What is Influenza?
Influenza, also commonly known as the flu, is caused by an infection of the respiratory tract by influenza viruses that infect the nose, throat, and sometimes the lungs. These viruses are classified into 4 types: A, B, C, and D, however only types A and B commonly cause illness in humans. The circulation of the virus types and subtypes occurs worldwide, varying by strain and intensity from year to year, between geographic areas and time of year.
The main mode of transmission of influenza viruses between people is predominantly through respiratory droplet transmission (when an infected person coughs or sneezes near a susceptible person). Indirect transmission can also occur, such as when a person touches a virus-contaminated surface and then touches his or her face, as well as airborne transmission via small-particle spray.
What are the symptoms?
After an incubation period of one to four days, flu usually comes on suddenly. Classic symptoms may include fever or feeling feverish/chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, fatigue (tiredness). Some people may have vomiting and diarrhoea, though this is more common in children than in adults. The cough can be severe and may persist for 2 or more weeks. Most people recover from the fever and other symptoms within a week without requiring medical attention.
Severe cases of the flu can lead to complications such as viral pneumonia, secondary bacterial pneumonia, parotitis (inflammation of the salivary glands), worsening of underlying medical conditions (such as pulmonary and cardiac disease), encephalopathy, myocarditis (inflammation of the heart muscle), inflammation of the muscles, co-infections with other viral or bacterial pathogens, and rarely, death.
Where is it found?
The circulation of influenza viruses varies geographically, however intensity is typically higher during the winter months: October to May in the Northern Hemisphere and April to September in the Southern Hemisphere. In many tropical or subtropical regions influenza activity can occur year-round.
Risk to travellers
The risk of exposure for travellers is dependent on the time of the year and destination.
Travellers are strongly recommended to receive the influenza vaccine, especially if flu infections are circulating in the destination region or if they are travelling in settings with increased risk of influenza circulation during the trip, such as:
- As part of large tourist groups (especially those including older people)
- Cruise passengers
- Those participating in mass gatherings (for example religious pilgrims or attending large sporting events
As most travel involves some modes of mass transport, it is not surprising that influenza is the number one vaccine-preventable illness diagnosed in travellers.
Australians who have received a current Southern Hemisphere influenza vaccine and are travelling later in the year to the Northern Hemisphere (during their usual October to May peak season) may receive a second dose of influenza vaccine within the same year.
In recent years, avian influenza strains that generally only afflict birds have caused infections in humans in various parts of the globe, including China, Indonesia and Egypt. Fortunately, transmission of these viruses between humans has been rare to date, but there remains a possibility that a mutation may allow for further spread. Advice for travellers to areas affected by avian influenza outbreaks includes: avoid animal markets, don’t touch animals/birds, eat only well-cooked chicken & eggs and practice good personal hygiene. Further information on avian influenza is provided by the Australian Department of Health.
How is Influenza treated?
Antibiotics are ineffective against the influenza virus and so in cases of severe flu and for people at risk of complications, antiviral medications may be prescribed. For most people however, rest, fluids and analgesia are all that is required (and of course appropriate measures to prevent spreading the infection to others).
Annual influenza vaccination is recommended for everyone ≥6 months of age and is particularly advisable for the following groups:
- children aged 6 months to <5 years
- adults aged ≥65 years
- Aboriginal and Torres Strait Islander people
- people with medical conditions that increase their risk of influenza
- homeless people
- pregnant women
- healthcare workers, carers and household contacts of people in high-risk groups
- residents, staff, volunteers and visitors to aged care and long-term residential facilities
- commercial poultry and pork industry workers
- people who provide essential community services
- people who are travelling during influenza season
Twice a year, the World Health Organization (WHO) convenes a group of experts to determine the anticipated strains and subtypes of flu viruses that will circulate in the different hemispheres during the following season. The vaccines are then manufactured to those specifications which should offer optimum protection for the circulating strains (but possibly not every strain causing illness in a community).
NB - Vaccination is recommended every 12 months. If travelling to a different hemisphere, the types or strains of flu virus circulating may differ from the hemisphere from which you have travelled. If you have been vaccinated recently against influenza, it is advisable to check if the strain of the virus at your destination is covered by the vaccine you have received.
What is Influenza Vaccination?
Injectable inactivated viral vaccines. These inactivated vaccines cannot cause flu infection – sometimes the vaccine’s side effects may be misinterpreted as early flu symptoms.
The vaccine formulation depends on the person’s age:
- People aged 6 months to <65 years should receive the quadrivalent influenza vaccine (QIV).
- People aged ≥65 years should receive the adjuvanted quadrivalent influenza vaccine (QIV), but may receive a standard QIV if the adjuvanted QIV is unavailable. The formulation recommended for the ≥65 years cohort contains an additive that promotes an enhanced immune response to the vaccine in this higher risk age group. It is not suitable for anyone aged under 65 years.
Contraindications: People who have previously had an allergic reaction following an influenza vaccine or component of an influenza vaccine.
Those individuals with known anaphylactic hypersensitivity reactions to egg proteins (eggs or egg products) or chicken proteins should discuss with a doctor whether administration of this vaccine may be conducted under special conditions (as per the Australian Immunisation Handbook). Otherwise, a new formulation that is not produced using eggs will soon be available.
Most individuals should receive 1 dose of influenza vaccine each year, however, the following people should receive 2 doses, 4 weeks apart:
- children aged 6 months to <9 years receiving influenza vaccine for the first time
- people of any age receiving influenza vaccine for the first time after haematopoietic stem cell or solid organ transplant
Level of protection
The efficacy and effectiveness of influenza vaccines of similar composition depend on the:
- age and immunocompetence of the vaccine recipient
- degree of similarity between the virus strains in the vaccine and those circulating in the community
Possible Side Effects
Post-vaccination symptoms may mimic influenza infection, however are generally mild and may include: pain, redness and swelling at the injection site, fever, lethargy and muscle aches. As with all vaccines, there is a small risk of an allergic reaction.
NB: Contrary to popular myth, none of the influenza vaccines are live so cannot cause influenza.
As with all vaccines, there is a small risk of allergic reaction.
Australian Immunisation Handbook https://immunisationhandbook.health.gov.au/vaccine-preventable-diseases/influenza-flu
US Centers for Disease Control & Prevention https://www.cdc.gov/flu/about/keyfacts.htm
DFAT Smart Traveller https://www.smartraveller.gov.au/before-you-go/health/diseases#avian-influenza
World Health Organization https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal)