3.1. Clinical Relevance
Although persons >65 yr old comprise approximately 13% of the United States population and have low rates of infection with influenza virus (10% of total influenza infections), they account for nearly 50% of the hospitalizations (20,000 to >300,000 per epidemic) and 90% of the deaths (20,000 to >40,000 per epidemic) attributed to influenza. The highest estimated rates are among persons >65 yr old and persons of any age with underlying cardiovascular disease in combination with either diabetes melli-tus or chronic pulmonary disease (15).
Prevention of influenza virus infection by immunization is a formidable task in older persons. Influenza A viruses, the primary cause of severe illness, are classified into subtypes on the basis of their hemagglutinin (H) and neuroaminidase (N) antigens. Sufficient antigen variation or drift within the same subtype, e.g., A/Texas/77 (H3N2) versus A/Bangkok/79 (H3N2), may occur over time so that infection or immunization with one strain may not induce immunity to related strains of the same subtype. Major antigenic shifts, which herald pandemic influenza, produce "new" viruses to which the population has no immunity, e.g., the shift in 1957 from H1N1 to H2N2. Influenza B viruses also cause disease in older persons and, although they are much more antigeni-cally stable than influenza A viruses, antigen variation does occur. Consequently, influenza vaccine presently must be administered each year and include the inactivated expected virus strains.
Inactivated influenza vaccines consist of highly purified egg-grown inactivated viruses in either a whole virus or split (subvirion) virus trivalent preparation containing two type A strains and one type B strain. For the 1999-2000 influenza season in the
United States, the standard trivalent inactivated influenza vaccine contained 15 ^g each of A/Beijing/262/95-like (H1N1), A/Sydney/5/97-like (H3N2), and B/Beijing/184/93-like hemagglutinin antigens in each 0.5 mL dose of vaccine. A single intramuscular 0.5 mL dose is required each year for older persons (15).
Acute local reactions with mild to moderate soreness around the vaccination site occur in approximately one-third of vaccines and last 1-2 d. Systemic reactions, including fever with or without a flu-like illness, occur in less than 1% of vaccinees, begin 6-12 h postvaccination, and persist for 1-2 d but are not associated with higher rates of systemic symptoms in older persons compared to placebo injections (16).
Precautions to vaccination include immediate hypersensitivity reactions or documented immunoglobulin E (IgE)-mediated hypersensitivity to egg protein; older persons with these findings should consult a physician for appropriate evaluation to help determine if vaccine should be administered. Persons with a previous history of Guillain-Barré syndrome (GBS) have an increased likelihood of coincidentally developing GBS after influenza vaccination, but whether influenza vaccination specifically might increase the risk for recurrence of GBS is not known. However, many experts believe that for most older persons who have a history of GBS and are at high risk for severe complications from influenza, the established benefits of influenza vaccination justify yearly vaccination. There are no studies to suggest an increased risk of reaction in persons with multiple sclerosis or other chronic neurologic demyelinating diseases. Although influenza vaccinations can reduce the clearance of theophylline and warfarin, studies have not demonstrated any adverse clinical consequences attributed to these drugs in vaccinees (15).
The proportion of elderly vaccinees who develop "protective" antibody titers postvaccination, i.e., serum hemagglutination inhibition (HAI) antibody titers of 1:40 or greater, ranges from more than 85% for H3N2 vaccine antigens in healthy community-based persons to 46-100% for H3N2 antigens and 20-69% for B antigens in ambulatory older persons in long-term care facilities. The antibody titers postvaccination for elderly nursing facility patients demonstrate considerable heterogeneity and are significantly lower than healthy young and healthy older persons (17). This suggests that older persons with chronic diseases, medications, or other conditions frequently associated with residence in long-term care facilities may be expected to respond less satisfactorily to inactivated influenza vaccines. T-cell-mediated immune mechanisms, e.g., influenza A-specific cytotoxic T lymphocyte activity, are also important in influenza viral clearance and appear reduced in older adults (18). Recent successful investigational efforts to improve the immunogenic response of older persons to influenza vaccine have included a concurrent four-week series of subcutaneous injections of thymic hormone, thymosin a1; the use of conjugated diphtheria toxoid-hemagglutinin vaccine; and combined live intranasal and inactivated influenza vaccines. However, the only currently licensed vaccine is the annual inactivated influenza virus formulation (19).
How effective is the vaccine? In a prospective cohort observational study of community-residing older persons during an influenza A epidemic, vaccination was associated with a reduction in the rates of hospitalization for pneumonia and influenza (57%), all acute and chronic respiratory conditions (39%) and congestive heart failure (43%) (20). The first randomized double-blind placebo-controlled trial in a similar population in the Netherlands demonstrated a risk reduction of nearly 60% for clinical influenza with serological confirmation (21). A case-control observational study of influenza outbreaks in nursing facilities indicated that, although the efficacy of influenza vaccine in preventing uncomplicated illness was relatively low (28-37%). The vaccine was substantial in reducing complications, including hospitalization or pneumonia (50-60%) and death (80%) (22).
Presently, all persons 65 yr of age and older should receive annually the inactivated parenteral influenza vaccine. The elderly subgroups that should be prioritized for organized vaccination programs include: (1) persons with chronic disorders of the cardiovascular or pulmonary systems, (2) residents of nursing facilities and other long-term care facilities, (3) persons who require regular medical follow-ups or hospitalization during the preceding year because of chronic metabolic conditions (including diabetes mellitus), renal dysfunction, hemoglobinopathies, or immunosuppression (including immunosuppression caused by medications) and (4) otherwise healthy older persons. Because most residents in long-term care facilities have reduced antibody responses, it is important that this elderly high-risk group receive influenza vaccine no earlier than mid-October. In addition, all persons who have extensive contact with these higher-risk older persons should also receive influenza vaccination annually. These people include physicians, nurses, and other health-care team personnel, as well as formal and informal providers of care in the home setting (15).
Strategies for implementing current recommendations and improving utilization of influenza vaccine (together with pneumococcal vaccine) will be discussed as in Subheading 4.4.
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