Clinical Info

Influenza disease

Influenza is caused by different strains of influenza viruses. Symptoms may vary with age, immune status and health of the individual, and include fever, sore throat, muscle aches, headache, cough and severe fatigue. The fever and body aches can last 3–5 days and the cough and fatigue may last for two or more weeks.

During seasonal increases, most influenza diagnoses are based on symptoms. The definitive diagnosis of influenza can only be made in the laboratory, usually from PCR testing of secretions from a nasopharyngeal swab. Samples should be collected within the first four days of illness.

Not everyone with influenza has symptoms or feels unwell. However, asymptomatic individuals can still transmit the virus to others.

Influenza can be difficult to diagnose based on clinical symptoms alone because influenza symptoms can be similar to those caused by other infectious agents including Neisseria meningitidis, respiratory syncytial virus (RSV), rhinovirus and parainfluenza viruses.

The Southern Hemisphere Influenza and Vaccine Effectiveness Research and Surveillance (SHIVERS) study, based in Auckland, identified around one in four people were infected with influenza during the 2015 influenza season. Data showed that four out of five children and adults (80%) with influenza did not have symptoms.

In an earlier study following the 2009 New Zealand influenza season, almost one quarter of adults who reported that they had not had influenza in 2009 had serological evidence of prior infection (21% [95% confidence interval 13–30%]). Conversely, almost one quarter of adults who reported having had influenza during 2009 had no serological evidence of prior infection (23% [95% confidence interval 12–35%]).

The SHIVERS hospital-based surveillance for severe acute respiratory infections in Auckland during 2017 identified that adults aged 80 years or older had the highest severe acute influenza respiratory infection hospitalisation rates of all age groups, 283 cases per 100,000 people compared with 97/100,000 for adults aged 65–79 years, 17/100,000 for midlife adults and 145/100,000 for infants aged under 1 year.

Pacific peoples (83/100,000) had higher hospitalisation rates for severe acute influenza respiratory infection than Mäori (45/100,000), and both groups had higher hospitalisation rates than Asian, European and other ethnicities.


The influenza virus is transmitted among people by direct contact, touching contaminated objects or by the inhalation of aerosols containing the virus. Therefore, thorough handwashing is an important preventative method. Symptomatic and symptomatic influenza cases can transmit the virus and infect others at home, in the community, at work and in healthcare institutions. Healthy adults with influenza are infectious for up to five days, and children for up to two weeks.

Handwashing is an important and effective way of reducing the spread of influenza.

When should people be vaccinated?

It is possible to come in contact with influenza viruses all year round. However, the likelihood of influenza viruses circulating in the community significantly increases during winter.

For most people, the best time to be vaccinated against influenza is before the start of the winter season. It can take up to two weeks for the vaccine to provide the best influenza protection. However, influenza vaccinations can be given when influenza virus activity has been identified as protective antibody levels have been observed to develop rapidly from four days after vaccination.

The funded INFLUVAC® TETRA quadrivalent vaccine for eligible adults and children aged 3 years or older is expected to be available from early-April until 31 December 2018. 

The funded INFLUVAC® trivalent vaccine or FLUARIX® TETRA quadrivalent vaccine for eligible children aged under 3 years, i.e. aged 6–35 months, is expected to be available from mid-April until 31 December 2018.

Why influenza vaccination is needed every year?

Annual influenza vaccination is required for two important reasons:

• Protection from the previous vaccination lessens over time
• The circulating influenza viruses can change and the strains in the vaccine usually change each year in response to the changing virus pattern

For 2018, the three influenza strains included in both the trivalent and quadrivalent funded vaccines are:
     • A/Michigan/45/2015 (H1N1) pdm09-like virus
     • A/Singapore/INFIMH-16-0019/2016 (H3N2)-like virus
     • B/Phuket/3073/2013-like virus

The additional influenza strain included in the quadrivalent vaccine is:
     • B/Brisbane/60/2008-like virus

Influenza survivor, Community clinic nurse Sam Pohe’s job was to endorse the flu vaccine to her high-risk patients but, as she lay in a coma at death’s door, her body riddled with complications deriving from influenza, it became obvious she’d forgotten to get one herself.

The Whangarei 45-year-old was usually one to practise what she preached but, last year, got so busy vaccinating her patients, she forgot to get immunised herself. This is Sam's story on YouTube - (courtesy of Northland District Health Board - Media Release)

Further information