As described by Microsoft here, there is a new 0-day vulnerability which is being used in targeted attacks. It also would appear difficult for traditional AV products to detect the difference between legitimate text documents and ones which may contain the exploit.
In a study completed and published by Avira ( http://www.avira.com/en/company_news/recognition_performance_virus_protection.html ) The results of the survey showed that for 34 percent (3,207 respondents) a long-established, trustworthy brand was key. Almost as many users, 33 percent (3,077 respondents), based their decision on the virus detection rates achieved in independent tests. Detection rates - lets call this effectiveness of the control - as this is the key metric used to measure effectiveness. This is a skewed metric as for the large majority of evaluations (ICSALabs, VB100, etc) use the "in-the-wild" or ITW list of viruses to perform the evaluations. There is no evaluation of these product's ability to respond or even detect newly released virus and malware. In all honesty really what we are dealing with here is preventative vulnerability management not virus detection and correction, and in my opinion there are four types of preventative protections required f