Why vaccinate school-aged children against influenza?

Many people consider influenza a nuisance that comes every winter. It is part of the normal cycle of colds that begins after school starts, causing havoc and disruption in the routine activities of family living. What people do not know or have forgotten are the following facts about influenza and influenza vaccines.

• Every year, an influenza epidemic strikes the United States, infecting 5% to 20% of the population. Preschoolers and school-aged children are the age groups most likely to be infected with influenza.

• Influenza causes more hospitalizations and deaths in children and adults than any other vaccine-preventable disease in the United States. In the 2012-2013 influenza season, more than 150 children were documented to have died from influenza; many of them were healthy and not vaccinated against influenza.

• Many more children die from influenza each year, but go unrecognized. The Centers for Disease Control and Prevention estimated that 1,280 children died during the 2009 H1N1 pandemic; however, only 348 children were reported to have died from laboratory-confirmed influenza.

• Influenza vaccines are safe and provide a good level of protection against influenza.

• There are 2 types of influenza vaccines approved for use in US children: the live attenuated influenza vaccine (LAIV) that is approved for healthy children aged 2 through 17 years; and the inactivated influenza vaccine (IIV) that is approved for children aged 6 months through 17 years and with at-risk conditions.

• Quadrivalent influenza vaccines will make their debuts in 2013-2014. These vaccines contain vaccine strains for both influenza A subtypes (H3N2 and H1N1pdm09) and both influenza B lineages (B-Yamagata and B-Victoria). The quadrivalent vaccines will provide broader coverage against human influenza viruses compared with the standard trivalent formulation that contains vaccine strains for both influenza A subtypes and only 1 influenza B lineage.

• Improving influenza vaccination coverage in children can reduce influenza illness in family members and other susceptibles in the community by reducing the risk of exposure and subsequent influenza infection and related complications. This concept is called indirect or herd protection.

A national wake-up call

The burden of influenza in children was unrecognized for many years.¹ The substantial number of influenza-associated deaths among US children in the 2003-2004 influenza season brought attention to the severity of influenza in healthy children. Sixty-seven percent of the 153 children who died from influenza that year did not have an at-risk medical condition recognized by the Advisory Committee on Immunization Practices (ACIP) for receiving influenza vaccine. Thirty-seven percent of the children who died were aged older than 5 years.²

Infants and children are highly susceptible to influenza because they either have not been infected or have been infected less often with 1 of the major circulating influenza viruses (H1N1, H3N2, B-Yamagata-like, or B-Victoria-like) compared with adults. School-aged children also experience high rates of influenza infection, febrile illness, and school absenteeism. During an influenza outbreak, 63 school days were missed for every 100 children.³ An increase in work-related absenteeism also occurred among the parents who missed approximately 1 day of work for every 3 days of school missed by their children.

Influenza causes secondary bacterial pneumonia and serious disease associated with organ systems other than the respiratory tract. Hospitalization due to acute febrile illness and central nervous system disease adds to the spectrum of serious disease in children attributed to influenza. Influenza infection in children also exerts a significant direct and indirect financial cost. Only in the last decade has ACIP appreciated the burden of influenza in children; it now recommends influenza vaccine for all children aged 6 months and older.⁴

Children are frequently cited as the major vector for the transmission of influenza virus within families, schools, and communities because children have high infection rates, prolonged viral shedding with large amounts of infectious virus, and come in close contact with susceptible classmates. Models suggest that the likelihood of an individual becoming infected with influenza increases approximately 2-fold if the household includes a member aged younger than 18 years compared with those without children.⁵

“Herd protection” works

Monto and colleagues were some of the first to recognize that children played an important role in the dissemination of influenza virus.⁶ They reasoned that, because school-aged children had high attack rates, infected school children would be the source of secondary infections to susceptible household members. Thus, vaccination of large numbers of children in a community might prevent or reduce the transmission of influenza to susceptible members of that community. In a proof-of-concept study, approximately 86% of school-aged children residing in a small community were vaccinated with a single dose of IIV in 1968. A significant overall reduction in influenza-associated illnesses was observed in the intervention community. Age-specific indirect effectiveness observed in unvaccinated adults was comparable to the total effectiveness observed in school-aged children.

The development of the Russian LAIV allowed the former Soviet Union to consider mass immunization of school children. Thirty schools in a major city in Russia participated in the study.⁷ Each school received the IIV or the Russian LAIV, or placebo. Participation ranged from 23% to 95% of the eligible school-aged children in the participating schools. During the 2 study years, both the Russian live and inactivated influenza vaccines protected against influenza-related illnesses. The Russian LAIV, but not the IIV vaccine, was associated with an indirect effect (herd protection) in unvaccinated students and staff.

