Seasonal influenza vaccine effectiveness in children during 2010–2011 season

Keywords: :

• China
• influenza vaccine
• vaccine effectiveness

To The Editor

Influenza causes more morbidity than any other vaccine-preventable illness and leads to an estimated 50 to 95 outpatient visits, 6 to 27 emergency department visits, and 30 to 90 courses of antibiotics each year per 1000 children aged less than 5 y.^1,2 Immunization recommended each year is the major public health measure for the prevention of influenza virus infection. The differences in match between vaccine and circulating strains make it necessary to assess influenza vaccine effectiveness (VE) each year.^3,4 In this study, we assessed the effectiveness of the trivalent inactivated influenza vaccine against laboratory-confirmed influenza for the 2010–11 influenza season among children in Guangzhou, the central city in southern China.

Results

During 2010–2011 season, 308 and 774 children were enrolled into case group and subset group respectively. No child in subset group was found to develop seasonal influenza. Demographic characteristics (gender, residence of area and age) are simliar between the two groups(x² = 0.035,p = 0.851; x² = 0.471,p = 0.790; t = 0.729,p = 0.466).

The influenza vaccination rate was 16.9% in the case group compared with 36.0% in the subset group. Vaccination and full vaccination provided over 55% protection to children aged 6 to 59 mo, better protection offered by full vaccination as compared partial vaccination. However, vaccine effectiveness did not differ from children aged 6 to 23 mo and 24 to 59 mo (Table 1).

Table 1. Effectiveness of influenza vaccine for children aged 6–59 mo in 2010–2011

	Full vaccination	Partial vaccination	Vaccination
6–23mo	55.0(36.6,68.1)*	36.1(-0.8,59.5)	59.3(39.2,72.7)*
24–59mo	58.8(35.6,73.6)*	49.4(13.8,70.3)	66.2(46.0,78.8)*
6–59 mo	55.8(42.1,66.3)*	41.5(17.6,58.5)*	61.9(48.5,71.8)*

*p < 0.001; Vaccine effectiveness was adjusted by gender, residence of area and age.

Methods

Children in Guangzhou aged 6 to 59 mo during the 2010–11influenza season (lasting from 2010 September to 2011 August) were included in this study. Influenza cases confirmed by laboratory in 2010–2011 were enrolled as case group as described previously.⁵ With cluster sampling method, children were selected in Guangzhou as a subset group and followed up a year. Case-cohort study was conducted between these two groups. Immunization records were obtained from Children EPI Administrative Computer System.^6,7 Cohort subjects did not have ILI during the study influenza season, which was confirmed by a phone call by physicians from GZCDC.

Influenza vaccination status was categorized into full, partial or unvaccination. Children were considered fully vaccinated if during the influenza seasons included in the study period they received ≥ 2 age-appropriate doses of seasonal influenza vaccine 4 or more weeks apart, with the second dose given ≥ 2 weeks before acute respiratory symptom onset. Children were also considered fully vaccinated if they received ≥ 2 doses in 1 season and 1 dose in the next study season. Children were counted as partially vaccinated if they received 1 dose in the study season or ≥ 2 doses in the season immediately before the study season.⁸ We analyzed relative risk(RR) by COX proportional hazards model analysis. VE was calculated as (1-RR)*100%. Study approval was obtained from the GZCDC ethics committee.

Comment

The degree of antigenic matching between circulating and vaccine strains affects vaccine effectiveness. The influenza vaccine demonstrated immunological effectiveness among children aged 6 to 59 mo. The protection of full vaccination is higher than partial vaccination; full vaccination should be encouraged in children. Continued future annual assessments of seasonal vaccine efficacy and effectiveness are necessary in order to better guide vaccination policies and influenza infection control efforts.

Acknowledgments

This work was supported by grants from the Department of Health of Guangzhou (201102A213026), Department of Health of Guangdong (A2011506), Guangdong Provincial Department of Science and Technology (2011B050300001 and 2012B091100045), Science and Information Technology of Guangzhou (2012J5100005).

Submitted

12/22/12

Accepted

01/02/13

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.