Clinical Info

Influenza the 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.

Not everyone with influenza has symptoms or feels unwell. However, asymptomatic individuals can still transmit the virus to others.1 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% CI 13–30%]).

Almost one quarter of adults who reported having had influenza during 2009 had no serological evidence of prior infection (23% [95% CI 12%–35%]). 

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.

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. Preliminary data suggests that 80% of children and adults with influenza did not have symptoms.3 Asymptomatic but infectious influenza cases can increase the spread of disease in the community and healthcare institutions.

The SHIVERS hospital-based surveillance for severe acute respiratory infections in Auckland during 2015 identified that infants aged less than 1 year had the highest severe acute influenza respiratory infection hospitalisation rates than all other age groups, 289 cases per 100,000 people compared with 20/100,000 for midlife adults and 141/100,000 for adults aged 80 years or older.

Pacific peoples (115/100,000) had higher hospitalisation rates for severe acute influenza respiratory infection than Maori (51/100,000), and both groups had significantly higher hospitalisation rates than Asian, European and Other ethnicities. Individuals living in an area identified as having the highest deprivation levels, NZDep9–10, were over represented with hospitalisation rates at 72/100,000 people compared with 18–33/100,000 people across the other four quintiles.


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 asymptomatic influenza cases can transmit the virus and infect others. Healthy adults with influenza are infectious for up to five days, and children for up to two weeks. Extended periods in an enclosed, poorly ventilated space with an infected person increases the chances of acquiring infection.

When should people be vaccinated?

The best time to be vaccinated is prior to influenza entering the community, before the start of the winter season. 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 vaccination will be available for 65 years and older, pregnant women and under 65 years old with certain medical conditions from early March until 31 December.

Why influenza immunisation is needed every year?

Annual immunisation is required for two important reasons:

  • Protection lessens over time 
  • The circulating influenza strains change each year and may not have been included in the previous year's vaccine


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