Influenza

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.

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.

 

A recent meta-analysis of influenza disease found that approximately 20% of children and 10% of adults who did not receive an influenza vaccination were infected annually, around half of those infected were asymptomatic.

Transmission

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.

Symptomatic and asymptomatic 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. The influenza virus is transmitted among people by direct contact, touching contaminated objects or by the inhalation of aerosols containing the virus. Influenza virus can be aerosolised without sneezing or coughing. Sneezing is more likely to contribute to contaminated surfaces and objects. 

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

Influenza transmission by infected but asymptomatic people

The ability to spread influenza viruses and infect others is usually equated to how much influenza virus is shed by an infected person. Studies have compared viral shedding by asymptomatic individuals, i.e. those who are infected but have no symptoms, with symptomatic individuals, i.e. those who are infected and have symptoms such as fever, sore throat, muscles aches and pains.

Viral shedding by asymptomatic persons occurs for around 3–4 days and by symptomatic persons for around five days. The amount of virus shed by asymptomatic persons was only slightly less than that shed by those with symptoms. Asymptomatic persons shed influenza virus and can infect others despite a seemingly more robust immune response to infection that stops development of symptoms.

The Southern Hemisphere Influenza and Vaccine Effectiveness, Research and Surveillance (SHIVERS) Serosurvey, in 2015, provided information about the immunity that people in the community have against influenza. Data identified around one in four people were infected with influenza during the 2015 influenza season and that four out of five children and adults (80%) with influenza did not have symptoms. When the results were applied to the New Zealand population in 2015, around 1.1 million people (26%) would have been infected with influenza. Around 880,000 (80%) of these people were asymptomatic carriers who could have spread the virus among their family, co-workers, classmates and patients without ever realising it.

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. There were 283 cases per 100,000 people in adults aged 80 years or older with compared 97 cases/100,000 for adults aged 65–79 years, 17 cases/100,000 for midlife adults and 145 cases/100,000 for infants aged under 1 year.

Pacific peoples had higher hospitalisation rates for severe acute influenza respiratory infection than Māori, 83 cases per 100,000 people compared with 45 cases/100,000. Both groups had higher hospitalisation rates than Asian, European and other ethnicities.

To reduce the spread of influenza:
  • Cover your mouth and nose when you sneeze or cough
  • Wash and dry your hands often
  • Stay away from others if you are sick

Influenza survivor, community linic urse 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 - https://youtu.be/Vct-M9fz9ME (courtesy of Northland District Health Board - Media Release)

Further information