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Effective Immunization of Older Adults Against Seasonal Influenza

The burden of influenza in adults 65 years old and above compared with other age groups: cumulative hospitalization rates (per 100,000 population) for influenza seasons 2012-2013 to 2016-2017.

The 2017-2018 influenza season reminds us that it is important for health care professionals to be prepared for the annual onslaught of this contagious respiratory disease associated with potentially serious complications. Vaccination is by far the best method to prevent and control influenza, reducing illness, hospitalizations, and mortality. The highest rates of influenza-associated morbidity and mortality are observed in older adults. The immune function of older adults decreases with increasing age, a phenomenon termed immunosenescence. Immunosenescence not only confers increased susceptibility to influenza disease, but also renders vaccination less effective. To address the need for improved vaccines that provide enhanced protection to this high-risk group, 2 formulations—a high-dose vaccine and an adjuvanted vaccine—have been approved in recent years specifically for people aged 65 years and over. Here we discuss: the challenges of influenza immunization in those 65 years and older; the recent advancements in vaccines targeted at this age group; and the latest influenza vaccine recommendations for the 2017-2018 influenza season in the United States.

Introduction

The Centers for Disease Control and Prevention (CDC) estimates that influenza has resulted in 9-60 million cases of illness annually in the United States (US) since 2010.1 Influenza is an unpleasant but usually self-limiting disease in healthy people, with recovery generally in less than a week, often without the need for medical attention.2 However, influenza and its associated complications can be extremely serious in some people, resulting in hospitalization or even death; an estimated 140,000-710,000 hospitalizations and 12,000-56,000 deaths have occurred due to influenza annually since 2010 in the US.1 In particular, people aged 65 years and over bear the largest burden, accounting for an estimated 71%-85% of all seasonal influenza-related deaths and 54%-70% of all seasonal influenza-related hospitalizations.3 The best way of preventing influenza is through vaccination, reducing the risk of illnesses, medical visits, hospitalizations, and mortality.4

Influenza Viruses

Influenza viruses circulate every season across the northern and southern hemispheres, resulting in annual epidemics of influenza.5 In temperate climates, seasonal epidemics occur during the fall and winter, typically between October and April in the US, but the timing can be unpredictable.6

There are 3 types of influenza viruses, classified as A, B, and C, based on their protein composition. Only influenza A and B cause clinically significant human disease and seasonal epidemics.5, 7 Influenza A viruses are divided into subtypes based on 2 proteins on the surface of the virus, hemagglutinin (HA) and neuraminidase (NA), and are then further classified into different strains.5, 7 Three subtypes of HA (H1, H2, and H3) and 2 subtypes of NA (N1 and N2) are among the influenza A viruses that have caused widespread human disease—these correspond with the H1N1, H2N2, and H3N2 seasonal influenza virus strains.8 Whereas there can be multiple influenza A viruses, influenza B viruses can be divided into 2 main lineages, referred to as B/Yamagata and B/Victoria.5, 7

Due to antigenic shift and drift, influenza viruses are changing constantly. Antigenic drift, which occurs frequently, involves small mutations in the viral genes coding for antigenic cell surface proteins. These mutations result in the emergence of novel surface antigens and are a reason for annually reviewing circulating influenza strains for the selection of viruses to be represented in subsequent influenza vaccines. On the other hand, antigenic shift occurs in influenza A viruses and involves the combination of 2 or more different viral strains to form a completely novel subtype. Such changes occur rarely and may result in global pandemic outbreaks.9, 10, 11

The dominant viral strains circulating among humans vary from season to season and can be difficult to forecast. Overall, influenza A has been the predominant circulating influenza strain in the US over recent years (to varying extents, from almost 100% in the 2009-2010 influenza season to 70% in the 2016-2017 influenza season).12, 13,14, 15, 16, 17, 18, 19 Of the influenza A viruses, A(H3N2) has predominated in 5 of the last 8 years (in 3 consecutive seasons from 2010-2011 to 2012-2013, then in seasons 2014-2015, 2016-2017, and 2017-2018, varying from 62% to almost 100%). A(H3N2)-predominant seasons have been associated with increased hospitalizations and deaths (compared with seasons in which A(H3N2) was not the most common circulating strain), especially among adults aged 65 years and over (Figure 1).17, 19, 20 Therefore, an additional burden on the elderly is seen when A(H3N2) is the predominant circulating strain.

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-William Schaffner, MD, Wilbur H. Chen, MD, MS, Robert H. Hopkins, MD, Kathleen Neuzil, MD, MPH

This article originally appeared in the August issue  of The American Journal of Medicine.

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