"There's nothing really unusual about this year except that it's a high year in Australia, which is what you see every once in a while," said Dr. Anthony Fauci, director of the United States' National Institute of Allergy and Infectious Diseases.
Australia's curve is "clearly much higher than the curve of last year," he added, but that's not surprising because the US government's own webpage of five to six years' worth of influenza case curves shows clearly that they're all over the place: "There's a tremendous amount of variability," he said.
"All the flu-ologists, myself included, say the only thing that you can predict about influenza is that it's going to be unpredictable," Fauci said.
Hitting hard in New South Wales
Australia's state breakdown reveals that New South Wales, the nation's most populous, is battling the most flu cases: 60,000 as of August 31. August has proved to be the worst month on record for New South Wales; government health data
show 35,670 confirmed flu cases in that month, most of them in Sydney, the capital. Previously, the worst months for New South Wales had been July, with 16,686 cases; August 2016, with 13,602 cases; and August 2015, with 12,901 cases.
Dr. Vicky Sheppeard, director of communicable diseases, NSW Health, said in an email that year-to-year comparison of reported cases is "not a reliable way to judge the severity of a season unless other factors such as testing practices are taken into account." Since 2010, she said, NSW doctors and hospitals have been using more sensitive tests so flu cases that previously might have gone undiagnosed are now reported.
Still, Sheppeard noted the high number of cases this year, particularly on the eastern seaboard where New South Wales is located.
An earlier onset this season contributed in part to the increase, the Australian Department of Health said in its surveillance report.
Influenza A (H3N2) is the predominant circulating influenza A virus nationally this year, and most of the deaths were due to this strain (81%).
"There are the A group of viruses and the B groups," said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University. "The A groups are the ones that usually are responsible for large epidemics."
Though these A strains change periodically, they're the ones that cause pandemics, "whereas the B flu strains usually smolder along. They always cause illness -- it can be just as severe as the A strains -- but they don't produce large outbreaks," he said.
In Northern Hemisphere flu seasons, A starts early, causes most of the cases and then usually abates. If we look at late season influenza, it's most often B strains, he added.
Each year, a flu vaccine is made for the Southern Hemisphere and for the Northern Hemisphere from combinations of A and B strains. Committees of scientists consider which viruses are making people sick, where those viruses are spreading and how well the previous season's vaccine protected against them. Sometimes, each hemisphere's formulation is different, but sometimes, they are identical.
In Australia, the early data "indicates the four flu strains in the vaccine are well-matched to circulating viruses," said Sheppeard. "It is known that one of the strains in the vaccine [H3N2] is less effective in preventing infection, despite a good match."
The vaccine formulation to be used in the US this year is "essentially identical" to what Australia is using now, Fauci said.
Boding ill for the Northern Hemisphere
Does Australia's bad flu season bode ill for Northern Hemisphere nations, including the US, Canada and across Europe?
"In general, we get in our season what the Southern Hemisphere got in the season immediately preceding us," Fauci said. An "intelligent guess," therefore, is that the north will probably have a bad flu season.
"With influenza, it is never 100%," he said.
Adolfo García-Sastre, a professor and director of the Center for Research on Influenza Pathogenesis at the Icahn School of Medicine at Mt. Sinai, said the important question is "what type of strain is causing the outbreak, whether it is one that is similar to ones that have been circulating before or not."
"Now, If the strain has changed, that's a different story," he said. "The high incidence may have been due to changes in the antigenic makeup": the part of the virus that makes it easier to infect people who have been exposed to the strain or have been vaccinated. If the influenza vaccine is not matching the circulating strain, "that could explain larger incidence," García-Sastre said.
How many people were sick last year also matters, Fauci said.
If the circulating strains this year are the same as last year and relatively few people became ill last flu season, then there will be fewer people with natural immunity in Australia, so "they wound up getting hit now," he said.
Meanwhile, other countries may have had more people sick during the previous flu season, so they might have more naturally immune people and will not get hit as hard this year, he said.
"So when you talk about influenza, almost nothing is absolutely precision," Fauci said. "In general, one can say we usually see here what they see there in their season."
Schaffner agrees: "There's not a one-to-one correlation." Still, hearing about Australia's high number of flu cases, he said, "I started to tighten my belt."