This winter’s flu season is turning into a “moderately severe” one that might get worse because of an imperfect vaccine and steady cold weather, flu experts and public health officials said this week.
The flu is now widespread across the country and the peak of transmission probably occurred during the Christmas-New Year’s holiday week, just as many people were crowded into planes, buses and cars or in large family gatherings, said Dr. Daniel B. Jernigan, director of the influenza division of the Centers for Disease Control and Prevention.
About 80 percent of cases are of the H3N2 strain, which caused many hospitalizations and deaths this year in Australia, where winter comes in July and August.
“H3N2 is a bad virus,” Dr. Jernigan said. “We hate H3N2.”
Compared to H1N1, the other seasonal Type A strain, and to B strains that usually arrive late in the season, H3N2 tends to kill more of the very young and very old, he said.
However, those fears are probably exaggerated because of two important differences between this country and Australia, said Dr. Anthony S. Fauci, director of the National Institute for Allergy and Infectious Disease.
Far fewer Australians are immune because flu shots there are recommended only for health care workers and people at high risk — those who are pregnant, have diabetes, obesity, lung problems, compromised immunity or other factors. Health authorities in the United States recommend flu shots for everyone older than six months.
Also, the same H3N2 strain that hit Australia hard circulated last winter in this country, so anyone who caught it then should be immune this year, Dr. Fauci said.
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.
“It’s far from a perfect vaccine, but we can still do a lot of good with a pretty good one,” said Dr. William Schaffner, head of preventive medicine at Vanderbilt University’s medical school.
Flu remains a major killer. Last month, health agencies in 47 countries, including the C.D.C., published a study in the Lancet with new, higher mortality estimates.
Even in the absence of a pandemic, a severe flu year kills nearly 650,000 people worldwide, while a mild one kills just under 300,000, the study concluded.
In recent years, the C.D.C. estimates, flu has killed about 12,000 Americans in mild years and 56,000 in moderately severe ones.
Some flu experts have privately complained that this year, the C.D.C. appears to be promoting vaccination less vigorously than usual, especially given the “Aussie flu” worries.
Dr. Jernigan expressed surprise at hearing that, but said changes in leadership within the Trump administration might have shifted media attention away from the issue.
Normally, the C.D.C. director holds a news conference each September to assess the coming season and urge Americans to get vaccinated. This year, Dr. Jernigan noted, the news conference was led by Tom Price, who was then the secretary of health and human services, which oversees the C.D.C.; Dr. Price, a physician, publicly got a flu shot.
But Dr. Price was at that time under intense scrutiny for his private jet travel, and political reporters were following his every move more closely than health reporters. He resigned under pressure the day after getting his shot.