After two years of very low to almost no transmission, influenza hit Australia in much higher case numbers this winter. Although no flu season is predictable, experts say some lessons can be learned from Australia.
Australia’s flu season usually runs from April to October and as of mid-September, there were 223,678 laboratory-confirmed cases of flu reported across Australia, according to data from the department of health’s surveillance system. 295 deaths and 1,763 hospitalisations had also been recorded since April.
The increase in cases wasn’t a shock to the experts here, and, in part, was due to the relaxing of public health measures put in place over the past two years to curb Covid-19.
When Australia’s first Covid-19 cases were recorded in early 2020, the government took the decision to shut the international borders in March that year. The borders remained closed, with some travel exceptions, until February 2022.
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“When we closed our borders, we stopped the possibility of flu coming in,” said Dr Sheena Sullivan, Senior Epidemiologist at the Doherty Institute in Melbourne.
Travellers who did leave the country, for whatever reason, were mandated to quarantine in a state-run facility, usually a hotel, for two weeks upon arrival back in Australia.
“International travel was heavily restricted for a while, and when it did occur travellers underwent a quarantine period. This would have significantly reduced the introduction of different influenza strains into the community,” said Dr Brendan Kennedy, infectious diseases physician at the Royal Adelaide Hospital.
In 2020 there were 21,235 laboratory-confirmed cases of influenza and 36 deaths reported.
The cases that were around in 2020 before the borders closed in March could not transmit easily because of the other public health measures in place for Covid, like physical distancing, mask wearing, and isolating when unwell with respiratory symptoms.
The next flu season in 2021, was almost flu-free. The borders were still closed and social distancing and other measures were still in place to limit the spread of Covid. The department of health recorded 598 notifications of flu that year.
One of the consequences of the low flu numbers in 2020 and 2021 was lower immunity for flu at a population level.
“If you had two years where no one got infected, you have this increased proportion of the population who are susceptible to infection. If you’ve got more susceptible people in your population it just makes it a bit easier for the virus to transmit,” Dr Sullivan said.
Australia’s flu season this year came on early, with case numbers and hospitalisations rising sharply at the beginning of the season. This put a strain on an already weary hospital workforce, said Dr Sullivan.
Things could have been very different if the circulating strains were associated with higher severity, including hospitalisations and deaths
“The problem is that you’ve got an exhausted healthcare workforce, you’ve got fewer healthcare workers … or if there’s Covid circulating you’ve got unfilled shifts because people are not permitted to come into work,” Dr Sullivan said.
“If you’ve got unfilled shifts, you don’t have as many beds available, but then you’ve got more people wanting those beds because they’ve got Covid, they’ve got flu, they’ve got RSV [respiratory syncytial virus] and they need to be hospitalised, and so that becomes a big strain on the hospitals.”
However, according to Dr Kennedy, in many ways, the flu season in Australia could have been a lot worse.
“I think there was a recognition that Australia was going to see a big rise in influenza cases in 2022 and preparations were made to limit the impact on the health system,” Dr Kennedy said.
At the Royal Adelaide Hospital and across the Central Adelaide Local Health Network, where Dr Kennedy works, a lot of planning and preparation was undertaken to address this, he said.
Although this season isn’t completely finished, Dr Kennedy said it wasn’t quite as severe as many expected.
“Things could have been very different if the circulating strains were associated with higher severity, including hospitalisations and deaths, but in Australia at least we have largely escaped what could have been an extremely challenging combination of Covid and resurgence of influenza.”
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Although the patterns of influenza across the southern and northern hemispheres don’t always follow each other, Dr Kennedy said they are usually a good guide.
“I think Europe can probably expect to see higher influenza cases based on what’s happened in Australia over our influenza season…the situation could also change if a different strain to the ones we’ve seen here in Australia emerge in Europe.”
Planning and preparation are important for the upcoming season, Dr Kennedy said, particularly with the potential for Covid cases to increase again and exert more pressure on health systems.
“It’s also a great opportunity to build on aspects of the Covid response that apply to influenza and other respiratory viruses, like access to testing, vaccination and treatment, and improve these across the board and get greater overall returns,” Dr Kennedy said.
In many ways, we’ve fallen back into our old bad habits when it comes to mask wearing, said Dr Sullivan. Not wearing masks makes it easier for the flu to circulate, however, Dr Sullivan said the main driver of high flu numbers in Europe this year is likely to be a more susceptible population.