Epidemiology
Epidemiology Influenza virus infections cause substantial annual morbidity and mortality worldwide including South Africa. [1-4] Annual influenza epidemics result in an estimated three to five million cases of severe illness, and about 290 000-650 000 deaths globally. [4] Influenza is an important cause of pneumonia or lower respiratory tract infection (LRTI) and approximately 8-10% o
Epidemiology
Influenza virus infections cause substantial annual morbidity and mortality worldwide including South Africa. [1-4] Annual influenza epidemics result in an estimated three to five million cases of severe illness, and about 290 000-650 000 deaths globally.[4] Influenza is an important cause of pneumonia or lower respiratory tract infection (LRTI) and approximately 8-10% of all patients with pneumonia test positive for influenza.[5] The burden of influenza in sub-Saharan Africa (and specifically in South Africa) is substantial, with some studies suggesting elevated influenza-associated mortality rates compared to other regions. [6,7] During the influenza season (usually between May and September) in South African hospitals, approximately 14% of inpatients with lower respiratory tract infection and 25% of patients with influenza-like illness will test polymerase chain reaction (PCR) positive for influenza.
A modelling study from South Africa using case-based and ecological data estimated that during 2013-2015 on average 10 737 847 (19.8%) of South African population was affected by influenza-associated illness annually. Of which 10 598 138 were mild, 128 173 were severe- illness episodes and 11 536 (0.1%) were fatal.[8] The highest number of mild cases occurred among individuals aged 5–24 years (5 745 544, 54.2%). An estimated 4 195 (36.1 %) of influenza-associated deaths were in individuals ≥ 65 years. Overall, the rates of influenza-associated mild illness were highest among individuals aged 5- 24 years (28 935.2 per 100 000 individuals in the population), rates of influenza-associated severe non-fatal illness were higher among individuals aged <1 year (1550.9 per 100 000 individuals in the population) and ≥65 years (761.3 per 100 000 individuals in the population), and rates of influenza associated deaths were higher among infants<1 year (80.3 per 100 000 individuals in the population)and persons aged ≥ 65 years (137.9 per 100 000 individuals in the population).[8]
In South Africa, among individuals aged ≥5 years, an estimated 30% of influenza-associated deaths are in HIV-infected individuals. [9] Pregnant women also constitute an important risk group for influenza- associated mortality. Among an estimated 646 - 1 428 seasonal influenza-associated deaths in women of childbearing age in South Africa in recent years, the majority (~90%) occurred in HIV- infected individuals and the influenza-associated mortality was three-fold higher (Relative risk(RR) 2.8, 95% confidence interval (CI) 1 .7 – 3.9) in pregnant compared with non-pregnant women. [10]
The highest rates of influenza-associated hospitalisation are in those aged ≥65 years, HIV-infected individuals and children <5 years (in particular children < 1 year).[1, 9, 11-13] Data from South Africa showed that extremes of age (<6 months [adjusted odds ratio (aOR), 37.6], 6–11 months [aOR, 31.9], 12-23 months [aOR, 22.1], 24–59 months [aOR, 7.1], and ≥65 years [aOR, 40.7] compared to those aged 5-24 years), underlying medical conditions (aOR, 4.5), HIV infection (aOR,4.3) and history of working in mine (aOR, 13.8) were significantly associated with increased risk of influenza associated hospitalization.[14] Influenza infection may trigger exacerbations of diabetes, pulmonary (e.g.asthma) or cardiovascular disease. For this reason, people with underlying chronic medical conditions are at high risk of influenza complications, often resulting in hospitalisation and even death. Surveillance data from South Africa showed that having underlying illnesses (other than HIV) was a risk for influenza-associated mortality (odds ratio (OR) 2.9, 95% CI 1.2 - 7.3). [1] Individuals with tuberculosis may also be at increased risk of influenza-associated death. [15, 16] The burden of hospitalisations and deaths due to influenza can vary substantially from year to year depending on the transmission and virulence characteristics of the circulating strain(s).
A recent study in South Africa assessing the mean annual national burden of medically and non-medically attended influenza-associated mild, severe-non-fatal and fatal illness among potential target groups for influenza immunization in South Africa during 2013-2015, reported that rates of influenza-associated illness were highest in children aged 6-59 months (23,983 per 100,000 population) for mild illness, in pregnant women (930 per 100,000 population) for severe-non-fatal illness and in individuals aged ≥65 years (138 per 100,000 population) for fatal illness.[17] In tropical areas, influenza occurs throughout the year. In temperate areas, influenza is highly seasonal and typically occurs during winter months like in South Africa. However, In 2020 there were very few influenza detections, mostly associated with an outbreak of influenza in Western Cape Province which was likely related to international travellers from the northern hemisphere.[18] Transmission of influenza in 2020 remained below the epidemic seasonal threshold despite continued testing and surveillance, likely related to non-pharmaceutical interventions implemented to control severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2).[19]
Influenza: NICD recommendations for the diagnosis, management, prevention and public health response, National institute of communicable diseases, 2021