Health experts are warning of a renewed spread of seasonal flu after two years of COVID-19 restrictions left fertile ground for the potentially deadly bug to spread.

The number of people hospitalised or killed by flu drastically declined during the pandemic, with zero flu-related deaths recorded in 2021.

However, the rates of influenza vaccination coverage also fell by more than 25 per cent from 2020 to 2021, leaving Australians particularly vulnerable to new strains, according to the 2022 Influenza Report Card, compiled by Sanofi Australia and New Zealand.

In 2019, about 50 per cent of Australians had their flu shot, a number that grew to 60 per cent in 2020 during the pandemic.

That number fell to just 34 per cent in 2021, and just 11 per cent of people have had a flu shot so far this year.

“Influenza vaccination coverage dropped dramatically during the COVID-19 pandemic, leaving Australians susceptible to influenza,” Infectious Diseases Physician Paul Griffin from the University of Queensland said in the report.

A side effect of the pandemic was that the number of individual cases and fatalities from flu fell dramatically.

While the impact of flu varies from year to year, last year nobody died from flu, and only 600 cases were recorded.

That compares with more than 21,000 flu cases and 36 deaths in 2020, where case numbers were affected by closed borders and other pandemic restrictions.

Comparatively, in 2019, some 313,033 cases were reported, and 902 people died.

“Each season the impact of influenza differs,” Infectious diseases paediatrician Robert Booy said.

“We expect that with global travel resuming, influenza strains will also travel and infection rates will rise,” he added.

Acting Australian Chief Medical Officer Sonya Bennett had warned that both flu and COVID-19 were highly contagious, serious viruses and everybody eligible should be vaccinated.

“This year’s winter season will likely see both an increase in transmission of the coronavirus and, for the first time since 2019, a resurgence in influenza,” Dr Bennett said in a statement.

Anyone over the age of six months is encouraged to get an annual flu shot.

She said people should note flu vaccines do not prevent COVID-19, and COVID-19 vaccines will not protect people from the flu.

“For this reason, it is vital people get a flu vaccine and are also up to date with their COVID-19 vaccinations, including having a COVID-19 winter booster dose if they are eligible,” she said.

“If you have been sick with COVID-19, you can receive a flu vaccine as soon as you feel well.”