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.
Place the roast in a roasting pan fat-side up and bake in a preheated oven for 15 minutes. Reduce the oven temperature to 325 degrees F and roast for 1 1/2 to 1 3/4 hours. Check the internal temperature of the roast.