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Influenza (flu) can be a serious illness, especially for older adults. People aged over 65 years get the greatest burden of severe flu disease. It is transmittable disease which attacks epithelial cells lining the respiratory tract and lead to local inflammation. It is transferred through droplet infection during coughing, talking or sneezing by infected person [1, 2].
70-90 % of seasonal related flu deaths were resulted in people over 65 years and it leads to 50-70 percent of seasonal flu related hospitalizations as well in the same age-group [3].
Immunity is the ability of the body to combat disease [4]. It consists of lines of defence and can fight responses by cellular and humoral mechanisms. It can be classified into three line of defence [5]. First line and second line defence belong to innate/non-specific and third line is specific. The first line defence includes skin and mucous membrane, hair, cilia, gastric PH [6]. Likewise, second line includes phagocytosis and inflammation intervene by specialised cells i.e. neutrophils, monocytes and natural killer cells whereas soluble proteins include complement, acute phase proteins [5]. Lastly, third line has two main responses which includes humoral response mediated through B-lymphocytes which helps in producing plasma cells that secrete antibodies and the cells mediated response mediated by T-lymphocytes which help to identify self from other, activation of macrophages, maintain an inflammatory response [5]. The immune system protects from disease causing micro-organisms with these line of defences in different ways. It is an interactive network of many organs and billions of freely moving cells and trillions of the free-floating molecules which are present in different parts of the body [4]. It has two responses i.e. innate and adaptive. The innate system is a type of immunity that does not require prior exposure to a harmful organism and present since birth. It refers to general protection rather than specific protection [4]. Likewise, the adaptive immune response involves a pathogen and antigen specific response and involves leukocytes i.e. B lymphocytes and T lymphocytes. It secrets antibodies which acts in different ways to combat with harmful pathogens [6].
The compositions of vaccine are updated annually by World Health Organization depending upon the virus strain. Mostly it consists of 3 or 4 strains (A/H1N1, AH3N2 and either or both two influenza lineages [2]. The immune responses to infection decrease with increasing age thus influenza vaccination is recommended for people aged over 65 years [7].
The effectiveness of influenza vaccine substantially varies every year due to complex and dynamic nature of virus that effect mortality and morbidity of the elderly resident. As such, new vaccine is manufactured to match the circulating strains which sometimes fails, results in higher number of death and hospitalization [8, 9].Flu can make long-term health problems worse
Especially, the elderly over 65 suffered from social isolation, depression, mental illness, malnutrition and frailty which has a direct impact on vaccine-induced immunity and is on the risk of adverse events [8].
In 1997-2016, 1847 deaths ~80% of people aged over 65 years and over [1]. Vaccination is the most effective method of preventing influenza. The influenza vaccines focused on lowering hospitalization and death rate in older people which is found to be more cost effective [9]. In 2017-2018 flu vaccine effectiveness was estimated at 40%
This is because the various strains of flu change from year to year and vaccines are designed to protect against three or for strains. A variety of vaccines are available, including high-dose version designed for people 65 and older. A cell-based vaccine for people who have severe allergic reactions to eggs (most flu vaccines are grown in eggs) [3]
During the 2016-2017 influenza season flu vaccine prevented 5.3 million illness 2.6 million medical visits 85,000 hospitalizations associated with influenza [3].
Haemagglutinins (HA) and Neuraminidase (NA) are two types of antigens found on the surface of the influenza virus which is vital for virulence of the organisms. HA has two parts: head and stem. The head helps in combining to the sialic acid receptors, while the stem helps in the fusion between viral and cellular membrane [10]. NA acts as a receptor destroying enzyme, which removes the sialic acid residue from the surface of infected cells thereby allowing the release and spreading of budding virions. Both humoral and cell mediated immune response is activated two weeks after the vaccination [11]. Corti informed that the effectiveness of vaccine depends on several host factors such as age, underlying health status, genetic status and antigenic matches between the vaccine and circulating virus.
The antibodies produced post-vaccination targets the head of hemagglutinin, bind to the head of HA , and recognize the homologous strains within the given subtype, and neutralise virus infectivity by blocking sialic acid receptor binding either directly by interacting with the receptor binding site at the tip of the molecular or indirectly, by projecting over the binding site thereby rendering it inaccessible [12]. These antibodies are involved in the selection of virus with variant Has in the process of antigenic drift, necessitating the annual re-development of influenza vaccines [10].
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