For the first time in over a decade, a total of 49 U.S. states are facing what the Centers for Disease Control calls "significant" flu activity. Compounding the trouble, this year’s flu vaccine is relatively ineffective, with just a 30 percent success rate. It's bad this season, but a pandemic could be much worse, and a new study found those flus tend to occur after the seasonal flu has peaked.Pandemic flus have occurred only a handful of times in the last century, most recently the 2009 swine flu that claimed over 18,000 lives across more than 50 countries.According to the study published in the Public Library of Science, flu pandemics are most likely to occur during the late-spring to early-summer months, after the seasonal flu has peaked. Though this doesn’t mean researchers can predict when the next pandemic will hit, understanding the time window during which these viruses are likely to strike could help health agencies be more prepared for health crises in the future.