Pandemic waves can have drastically different durations and periods between them. For instance, the Science Museum of London writes that the Black Death that decimated Europe between 1347 and 1351 was the first wave of a centuries-long plague pandemic. The second, nastier wave didn’t hit until the 1500s, and the third occured in the 1800s. By contrast, the waves of the 1918 flu pandemic happened within months of each other, per the journal Emerging Infectious Diseases. The first occurred in the spring of 1918 followed by the second in the fall of that year. A 2013 study in PLOS One points out that the 2009 flu pandemic exhibits a similar pattern, with the first wave starting in March and the second in October. Though, not every country that was affected experienced a second wave (China, for instance, didn’t).

After analyzing the spread of the 2009 virus, researchers identified five dynamics that seem to generate second waves of flu pandemics. Among them is the seasonality of social interactions. School attendance, for instance, may cause a second surge in cases when students resume classes in the fall while things die down during summer break. Another notable possibility is that two vulnerable populations have little interaction with each other, causing the pandemic to surge unevenly across groups. Consistent with that view, a 2018 study of the 1918 flu pandemic suggests that the first wave affected poor people while the second wave overwhelmingly affected wealthier people.