Testing rates, Covid and the lockdown effect: what’s driving Australia’s big flu season?

Australia is experiencing some of the highest levels of confirmed influenza cases, fever and cough in the last five years, but this hasn't translated to an "unprecedented" number of hospitalisations.

Experts say one reason cases increase year-to-year is because testing is becoming more accessible. This is especially so now, after two years of Covid lockdowns and isolation.

"Influenza case numbers are always hard to interpret," said Dr Rob Moss from the the University of Melbourne's school of population and global health. "They generally represent a small (but unknown) proportion of the people who have an influenza infection."

Dr Moss said hospitalisations are a "much more stable" indicator of influenza activity. "Although it's too early to draw any confident conclusions, FluCAN [hospital] data suggest that we're not seeing an unprecedented number of influenza hospitalisations."

There have been almost 90,000 confirmed cases of influenza so far this year, according to department of health data, while 700 people have been admitted to hospital since the beginning of April.

"We can also look at influenza-like illnesses (ILI). This is a broad category that covers a range of pathogens that cause symptoms such as fever, cough, and headache," said Moss, who notes that the category includes a reasonable proportion of Covid-19 cases, making interpretation a little more challenging than it otherwise would be.

"Nevertheless, figures [from the] Australian Influenza Surveillance Reports suggest that we're seeing a similar amount of ILI to what we saw in 2019.

"This is perhaps alarming, since 2019 was a very large influenza season, but we can certainly attribute at least a moderate amount of this to Covid-19."

The data from a weekly FluTracking survey of over 60,000 people shows 2.5% of respondents were experiencing fevers, coughs or were absent from work. This was significantly higher than during an equivalent period in the past two years, or the average from 2015 to 2019.

Compared to the almost 90,000 confirmed cases in the year to 5 June, there were just 326 cases reported to the National Notifiable Disease Surveillance System (NNDSS) in the year to 6 June 2021. There were over 20,000 cases in the equivalent period for 2020 and over 90,000 in 2019.

The number of influenza cases in previous years peaked a lot later than it appears to be doing so this year. And while there might have been less influenza circulating in 2020 and 2021, several experts have said that the numbers are likely influenced by the pandemic. Lockdowns and isolations would have stopped some patients visiting doctors and getting tested.

The higher case numbers aren't evenly distributed across the country. In the year to 5 June, there were more than 40,000 laboratory-confirmed cases of influenza in New South Wales. There were a further 20,000 cases in Victoria and 16,000 in Queensland. The Northern Territory had 2,732 confirmed cases, but the highest rate - 1,109 per 100,000 population.

All states have seen significantly more instances of reported symptoms than in 2020 and 2021. Most states have also seen bigger and earlier peaks than the average over the past five years.

There were 733 hospital admissions for influenza between April and 5 June this year. The equivalent figure for 2019 was 840. But even as national hospital numbers may not be higher than 2019, some hospitals are seeing large numbers of cases.

Data from the latest Queensland Weekly Influenza Surveillance Report shows the Torres and Cape, Cairns and Hinterland, and West Moreton hospital regions are all experiencing more than twice as many hospital admissions with influenza than the average for the previous five years.

The Queensland government made flu vaccinations free in late May after the number of cases jumped 132% in just one week. Other states have also considered doing this as they see surges in hospital cases while already dealing with high Covid numbers.