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 on your prepared baking sheet and place it in the oven. Cook for about 1 hour and 20 minutes. Use a meat thermometer to check the temperature of the roast. At this stage, it should be 115 degrees Fahrenheit (46 degrees Celsius).