Complicating the process is that scientists don’t have a sense yet — after only two winters with SARS-2 — about just when the virus might peak, and how strong the seasonal factors are. The virus has been spreading incredibly effectively throughout this summer, but many experts do anticipate even more elevated transmission at some point this fall and winter, at least in colder parts of the country.

Even with flu, it’s still a bit of a guessing game as to which month the virus will peak, particularly as Covid-mitigation efforts have thrown off the regular behavior of other viruses. And while the past two flu seasons have been tamed by the efforts to slow Covid, Australia is in the midst of a severe flu season, which can often portend what the U.S. season will look like.

“We’ve got a narrow window,” Patsy Stinchfield, a pediatric nurse practitioner and the president of the National Foundation for Infectious Diseases, said about the annual flu shots. “We want to make sure we’re not vaccinating too early, because then you risk a late season outbreak.”