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.
Preheat the oven to 475 degrees. Place the roast into a shallow roasting pan with the fat side up. Place the roast into the oven and cook for 30 minutes. Remove the roast from the oven and let it rest for an additional 30 minutes. Reduce the heat of the oven to 325 degrees.