised immune systems ought to get two flu shots per season, since a shot they got in October may wear off by March or April.