I haven't heard anything definite regarding a World War Z sequel novel. However, considering the popularity of the original, it would seem like a great opportunity for the author or someone else to continue the story. Maybe they are working on it secretly, but without any official word, it's all speculation. The first novel was so successful in exploring the post - apocalyptic world during a zombie outbreak, a sequel could expand on that universe further, perhaps showing how different parts of the world are rebuilding or facing new threats.
There may or may not be a World War Z sequel novel. While the concept has a lot of potential for continuation, with the rich world that was created in the first novel, it's difficult to say. The original book was adapted into a movie, which also had an impact on the franchise. Some factors that could influence the creation of a sequel novel include the availability of the author, the market demand as determined by the publishers, and whether there are new and interesting ideas to explore within the World War Z universe. It could be that the author is taking a break from the series or is waiting for the right inspiration to strike.