the fact = ?

'This is an antibody that evolved after the fact. That is part of the problem we have in dealing with HIV ?

I think the author refers "this" to " that one of them neutralised 91 per cent of HIV strains". But this antibody "gets ahead of the virus". No problem here, so why the author says "this is part of the problem"?

Context:
Now researchers have revealed three powerful antibodies have been discovered. Writing in the journal Science today, they said that one of them neutralised 91 per cent of HIV strains.
Study author Dr Gary Nabel of the National Institute of Allergy and Infectious Diseases,said: 'This is an antibody that evolved after the fact. That is part of the problem we have in dealing with HIV - once a person becomes infected, the virus always gets ahead of the immune system.
'What we are trying to do with a vaccine is get ahead of the virus.'
In the UK, there are 73,000 people living with HIV - the virus that progresses into AIDS - and experts believe that 30 per cent of people with HIV do not know they have the condition.


Read more: http://www.dailymail.co.uk/health/article-1293338/Super-antibodies-pave-way-effective-AIDS-vaccine.html#ixzz0tTP9cFOd