Seasonal Influenza Medical Briefing

Seasonal Influenza

Written by Dr Finn Morgan, Group Medical Director

Updated: June 2019

“Flu” is short for influenza, which is a highly contagious viral illness which affects the lungs and throat.

In the English-speaking world there is a tendency to refer to commonplace coughs and colds as “flu”. In fact these are caused by other viruses; true influenza is distinct from these illnesses and usually causes symptoms which are significantly worse.

Influenza is a common disease which is present all year round, all over the world. However in temperate zones most influenza activity occurs during the winter months. This means that there are two flu seasons every year: one in the Northern Hemisphere and one in the Southern Hemisphere (tropical regions experience similar rates of influenza all year round). Flu seasons vary widely in their severity so some years will have many more cases than others.

The influenza virus

The influenza virus is actually a group of viruses. They are classified into influenza A, B and C. Most years, influenza A causes more disease than influenza B while influenza C is of relatively little importance. Influenza A viruses are often further described by ‘H’ and ‘N’ numbers which refer to proteins found in the viral coat – for example the “swine flu” which appeared in 2009 was influenza A H1N1.

One of the most notable features of influenza viruses is that they have a tendency to change a little bit every year – new strains of influenza virus are frequently identified. People who have had influenza before, or who have been vaccinated against influenza, will have immunity to the influenza viruses they have been exposed to. But as the circulating influenza viruses change over time, that immunity becomes less effective. This is one of the reasons why some flu seasons are more severe than others. Occasionally an influenza virus will appear which is markedly different from those which have gone before. When this happens, the population has little immunity to the new virus and an influenza pandemic can result. This is a much-feared event which in the 20th century was responsible for large-scale loss of life.

TRANSMISSION | How do you get seasonal influenza?

Influenza spreads very easily from person to person. Infected individuals will usually begin shedding viruses the day before they feel unwell until 5 to 7 days after they recover. Coughing, sneezing and even talking launches the virus particles into the air whereupon they can be inhaled by another person. Influenza virus can also survive for some time on hard surfaces such as door handles so it is possible for someone to get the virus on their hand and then become infected by touching their mouth.

SYMPTOMS | What are the symptoms of seasonal influenza?

1 to 4 days after becoming infected, patients develop a range of symptoms which will include several of the following:

  • Cough
  • Runny nose
  • Feeling weak or tired
  • Red and/or watery eyes
  • High temperature (often experienced as chills and/or shivering)
  • Sore throat
  • Muscle aches and pains
  • Headache

Although the list of symptoms is similar to those experienced by patients suffering ordinary coughs and colds, it should be noted that patients with influenza usually feel much, much worse than they would do from a simple cold. Influenza symptoms also characteristically start a lot more suddenly. The principal initial symptoms are usually fever, muscle pains and weakness. Cough and runny nose are less notable at the beginning but often develop later on. Children with influenza may also have diarrhoea. Patients with influenza will typically remain unwell for 5 to 7 days although the length of illness can range from 3 to 12 days.

The prognosis in influenza is usually good: previously healthy patients who contract influenza are highly likely to make a full recovery from the illness. However, it is possible for a simple case of influenza to develop into an illness which is more serious and which can even be life-threatening. The most feared complication of the disease is pneumonia. This can either be caused by the influenza virus directly, or the influenza can predispose the patient to becoming infected with another bug which then causes pneumonia. Influenza may also cause serious illness by exacerbating medical conditions which patients already have (e.g. heart failure).

The following groups of patients are at increased risk of developing serious complications of influenza:

  • Adults over the age of 65
  • Pregnant women
  • Children under the age of 5 (and especially under the age of 2)
  • Residents of long-term care facilities

In addition, patients with the following medical conditions are at increased risk of serious complications of influenza:

  • Severe obesity (defined as a body mass index greater than 40)
  • Chronic lung disease (e.g. chronic bronchitis, emphysema, chronic obstructive pulmonary disease, cystic fibrosis). Asthma counts if inhaled steroids have ever been needed
  • Problems with the spleen or if the spleen has been removed. Sickle cell disease can damage the spleen and therefore is included in this category
  • Chronic liver disease (e.g. cirrhosis or hepatitis)
  • A weakened immune system. Examples include HIV disease or any condition treated with steroids or chemotherapy
  • Chronic heart disease
  • Chronic kidney disease
  • Chronic neurological conditions (e.g. Parkinson’s disease, previous stroke, muscular dystrophy)
  • Diabetes

This is not an exhaustive list; anyone with a serious ongoing medical condition may be at increased risk of serious complications of influenza.

COMPLICATIONS | Are there any danger signs to watch out for?

Most patients with influenza will feel very unwell and yet will make a full recovery. The following features suggest that an individual’s case is more serious and should prompt you to seek immediate medical advice.

