Elsevier

Vaccine

Volume 27, Issue 33, 16 July 2009, Pages 4457-4461
Vaccine

Effectiveness of inactivated influenza vaccine in children less than 5 years of age over multiple influenza seasons: A case–control study

https://doi.org/10.1016/j.vaccine.2009.05.038Get rights and content

Abstract

The effectiveness of influenza vaccine over multiple influenza seasons in children less than 5 years of age has not been well studied. This is especially important to assess because of the recent recommendation for routine influenza vaccination in childhood.

We conducted a matched case–control study to assess the vaccine effectiveness of Trivalent Inactivated Influenza Vaccine (TIV) against laboratory-confirmed, medically attended influenza among children 6–59 months of age at the start of each influenza season from 1999–2000 through 2006–2007 in Olmsted County, MN, USA.

The children vaccinated against influenza accordingly to the 2007 ACIP guidelines had a lower risk of laboratory-confirmed medically attended influenza illness (Odds Ratio: 0.14, 95% Confidence Interval: 0.03–0.71) than the unvaccinated children. TIV provided strong protection against laboratory-confirmed medically attended influenza in children 6–59 months old in the fully vaccinated group. This trend continued in the partially vaccinated group (Odds Ratio: 0.27, 95% Confidence Interval: 0.07–0.97) but the protection provided maybe suboptimal.

Introduction

Immunization is the major public health measure for the prevention of influenza virus infection [1], [2]. Recent data show that childhood influenza causes excess hospitalizations, medical visits, and antibiotic prescriptions in healthy children, especially those younger than 2 years of age [3]. Influenza causes more illness each year than any other vaccine preventable illness [4].

For the 2007 influenza season, the Advisory Committee on Immunization Practices (ACIPs) reemphasized the importance of administering 2 doses of vaccine to all children aged 6 months to 8 years if they had not been vaccinated previously against influenza [5], [6]. ACIP also recommended that children aged 6 months to 8 years who had received only 1 dose in their first year of vaccination receive 2 doses the following year [6]. The efficacy (prevention of illness among vaccinated persons in controlled trials) and effectiveness (prevention of illness in vaccinated populations) of influenza vaccines depend primarily on the age and immunocompetence of the vaccine recipient, the degree of similarity between the viruses in the vaccine and those in circulation, and the outcome being measured [6].

Few studies have been published on the effectiveness of influenza vaccine among children less than 5 years of age over multiple influenza seasons. One or more mismatches between the vaccine strain and the circulating strain may contribute to lower vaccine effectiveness in a particular influenza season. Conversely, antigenic similarity between vaccine strain and circulating strain is likely to increase vaccine effectiveness.

Many studies in children less than 2 years of age have demonstrated suboptimal protection with a single dose in naïve children. We conducted a case–control study to evaluate the effectiveness of Trivalent Inactivated Influenza Vaccine (TIV) against medically attended, laboratory-confirmed cases of influenza illness in Olmsted County residents who sought care at the Mayo Clinic and associated hospitals between 6 and 59 months of age over eight influenza seasons.

Section snippets

Study population

Children between 6 and 59 months of age at the start (1 November) of the influenza seasons (1999–2000 to 2006–2007) were included in the study. The influenza season was defined from 1 November to 30 April for the respective season for all the seasons in the study period. Only Olmsted County residents were included in the study to ensure accurate ascertainment of immunization history. We obtained the unique patient identifier numbers of all influenza positive polymerase chain reaction (PCR) and

Results

There were 103 matched pairs of cases and controls over eight influenza seasons.

Table 2 shows the demographic distribution of the cases and controls, 75 (72%) of the cases were unvaccinated, 13 (14%) were partially vaccinated and 15 (15%) were fully vaccinated. In comparison, for the control subjects, 46 (45%) were unvaccinated, 23 (22%) were partially vaccinated and 34 (33%) were fully vaccinated. Fig. 2 shows the distribution of the cases across the eight influenza seasons.

Discussion

This analysis shows that, when given as recommended by the ACIP, TIV is highly effective in preventing laboratory-confirmed influenza illness in children <5 years old.

The estimated VE in the literature for children less than 9 years of age is about 30–90% [3]. We found higher effectiveness over the 8-year period of our study. In this study period, the circulating strain and the vaccine strain were closely matched in ∼45–50% of all isolates (50% for H1N1, 37% for H3N2 and 50% for Influenza B).

Acknowledgement

Funding: None.

Financial Disclosures: TGB is on the speaker's bureau for MedImmune. No disclosures for other authors.

References (17)

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