“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.
“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.