Depending on how developed the baby's lungs were at birth, it can take up to 2 years for his lungs to catch up. You'd think that they'd be just a little bit behind, but those last few weeks in utero are so important for lungs... and it takes months/years outside the womb for lungs to grow the same amount as they would have in a few weeks before birth. I don't know about not allowing kids near the baby for a full year or avoiding people who've been traveling, but precautions like hand washing and staying away from crowds are a very good idea.
My twins were born four weeks early. The hospital sent me home with guidelines for six weeks early (wrong sheet); it said to keep them strictly at home for 3 months with limited visitors, avoid indoor crowds like the mall for 6 months, get the RSV shots the next winter, and wash hands and take off shoes the instant we entered the house.