Our experience in central Texas suggests that influenza vaccination coverage as low as 20% to 25% in children, primarily with the LAIV delivered by nasal spray, can significantly reduce medically attended acute respiratory illness (MAARI) in adults.⁸ A recent study in Russia demonstrated that mass influenza vaccination of children with the IIV reduced influenza-associated morbidity by 2- to 3.4-fold in unvaccinated, noninstitutionalized elderly adults.⁹ In this study, approximately 57% of children (aged 3-6 years) in kindergartens and 72% of children (aged 7-17 years) attending schools were vaccinated. King and colleagues conducted a multicenter, school-based vaccination study to reduce influenza-related illnesses.¹⁰ Forty-seven percent of the students in the intervention schools received LAIV. A significant reduction in influenza-related outcomes occurred in families of children attending the intervention schools. In Ontario, Canada, the high-risk based influenza immunization program was expanded to a universal influenza immunization program in October 2000.^11,12 In the United States, a universal influenza immunization recommendation for children was made in 2008 and expanded to all persons aged 6 months and older in 2010.^4,13

Universal influenza vaccination has the potential to substantially reduce overall morbidity, mortality, and health care cost related to influenza. To achieve a substantial direct and indirect impact, high vaccination coverage is needed. Preliminary estimates for influenza vaccination coverage in 2012-2013 were about 55% for all children and about 35% for all adults.¹⁴ The highest coverage was achieved in children aged 6 months to 4 years at approximately 68% and the lowest in children aged 13 to 17 years at about 40%, a coverage level that approached those in adults.

The medical home has been the primary site for administering influenza vaccines to children; however, many children do not have a medical home. Complementing strategies and equitable reimbursement to improve influenza vaccination coverage among all children in the community are needed. A school-based vaccination program is 1 such strategy that can enhance access to vaccines and complement the influenza vaccination program administered at the health care practices.

REFERENCES

Poehling KA, Edwards KM, Weinberg GA, et al; New Vaccine Surveillance Network. The underrecognized burden of influenza in young children. N Engl J Med. 2006;355(1):31-40.

Bhat N, Wright JG, Broder KR, et al; Influenza Special Investigations Team. Influenza-associated deaths among children in the United States, 2003-04. N Engl J Med. 2005;353(24):2559-2567.

Neuzil KM, Hohlbein C, Zhu Y. Illness among schoolchildren during influenza season: effect on school absenteeism, parental absenteeism from work, and secondary illness in families. Arch Pediatr Adolesc Med. 2002;156(10):986-991.

Centers for Disease Control and Prevention (CDC). Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP)-United States, 2012-13 influenza season. MMWR Morb Mortal Wkly Rep. 2012;61(32):613-618.

Longini IM Jr, Koopman JS, Monto AS, Fox JP. Estimating household and community transmission parameters for influenza. Am J Epidemiol. 1982; 115(5):736-751.

Monto AS, Davenport FM, Napier JA, Francis T Jr. Modification of an outbreak of influenza in Tecumseh, Michigan by vaccination of schoolchildren. J Infect Dis. 1970;122(1):16-25.

Rudenko LG, Slepushkin AN, Monto AS, et al. Efficacy of live attenuated and inactivated influenza vaccines in schoolchildren and their unvaccinated contacts in Novgorod, Russia. J Infect Dis. 1993;168(4):881-887.

Piedra PA, Gaglani MJ, Kozinetz CA, et al. Herd immunity in adults against influenza-related illnesses with use of the trivalent-live attenuated influenza vaccine (CAIV-T) in children. Vaccine. 2005;23(13):1540-1548.

Ghendon YZ, Kaira AN, Elshina GA. The effect of mass influenza immunization in children on the morbidity of the unvaccinated elderly. Epidemiol Infect. 2006;134(1):71-78.

King JC, Stoddard JJ, Gaglani MJ, et al. Effectiveness of school-based influenza vaccination. N Engl J Med. 2006;355(24):2523-2532.

Groll DL, Thomson DJ. Incidence of influenza in Ontario following the Universal Influenza Immunization Campaign. Vaccine. 2006;24(24):5245-5250.

Glezen WP. Benefits of a universal influenza immunization program: more than the reduction in the use of antibiotics. Clin Infect Dis. 2009;49(5):757-758.

CDC’s Advisory Committee on Immunization Practices (ACIP) recommends universal annual influenza vaccination [press release]. Centers for Disease Control and Prevention (CDC). February 24, 2010. http://www.cdc.gov/media/pressrel/2010/r100224.htm. Accessed July 22, 2013.

Kennedy ED; Centers for Disease Control and Prevention (CDC). Influenza vaccination coverage: How well did we do in 2012-13. Presented at: National Adult and Influenza Immunization Summit; May 15, 2013; Atlanta, GA. http://www.cdc.gov/flu/pdf/fluvaxview/kennedy_2013_summit_slides2.pdf. Accessed July 22, 3013.

DR PIEDRA is professor of Molecular Virology and Microbiology, and Pediatrics, Baylor College of Medicine, Houston, Texas, and director, Viral Diagnostic Laboratory, Houston. He has been principal investigator for clinical trials studying viral infections, immunology, and vaccine safety. He is also a physician at Texas Children's Hospital, Houston.

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