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 475F.  Once the roast has rested, the oven has preheated and once you have everything else for your dinner just about ready, put the roast into a clean roasting pan or onto a baking sheet and put it into the hot oven for 13-17 minutes. You want the outside to get nice and brown.