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.
Put the roast directly onto the oven rack once the oven has finished heating. Place a tray beneath the roast to catch the drippings. The fat side of the roast must face upward so that the flavor of the meat drips down as the roast is cooking. Let the roast cook for 20 minutes or until browned. Lower the heat of the oven to 200 degrees (conventional ovens would use 225 degrees). Cook for an hour and a half, then check the temperature of the middle of the roast. It should be at 135 to 140 degrees.