  • Patients who appear to be getting better but who then become worse with fever and coughing
  • Bluish discolouration of the skin. This is often most noticeable in the lips and fingernails and can also be seen in the tongue
  • Patients who have a fever with a skin rash
  • Any difficulty in breathing, or children who seem to be breathing more quickly than usual
  • Children who are too irritable to be held and comforted
  • Children who are not drinking enough fluids, or who have persistent or severe vomiting and/or diarrhoea
  • Drowsiness or withdrawal from interaction with others
  • Any of the features listed in “CHILDREN”
  • Refusal to feed
  • Has no tears when crying
  • Seems to have fewer wet nappies than usual
  • Any difficulty in breathing or feeling short of breath
  • Severe or persistent vomiting
  • Patients who appear to be getting better but who then become worse with fever and coughing
  • A feeling of chest tightness, or pain in the chest or abdomen
  • Confusion
  • Sudden dizziness

TREATMENT | How is seasonal influenza treated?

Most cases of influenza can be treated at home without needing to see a doctor. Patients should do the following:

  • Drink plenty of fluids to avoid dehydration. If your urine is pale in colour then you are drinking enough
  • Get plenty of rest
  • Use paracetamol (also known as Tylenol or acetaminophen) and/or ibuprofen to treat symptoms of fever and muscle pains
  • Keep warm.

At the same time, you should try your best not to pass on the virus to others – remember it is highly contagious. You should do the following:

  • Wash your hands often with soap and water. An alcohol-based hand rub is also excellent to have at the bedside so you can disinfect your hands without having to get up
  • Cover your nose and mouth with a tissue if you cough or sneeze. Get rid of the tissue as soon as you can afterwards, ideally by flushing it down the lavatory
  • Try to limit your contact with other people unless absolutely necessary. This should continue until 24 hours after any fever has gone and you have stopped taking medication such as paracetamol (also known as Tylenol or acetaminophen) or ibuprofen
  • Clean and disinfect surfaces and objects which may have become contaminated.

If you have any of the danger signs listed in the section above, you should seek immediate medical attention. You should also see a doctor if you are still unwell after 7 days.

If you are in one of the patient groups which are at increased risk of complications of influenza (see above) you should see a doctor as soon as possible after you become unwell. This is because your doctor may want to give you an antiviral medication.

PREVENTION | What precautions should I take to prevent getting seasonal influenza? Is there a vaccine?

Simple hygiene measures will reduce your chances of getting influenza – see the section above for more details. If you come into contact with someone who has symptoms of influenza, or if you are caring for someone, you should wash your hands frequently with soap and water or use an alcohol-based hand rub.

The best way of avoiding influenza is to be vaccinated. Governments of different countries make different recommendations about who should be vaccinated. In many European countries vaccination is targeted at patient groups who are at increased risk of influenza complications (see above). However in the United States the CDC recommends that everyone over the age of 6 months be vaccinated.

Flu vaccination should be done every year. Ideally the vaccine should be given in advance of the flu season but there is no reason not to vaccinate once the season has started. Just one dose of the vaccine is required, with immunity developing within two weeks. Vaccination is usually given as an intramuscular injection. In some countries influenza vaccine is given to children as a nasal spray but this is not currently recommended in the United States by the CDC.
Influenza vaccination is never 100% effective because it cannot provide protection against every type of influenza which is circulating. In fact, virologists have to make a decision about six months in advance of the flu season which strains of virus to include in the vaccine – in other words they have to guess which strains will be most prevalent during the coming winter. The accuracy of these predictions influences the effectiveness of the vaccination which is usually judged to provide 40-60% protection. Influenza vaccine is safe for nearly everyone.

The following patients should not be vaccinated:

  • Infants under the age of six months
  • Patients known to have severe, life-threatening allergies to influenza vaccination or to one of the ingredients in the vaccine.

The following patients should seek medical advice before being vaccinated:

  • Patients with a known allergy to eggs
  • Patients with an allergy to an ingredient of the vaccine
  • Patients who have previously had Guillain-Barré syndrome.

If you are unwell on the day of your influenza vaccination you should delay having the vaccine until you are feeling better.

What should I do if I am worried about getting flu?

Individuals and organisations located in areas of high transmission should consider now what they can do to avoid being affected by influenza.

Individuals can:
  • Get a flu vaccination if they haven’t had one this season. Regardless of the 30% reported effectiveness, the vaccination is still recommended as the most effective defence against developing influenza
  • Consider if they, or a member of their household, are in a high risk group for influenza complications. If so, influenza vaccination is strongly recommended
  • Take note of the hygiene tips detailed above to decrease the chance of catching the disease from others.
Organisations can:
  • Communicate with staff to raise awareness of the ongoing influenza season and promote good hygiene practices
  • Provide, or fund, influenza vaccination for staff and household members
  • Ensure that staff who are ill do not come into work.

It is important to note that travellers who cross the equator and go from one hemisphere to another, for example from Europe to South America, will need the appropriate vaccine for that hemisphere – remember there is a different vaccine for northern and southern hemispheres. The vaccines come out at the start of the flu season which is leading up to the winter months. On arrival in-country, the traveller would need to seek out a healthcare provider to administer the current season’s flu vaccine.