Neither of these things are guaranteed to happen if you get the flu vaccine when you are sick, but they are possibilities. If you have just a mild illness, there is no reason to wait to get vaccinated.
Early in the day, let roast stand at room temperature at least 1 hour before cooking. Preheat oven to 375 degrees. Rub meat ... the outside and rare in the middle. Meat will be very brown ... stand at room temperature for at least one hour.
1 Preheat the oven to 500 degrees F (260 degrees C).  Bake the roast for 20 minutes in the preheated oven, then reduce the temperature to 325 degrees F (165 degrees C), and continue roasting for an additional 60 to 75 minutes. The internal temperature of the roast should be at 135 degrees F (57 degrees C) for medium rare.