According to the C.D.C.’s FluView index, which is updated each Friday, this season’s infection rate is closely echoing that of the 2014-15 season, which was also a predominantly H3N2 year and also rated “moderately severe.”

However, the hospitalization rate is already only half that of 2014-15, Dr. Jernigan said, so he expects fewer deaths.

Flu has been ticking up again in the wake of several mild seasons that followed the 2009-10 “swine flu” pandemic. In that year, an H1N1 virus with a mix of human, bird and pig genes that had never been seen before struck in the spring of 2009, disappeared over summer and returned in the fall. Although millions caught it, it turned out to be relatively mild, and few died.

The H3N2 component of Australia’s flu shot was reported to be only 10 percent effective at preventing infection and is the same as in North American shots. But both Dr. Jernigan and Dr. Fauci said they expected to see roughly 30 percent effectiveness when data is collected at season’s end, in part because more healthy people get their shots.

The vaccine mismatch was not caused by a genetic shift in the circulating flu, as happens in some years, but by changes in the “seed virus” used in the vaccine; as it grew in eggs, it picked up mutations foreign to human flu.

Dr. Fauci was one of the authors of an article published Thursday in The New England Journal of Medicine arguing that it is time to stop using a flawed 70-year-old technology — eggs — and pursue a universal flu vaccine that can be ready in less than the eight months it now takes.

Nonetheless, experts still recommend getting flu shots even at this late date because the season has three more months to run and because, even when shots fail to stop infection, they often prevent the worst complications: pneumonia and death.