Influenza Vaccine: Review of Effectiveness of the U.S
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Influenza Vaccine: Review of Effectiveness of the U.S. Immunization Program, and Policy Considerations David A. Geier, B.A. The Centers for Disease Control and Prevention (CDC) has recommended the administration of influenza vaccine during the Paul G. King, Ph.D. 2006-2007 influenza season to the following persons:5 Mark R. Geier, M.D., Ph.D. • children aged 6 to 59 months; • women who will be pregnant during the influenza season; • persons aged> 50 years; ABSTRACT • children and adolescents (aged 6 months to 18 years) who are receiving long-term aspirin therapy and, therefore, might be at A number of studies have reported that influenza vaccine (IV) risk for experiencing Reyes syndrome after influenza infection; administration has been less than optimally effective in certain • adults and children who have chronic disorders of the subpopulations. This study examines yearly influenza death rate, pulmonary or cardiovascular systems, including asthma yearly influenza case rate, and yearly rate of hospitalizations with (hypertension is not considered a high-risk condition); influenza as the first-listed discharge diagnosis. By these • adults and children who have required regular medical follow- measures, the yearly U.S. mass influenza vaccination campaign up or hospitalization during the preceding year because of has been ineffective in preventing influenza in vaccine recipients. chronic metabolic conditions (including diabetes mellitus), The use of antiviral drugs to treat influenza, in light of the potential renal dysfunction, hemoglobinopathies, or immunodeficiency for an influenza pandemic, needs further consideration. (including immunodeficiency caused by medication or human immunodeficiency virus); Background • adults and children who have any condition (e.g., cognitive dysfunction, spinal cord injuries, seizure disorders, or other th Vaccines are one of the great public-health triumphs of the 20 neuromuscular disorders) that can compromise respiratory 1 century. function or the handling of respiratory secretions, or that can th Vaccination began in the late 18 Century with Dr. Edward increase the risk for aspiration; Jenner’s inoculation of people with the much less deadly cowpox • residents of nursing homes and other chronic-care facilities that virus to protect against deadly smallpox because he noticed that house persons of any age who have chronic medical conditions; milkmaids, who had had cowpox, did not contract smallpox.2 The • persons who live with or care for persons at high risk for widespread use of vaccines to prevent infectious disease is based on influenza-related complications, including healthy household the long-known observation that, for many infectious diseases, contacts and caregivers of children aged 0 to 59 months; people who recover from having an infectious disease develop a • healthcare workers; long-term immunity against that disease.3 • any person who wishes to reduce the likelihood of becoming ill Influenza is inherently different from the common vaccine- with influenza or transmitting influenza to others should they preventable diseases. An individual who recovers from influenza become infected. usually does not acquire resistance to other strains of influenza Simonsen et al. from the National Institute of Allergy and virus. Through genetic drift, the immunological determinants Infectious Disease have recently studied influenza-related change, enabling the virus to escape the adaptive immunological mortality in the U.S. over the past three decades.6 They found that defenses of the host. Every few years, this produces a significant the influenza mortality rate in the over-65 age group has increased change called a genetic shift.4 despite a concurrent jump in vaccination rates in that group from The influenza vaccine program is the first that seeks to offer 15% in 1980 to 65% in 2001. The Simonson article used population-wide protection against a disease that undergoes such mathematical models to correct for the effects of aging of the rapid antigenic changes. It therefore differs from other vaccine population as well as for the variations in the virulence of the programs in many significant ways. First, because it takes many influenza strains prevalent in the population. Even with these months to produce sufficient vaccine to protect a large population, corrections, the authors concluded that the mortality-reduction an educated guess has to be made as to which strain(s) of influenza benefits of influenza vaccination may be substantially less than will be endemic in the following influenza season. This guess must previously thought. be made well before there is an outbreak of cases of influenza. The mortality reduction found