Herd immunity in adults against influenza-related illnesses with use of the trivalent-live attenuated influenza vaccine (CAIV-T) in children
Introduction
Influenza is a vaccine-preventable disease. Over 80 million doses of licensed inactivated influenza vaccine, trivalent (IIV-T) are produced annually in the United States [1]. The principal strategy advocated by the Advisory Committee on Immunization Practices (ACIP) for control of influenza has been the vaccination of persons at risk for serious complications from influenza, in particular, persons ≥65 years old. Yearly influenza epidemics, however, continue unabated and are responsible for significant health and financial burden to our society [1], [2], [3]. In the last 20 years (1976–1999), a significant increase has occurred in influenza-associated all cause excess deaths [1], [4]. From 1990 to 1999, the annual number of influenza-associated all cause deaths exceeded 50,000 [4]. Despite the increase in vaccine coverage of persons ≥65 years to 65% during the last decade, a corresponding reduction in influenza-associated all cause excess deaths has not been observed [1], [4]. Consideration of additional control methods is warranted.
A complementary strategy for the prevention and control of influenza is universal vaccination of healthy children [5], [6]. Children have high rates of infection, medically attended illness and hospitalization from influenza [7], [8], [9], [10], [11], [12], [13]. Children play an important role in the transmission of influenza within schools, families and communities [14]. Vaccination with IIV-T of approximately 80% of schoolchildren in a community has decreased respiratory illnesses in adults [15]. and excess deaths in the elderly [16].
The recently licensed cold adapted influenza vaccine, trivalent (CAIV-T) has been shown to be safe and highly efficacious in young children and healthy adults [17], [18], [19], [20]. CAIV-T is approved for use in healthy persons 5–49 years of age. The trial reported here was conducted prior to licensure of CAIV-T. The primary objective in this open-label, non-randomized, community-based trial was to determine the proportion of children 1.5–18 years of age who should be vaccinated to reduce the incidence of medically attended acute respiratory illnesses (MAARI) during influenza epidemics among susceptible contacts [21].
Section snippets
Study design
An open-label, non-randomized, community-based trial with an investigational influenza vaccine, CAIV-T, was conducted in healthy children 18 months through 18 years of age. Age-eligible children in the intervention communities, Temple and Belton (T-B), were offered CAIV-T each year (T-B are adjacent towns in Bell County). Age-eligible children in the comparison communities, Waco, Bryan, and College Station (W-B), were not offered CAIV-T (Waco, in McLennan County is approximately 40 miles north
CAIV-T coverage in intervention communities
Demographic characteristics of the intervention and comparison communities are presented in Table 2. Approximately 30% of the populations for the intervention and combined comparison communities were ≤19 years old. The intervention sites had a higher proportion of persons ≥65 years of age and the combined comparison sites a higher proportion of young adults. The average household size was larger in the combined comparison communities.
CAIV-T was not offered in the baseline year (1997–1998). In
Discussion
Herd immunity or indirect effectiveness due to influenza vaccine has been recognized and accepted for many years. Monto et al. [15] recognized that children play an important role in the dissemination of influenza viruses. They reasoned that because school-aged children usually had the highest attack rates, infectious school children were the likely source of secondary infections of susceptible household contacts. Vaccination of large numbers of children in a community, therefore, might prevent
Acknowledgements
We are thankful to Ira M. Longini for critical review of the manuscript. We appreciate the support provided by Linda Lambert, Ph.D., Influenza Program Officer at NIAID, and Paul Mendelman, MD and Pat Fast, MD from Medlmmune. Sonia Holleman, Hope Gonzales, Nancy Arden, Carolyn Kamenicky, Neil Pascoe and Dwayne Haught (NP and DH: Texas Department of Health) supported viral surveillance. This study would not be possible without the staff and physician support from Baylor College of Medicine, Scott
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