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.(1)
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.(2)
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.(3)
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. 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. (1) Not everyone with influenza has symptoms or feels unwell. However, asymptomatic individuals can still transmit the virus to others.
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.(4)
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 5 days, and children for up to 2 weeks.(1)
Not everyone with influenza has symptoms or feels unwell. However, asymptomatic individuals can still transmit the virus to others. (1,4-6)
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.(7)
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%]).(8)
During 2019, hospital-based surveillance for severe acute respiratory infections in Auckland identified that infants aged under 1 year had the highest severe acute influenza respiratory infection hospitalisation rate of all age groups. There were 326 cases per 100,000 people in infants aged under 1 year compared with 216 cases/100,000 for adults aged 80 years or older, 98 cases/100,000 for children aged 1–4 years, and 77 cases/100,000 for adults aged 65–79 years. Māori and Pacific peoples had higher hospitalisation rates for severe acute influenza respiratory infection than Asian, European and other ethnicities at 46 cases and 88 cases per 100,000 people respectively.(9)
During 2020 and 2021, patterns of influenza-like illness, severe acute respiratory infections, and confirmed influenza illnesses substantially differed to previous years in New Zealand.(10)
Surveillance systems in Australia, New Zealand and other Southern Hemisphere populations have shown curtailed spread of seasonal respiratory viruses (11), with New Zealand reporting no annual laboratory-confirmed influenza outbreaks or epidemic during the 2020 and 2021 winter seasons. (12) Similar patterns of reduced disease incidence were observed in the United Kingdom (13), U.S. (14), and Europe (15), with minimal respiratory illnesses reported during the 2020-2021 Northern Hemisphere winter season.
Recent studies have indicated that large compensatory influenza seasons are predicted to follow recent light seasons, and a higher influenza vaccine uptake is necessary to reduce the projected increase in disease burden. (16)
As COVID-19 restrictions ease and borders re-open in New Zealand in 2022, it can be expected that a resurgence of respiratory viruses will follow, and seasonal viruses may not follow typical seasonal patterns. Reduced residual immunity from the lack of seasonal infection in 2020 and 2021, in conjunction with the risk of COVID-19 co-circulation presents additional risk of serious disease.
Influenza and COVID-19 co-infection
Data from the United Kingdom’s winter of 2019-2020 suggest that COVID-19 and influenza co-infections have resulted in severe disease outcomes, with higher mortality rates in cases with co-infection, compared to those who only tested positive for COVID-19. Age-specific mortality rates were higher among older people with COVID-19 and influenza co-infection. (17,18)