As mentioned before, the whole point of recirculation is reducing and containing airflow, NOT filtering air. With the recirculation on, you will have a lower risk on catching something from a person 5 rows away. It will not change a thing when you are sitting next to him(her).
Air filtration is not the objective of recirculation. The filters only mitigate the risk of recirculating contaminated air (and apparently the HEPA filters are well suited for this).