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.
Back to heterologous immunity and the influenza virus. The story starts when the Canadians noticed in 2010 that the people who got annual flu shots were getting the pandemic strain A/H1N1-pdm09 more often than the people who got no shot at all: