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Are you clued up on the flu?

Written by Dr Claudia Pastides

, 6 min read

Are you clued up on the flu?

To stay one step ahead of flu-season, we’ve sourced some of the most frequently searched flu questions, and asked Dr Claudia to answer them.

How does the flu start?

The flu begins when the influenza virus makes its way into the back of your nose or throat. This usually happens by breathing in air that contains flu particles. The virus then goes on to infect the cells of your airways.

Another way is by touching a surface with flu particles on it and then putting your hand to your mouth or nose.

How does the flu spread?

Flu particles are propelled through the air when people infected with the flu virus sneeze, cough or even talk. They then land on surrounding surfaces where the virus is able to survive for up to 24 hours.

Once the flu virus infects cells in your airway, it begins to multiply. These newly multiplied viruses move on to infect more and more cells. This causes you to develop the symptoms of flu.

What are the symptoms of flu?

Flu symptoms tend to start a day or two after you’ve come in contact with the flu virus. They typically then go on to worsen very quickly. Symptoms include:

  • A sore throat
  • Dry cough
  • High fever
  • Muscle aches and pains
  • Feeling exhausted
  • Blocked nose
  • Headache

Which flu is worse?

There are 3 types of influenza viruses that affect humans; A, B and C:

Type A - some can cause pandemics. They typically cause more serious disease (for example Swine Flu, Avian Flu and the Spanish Flu)

Type B - don’t cause pandemics but still cause serious disease

Type C - tends to cause milder flu symptoms

Within influenza A and B there are also multiple different types, referred to as strains.

Can the flu kill?

Each year in the UK, around 600 people die due to complications from the flu. The World Health Organisation estimates that the flu kills 250,000-500,000 people around the world every year.

The flu is especially harmful to certain more vulnerable groups, although it can kill people who are normally fit and well too. It is a pretty unpredictable virus at times.

As a result, many countries throughout the world have annual vaccination programmes. The flu vaccine is the best defence we have against influenza.

How do flu vaccines work?

When the flu enters your body, your immune system realises that the influenza virus shouldn’t be there. Your body then mounts an immune response to kill the flu off and stop it from spreading further in your body.

Vaccines are made up of influenza viruses that are either already dead or that are severely weakened. Your body will respond to the influenza virus strains in the vaccine and mount an immune response. But these viruses won't make you ill.

If you then come in contact with the same influenza virus strain during flu season, your immune system is already primed and ready to fight the virus off.

In some cases this means you won’t get any flu symptoms at all. In other cases you might still get symptoms but they will be less severe. And you’ll be less likely to end up in the hospital or with major complications.

Which flu vaccine is best?

There are 3 flu vaccines commonly used in the UK. The one that’s recommended depends on your age/which group you’re in:

  • Children aged 2 to 17 - The intranasal flu vaccine. This vaccine is given as a nasal spray and is very effective in children. The viruses in this vaccine are weakened and not dead. As a result, there is a risk of this vaccine causing flu in people with a weakened immune system, so it is not suitable for everyone.
  • Children and adults aged 6 months to 64 years - The quadrivalent flu vaccine (QIV) This is an injected vaccine offering protection from 4 different flu strains.
  • People aged over 65 - The adjuvanted trivalent flu vaccine (aTIV). This vaccine is injected and has a chemical inside it called an adjuvant. The adjuvant helps to lengthen and strengthen the immune response in this age group. Without the adjuvant, the vaccine just isn’t as effective in over 65s.

The aTIV for over 65 year olds has 3 strains of influenza in it. The QIV and intranasal flu vaccine both have 4 different flu strains within them.

How are flu vaccines made?

The World Health Organisation (WHO) is always keeping an eye on which influenza viruses are hanging around. They predict which strains of influenza are most likely to cause flu during the upcoming flu season.

The problem is that it takes time to make vaccines. So the WHO have to make their recommendations 6 months before flu season begins.

Frustratingly, in those 6 months, there is a chance that the flu virus will change. Or sometimes a different strain will end up being responsible for the majority of infections. As a result, the annual flu vaccine isn’t always a perfect match.

Why do you need a flu vaccine every year?

Because influenza strains vary each year, it is best to have the flu vaccine annually. Every year, different strains of the virus are included in the vaccines.

Does the flu vaccine work?

Across all age groups including children, the flu vaccine prevented 15-52% of flu cases between 2015 - 2020.1

Are flu vaccines safe?

Yes. We’ve been using flu vaccines safely since the 1940s. Most people don’t go on to have any problems as a result of flu vaccination.

With all vaccines, there is a very small risk of allergic reactions. For this reason the vaccine is given by people trained in recognising allergic reactions. These healthcare professionals can also treat severe and life-threatening allergic reactions (known as anaphylaxis) using an adrenaline injection, similar to an epipen.

Anaphylaxis is very rare. Between 1997 and 2003 there were a total of 130 reports of anaphylaxis in response to all immunisations given in the UK. The UK administered 117 million vaccinations that year. This means the overall rate of anaphylaxis is around 1 in 900,000.2

That’s like 1 person in the whole of Liverpool, or 1 child out of all the children aged 15 and under in Scotland.

Will the flu injection make me sick?

No. Side effects can occur, but not because the flu vaccine has given you the flu. They occur because your body mounts an immune response to the dead or weakened virus you’ve been vaccinated with.

Common symptoms include:

  • Arm pain at the injection site
  • Runny or blocked nose
  • Loss of appetite
  • Feeling generally unwell
  • Headache
  • High temperature
  • Achy muscles

Do children ‘shed’ the nasal flu virus from their noses?

There is no evidence that healthy unvaccinated people can catch the flu from the nasal flu vaccine. Nor is there evidence that they can get the flu from children that have been vaccinated recently with the nasal flu vaccine. Children should however be kept away from people with severely weakened immune systems for 2 weeks (e,g, people receiving chemotherapy or who have recently had an organ transplant), as there is a very small risk of them passing the weakened flu virus to them.

Does the flu vaccine prevent colds?

No. The flu vaccine only helps protect you against influenza. Colds are caused by a wide variety of other viruses.

Am I eligible for a free flu vaccine?

Check out the NHS page on “Who should have the flu vaccine?”.



The information provided is for educational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Seek the advice of a doctor with any questions you may have regarding a medical condition. Never delay seeking or disregard professional medical advice because of something you have read here.

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