in several observational studies, Second, influenza vaccine has to be made and administered without which compare cohorts of vaccinated and unvaccinated people, specific efficacy testing, which, in other viral vaccines, includes reflects systematic bias, in the opinion of Simonsen et al. The some double-blind field trials against the wild-type disease. Third, unvaccinated groups had a disproportionate number of very ill there is no time to fully test each newly made influenza vaccine for people. People in fragile health right before the influenza season, its safety, especially with respect to its long-term rarer side effects. suggested Simonsen et al., are more likely to die and less likely to Journal of American Physicians and Surgeons Volume 11 Number 3 Fall 2006 69 Table 1. Raw Data Employed for Analysis in the Present Study SD * U.S. Census Bureau † Biologic Surveillance Summaries of the Centers for Disease Control and Prevention ‡ National Center for Health Statistics § Estimates for 1979 through 1998 use International Classification of Diseases, 9th Revision (ICD-9) coding ** Estimates for 1999 through 2000 use International Classification of Disease, 10th Revision (ICD-10) coding receive the vaccine. Additionally, Kristen Nichols, an Materials and Methods epidemiologist with the Veterans Administration in Minnesota, noted that measuring influenza deaths by looking at the “excess An ecologic method was used. The total number of doses of mortality” that occurs during a given influenza season is influenza vaccine distributed/administered each year was inappropriate because other respiratory illnesses that circulate determined from the Biological Surveillance Summaries of the during the same season can also cause death. Blood tests are rarely CDC for the periods 1979 through 1985, and 1990 through 2000 done to confirm that ailing persons actually had influenza.7 (CDC, personal communication 2002). In order to estimate Maeda et al. conducted a prospective trial of healthy infants and influenza vaccine coverage rates in the United States, the U.S. young children (6-24 months old) who had received inactivated Census Bureau estimates were used for the population of the United 8 influenza vaccine before influenza seasons. Age-matched children States from 1979 through 1985, and 1990 through 2000.9 Based were randomly assigned as the control. These children were upon these measurements, the influenza vaccine percent followed up from January to April in each year (2000, 2001, and population coverage (IVPPC) was determined for each year 2002). Influenza attack rates were not significantly different in the two groups. examined in this study ([Yearly Total Net Number of Influenza Because a number of previous epidemiological studies have Vaccine Doses Distributed / Yearly Estimated United States raised doubts about the effectiveness of influenza vaccines, large- Population] x 100 = IVPPC). scale population data need to be reviewed to assess the effectiveness The effectiveness of influenza vaccine administration in the of the mass influenza vaccine campaign in the United States. The United States was studied by evaluating the IVPPC in comparison present study examines the period from 1979 through 2000. to three outcomes: (1) yearly influenza death rate (1979, 1981, 70 Journal of American Physicians and Surgeons Volume 11 Number 3 Fall 2006 Table 2. Number of Influenza Deaths per Year in Children per 10,000 Persons Influenza First-Listed Hospital Discharges Influenza Vaccine Percent Population Coverage Figure 1. Yearly Rate per 10,000 Persons of Influenza First-listed Hospital SD Discharge Diagnoses in Comparison to the Influenza Vaccine Population Source: National Center for Health Statistics 1983, 1985, 1991, 1993, and 1995 through 2000); (2) yearly influenza case rate (1982 through 1996); and yearly rate of hospitalization with influenza as the first-listed discharge diagnosis (1990 through 2000). The data for these measures were obtained from the National Center for Health Statistics.10 Table 1 presents a complete summary of the raw data employed. Our sources did not provide data for the years that are not included in our analysis. Influenza Case Rate per 100 Persons The Simple Linear Regression statistical test from StatsDirect Influenza Vaccine Percent Population Coverage (Version 2.4.1) statistical package was utilized for data analysis. The null hypothesis employed in this study was that the IVPPC should have no effect on the measures of effectiveness; in other words, the slope of the line should not be different from zero. In this analysis, the equation of the regression line, the slope of the Figure 2. Yearly Rate per 100 Persons of Influenza Cases in Comparison to the Influenza Vaccine Population Percent Coverage regression line, the correlation coefficient (r), and