Its only a matter of time before the botnet network go back to its original size.
Although a quarter of the zombie network has been taken out of action, the upgraded version of the botnet will be more difficult to take down, said Ms Cox.
Taking down infected networks is a "thankless task", according to Sophos, a rival to Symantec.
"It's a bit like trying to deal with the rabbit problem in Australia - you know you're unlikely ever to win, but you also know that you have to keep trying, or you will definitely lose," said Sophos head of technology Paul Ducklin.