Flu: Why This Year's Outbreak Is One Of The Worst

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Rosie McCall 02 Feb 2018, 11:42

Enter ‘Aussie flu’

“Aussie flu” refers to one kind of influenza A virus strain, the H3N2 strain.

The southern hemisphere, including Australia, just experienced one of its worst influenza seasons in recent history and this is the virus that has reached British shores. But we don’t actually know where the virus originated from. All we can say is, it probably wasn’t from Australia.

One place it is more likely to have come from is the sub-tropical regions that do not have winter seasons. These regions do not suffer from the same large flu epidemics that temperate countries like the UK and Australia have (we don’t know why, but some scientists have suggested it’s to do with temperature or humidity), but have continuous lower-level circulation of flu that allows influenza viruses to persist between winters.

What’s worrying about this season is the experience Australia had last flu season. Australia was hit particularly hard by influenza virus H3N2. H3N2 is a typical seasonal flu strain – like H1N1 – but it tends to be more difficult to control.

There are three red flags this flu season, and they are that H3N2 viruses typically causes more hospitalisations and deaths in older people, there are difficulties in producing effective H3N2 vaccines (explained below), and there’s more than just H3N2 to consider, especially in the UK this year.

Although good against the other strains, this season the vaccine is about 20% protective against H3N2 viruses (not great, but better than nothing) as the virus changed unavoidably during production. This is due to a quirk of how flu vaccines are produced. They are grown in chicken eggs, and then inactivated before being used in vaccines.

Flu viruses mutate quickly and they mutate to adapt to their environment. Of course, a chicken egg is a different environment to a human body, so the end result may be a virus that’s not best suited to a flu vaccine. This appears to have been what happened with the latest H3N2 vaccine.

Seasonal flu epidemics are usually caused by a mixed bag of viruses. This year, the mix is so far mainly shared between H3N2 and influenza B.

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