Influenza, commonly known as ‘the flu’, is an illness caused by a group of viruses (the influenza viruses) that infect the respiratory tract. Influenza infection usually has different symptoms and causes a more severe illness than most other common viral respiratory infections and may be a life-threatening infection in certain people; it should not be confused with the common cold!
In most parts of Australia, influenza outbreaks are seasonal, occurring between late autumn and early spring. Seasonal outbreaks occur every year and vary from mild sporadic outbreaks to serious epidemics; it is estimated that between 5 and 20% of the population may be infected annually.
Occasionally severe worldwide outbreaks (pandemics) occur involving higher infection rates and more severe disease.
In adults, the symptoms of influenza can include fever, dry cough, muscle and joint pain, extreme tiredness, headache and sore throat. In children, influenza may present with a cough, high fever and listlessness. Children can also get diarrhoea and vomiting as a result of influenza infection.
How Influenza Spreads
Influenza viruses are mainly spread when infected people cough or sneeze, releasing small virus-containing droplets into the air which can be breathed in and infect the respiratory tract of the people around them. Contaminated respiratory secretions on hands and other surfaces can also transmit the infection by hand to mouth or eye infection.
It has been found that influenza viruses can survive for up to an hour in the air of an enclosed environment, more than eight hours on hard surfaces such as stainless steel and plastic, and up to five minutes on hands after transfer from other surfaces.
One very important factor in the spread of influenza is that early in infection, even before symptoms become evident, an infected person can be shedding influenza viruses and infecting the people around them. Good cough and sneeze etiquette and hand washing can contribute to limiting the spread of influenza and other respiratory virus infections.
It is generally believed that young children are the greatest spreaders of influenza because they generate more viruses in their respiratory tract and are less likely to practice good hygiene.
Complications from influenza can include:
- acute bronchitis
- acute otitis media
- cardiovascular complications, including myocarditis and pericarditis
- encephalitis and/or encephalopathy
- haematological abnormalities
- pneumonia (primary viral and secondary bacterial pneumonia*)
- Reye syndrome
*Secondary bacterial pneumonia is more common than primary viral pneumonia, and is a frequent complication in people with underlying medical conditions, often causing death.
Infections caused by A (H3N2) strains are more likely to lead to severe morbidity and increased mortality than influenza B or seasonal A (H1N1) strains.
Severe disease may also occur in otherwise healthy children and young adults. Annual attack rates in the general community are typically 5–10%, but may be up to 20% in some years. In households and ‘closed’ populations, attack rates may be 2–3 times higher. However, because asymptomatic or mild influenza illness is common and symptoms are non-specific, many influenza infections are not detected.
1. Kwok KO, Riley S, Perera RA, et al. Relative incidence and individual-level severity of seasonal influenza A H3N2 compared with 2009 pandemic H1N1. BMC Infectious Diseases 2017;17:337.
2. Neuzil KM, Zhu Y, Griffin MR, et al. Burden of interpandemic influenza in children younger than 5 years: a 25-year prospective study. Journal of Infectious Diseases 2002;185:147-52.
3. Hurwitz ES, Haber M, Chang A, et al. Effectiveness of influenza vaccination of day care children in reducing influenza-related morbidity among household contacts. JAMA 2000;284:1677-82.
Influenza vaccines are available which offer a high degree of protection against seasonal illness and the severe consequences of influenza however, these must be administered annually due to changes in the influenza viruses. Good hygiene is one of the most important ways to help prevent colds and flu.
Vaccinating pregnant women against influenza gives a 3 for 1 benefit:
- protects the woman during pregnancy and in the early months of motherhood
- protects the young infant by transplacental antibodies
- protects the young infant by antibodies in breast milk.
The World Health Organization (WHO) Strategic Advisory Group of Experts (SAGE) has identified pregnant women as the most important risk group for seasonal influenza vaccination. WHO.SAGE Meetings.2012: available from: https://www.who.int/influenza/vaccines/SAGE_information/en/
Recently some effective antiviral medications for influenza have been developed, based on original Australian research, and these can be used to treat influenza infections provided that they are taken in the early stages of the illness. Antibiotics only work for bacterial infections so they won’t work for colds and flu which are caused by viruses.