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Why are some flu outbreaks so much worse than others?

From BBC - January 18, 2018

Flu comes along every winter, but how many people it will infect - and just how poorly they will be - is incredibly difficult to predict.What makes one flu outbreak worse than another?

After Australia experienced its worst flu season in more than a decade, it was widely expected that the arrival of winter would see more people than usual fall ill in the UK.

That's exactly what has happened: this flu season is the worst for seven years.

The latest figures show a 40% increase in the number of people going to GPsin England with suspected flu in the last week. Wales, Scotland and Northern Ireland have also recorded increases.

Things are not expected to improve in the coming weeks - the UK has a nasty case.

Flu viruses are split into three different types: A, B and C Influenza, with lots of different strains within these.

In this year's flu season, there are four strains circulating - and about half of hospital cases are caused by the B strain.

A is usually the most lethal and is first transmitted from animals to humans.

This winter one of the dominant strains circulating in the UK is a strain of Influenza A called H3N2, or Aussie flu.

The H3N2 strain is not new, but is a more severe strain of flu than the H1N1 strain that has circulated over the last two years.

It also differs from the strains covered by the current vaccine that has been given to many people.

Why flu is so unpredictable

Flu viruses are always competing to infect and pass between people - the virus strains most successful at this are the ones that will become dominant.

Predicting which of these strains will become dominant in any given season is always a challenge.

If, like this year, the main flu viruses are sufficiently different to previous years and the strains in the flu jab, more people may be infected.

Flu viruses constantly evolve as they pass from person to person, changing their appearance so our immune system does not recognise them as easily.

As a good rule of thumb, if more people receive the flu vaccination, then fewer people will get flu.

The vaccine also reduces the severity of the symptoms - meaning people are less likely to go to hospital - and the risk of spreading it to other people.

Growing vaccine in eggs

However, some flu vaccines are more effective than others.

They work by getting the immune system to recognise parts of the influenza virus, so it responds more effectively when it encounters the real thing.

It takes eight or nine months to develop and manufacture enough flu vaccine for one winter.

Flu jabs are still made in the same way they have been for more than 60 years - by growing them in chicken eggs.

This means scientists have to decide which strains to include in a vaccine many months before the flu season actually starts.

A big problem is that the virus keeps evolving before the vaccine is ready.

If the virus changes more than expected, or a minor strain becomes unexpectedly common, the vaccine will be less effective.

While this year's vaccine is not as effective as hoped, it is still the best first defence available.

A universal flu vaccine that would protect against all strains of flu is still many years away.

When a flu outbreak becomes a pandemic

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