Britain’s media have made much of the “Aussie flu” sweeping across the northern hemisphere, but is there evidence the land Down Under is to blame?
There are a few things we know. Flu viruses circulate all year around, all over the world and they are constantly evolving through mutation. This means even viruses of the same subtype can be slightly different, and so vaccines against the subtype won’t necessarily work on newly mutated forms.
Flu viruses are also shared around the globe thanks to air travel. In temperate climates we see a short period in winter when flu cases leap up – a flu epidemic – that occurs every year.
Read more: Here's why the 2017 flu season was so bad
Not all flu cases in Britain, or anywhere else, are being caused by just one flu virus. Multiple flu viruses circulate each year and are broadly grouped into two types: A and B.
Influenza B viruses form two main groups, while the influenza A viruses are more variable. The most common influenza A strains are A/H3N2 and A/H1N1 (which may sound familiar as it was the strain that caused the 2009 “swine flu” epidemic).
The main influenza A subtype in the 2017/18 British season is the same as it was in the Australian season; the most rapidly mutating flu virus subtype, A/H3N2.
There is also some A/H1N1 and both forms of flu B virus getting around.
Tropical East-Southeast Asia, where flu season is more constant (as they don’t have a “winter” for it to peak), is often a global source for new seasonal flu A variants.
The UK flu report shows H3N2 is the main virus being detected, but intensive care admissions – a marker of serious disease – show flu B is having a big impact too, in all age groups but especially children.
There is no easy way to prove the viruses in Britain originated in Australia. Detailed genetic sequencing and detective work could help, but that work hasn’t been done and it’s an academic question anyway.
A quick analysis of the few available H3N2 hemagglutinin (a protein found on flu viruses) gene sequences from October to December 2017 reveal there was as much variability among this UK season’s H3N2 strains as seen between UK and Aussie strains. There are multiple strains.
Studies show the flu vaccine, which protects against three or four strains, performed unsatisfactorily against H3N2 in Australia last season. The vaccine strain was only 5-19% effective at protecting against circulating H3N2.
Vaccination remains our best defence against the multiple strains that surge during our flu season every year. Adding resources and pressure to calls for improved vaccines rather than where the vaccine target originated from are more likely to help to those who sicken and sometimes die from flu.
Update: since publication, the UK government released a new weekly national flu report. This shows a continuing rise in detection of flu B cases. Flu B viruses now clearly dominate among confirmed flu cases in the UK, which differs from most of the Australian season.