Its possible. The Malware in this world changes so fast that sometimes the anti-virus companies like symantec or kaspersky labs can't keep up on rolling out updates. If the software is updated, its chances of missing something specific are slim to none depending on what you are running (some are better than others, krebs on security with pcmag tests antivirus software all the times and ranks them) and I believe how deeply entrenched it is, though I am a bit unsure on that one. The chances of you getting a brand new piece of malware that anti-virus software doesn't protect against is pretty slim to none, but that can change depending on how your browsing habits are, firewall settings, and if your pc is used for something high profile, as that can make it a target for stuff like keyloggers or possibly trojans, bot software, spyware, etc. I wouldn't worry so much because if you are using something other than MSE or Windows Defender, then you are fine. Personally, I run Norton 360 which gives me my real time, and if I think it missed something, I have malwarebytes anti-malware free that will scan behind Norton and see if it missed anything. Give malwarebytes a try. I highly recommend it.