A combination of extreme weather and two circulating influenza A subtypes are likely behind the “exceptional” 2019 flu season in Australia, a study suggests.

There have been 486 flu-related deaths reported so far this year

In an analysis in Eurosurveillance, researchers, led by Professor Ian Barr from the Doherty Institute in Melbourne, also suggest that the mild 2018 season “never really stopped” and has contributed to the apparent early start of the 2019 season.

Nationwide, there have been 486 flu-related deaths reported so far this year, with 98% of these due to influenza A.

The number of laboratory-confirmed cases has reached 242,714, according to the National Notifiable Disease Surveillance System — one of the largest seasons on record and second only to the 251,159 cases in 2017.

According to Professor Barr’s team, the spike in flu cases reported in 2019 can be traced back to the mild 2018 season, which they say was late and “progressed with such minimal activity that it barely registered as a season”.

“However, several surveillance indicators suggested that the influenza activity seen in 2018, while low, never really stopped, as it was expected to, at the end of the Southern Hemisphere spring,” they wrote.

“Influenza activity was truly exceptional [during this period] and cannot be accounted for by merely increased availability of testing.

“Australia experienced an upsurge in influenza cases with a large wet-season outbreak in the tropical north, while southern Australia saw record numbers of laboratory-confirmed influenza notifications, increased hospitalisations and dozens of influenza-related deaths in late summer and early autumn, resulting in an early start to the 2019 influenza season throughout the country.

Both influenza A subtypes had co-circulated as the predominant strains, the researchers said, with influenza B strains circulating at lower levels.

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