Dr. Kevin Kelleher:

If I knew the answer to that question, I would be up for the Nobel Prize in physiology or medicine as a consequence, unless my colleagues are up to that. It is one of those complex issues. There are so many different factors that affect people in how they react to a disease. It is not even that they would react the same today as they would react in three weeks' time or they reacted two years ago. It is very different. It is a very complex issue. We can somewhat understand it and that is what we use to try to give our advice on, but we do not understand explicitly why some people contract the virus, except some people have got either natural or acquired immunity as a result of a vaccine for an infectious disease. Does Dr Doherty want to add to that?