Influenza epidemiology and vaccine effectiveness among patients with influenza-like illness, viral watch sentinel sites, South Africa, 2005-2009.

Amelia Buys,Jocelyn Moyes,Bernice N. Harris,Terry Besselaar,Barry D. Schoub,Johanna M. McAnerney,Cheryl Cohen,Dhamari Naidoo,Genevie M. Ntshoe,Lucille Blumberg,Stefano Tempia,Marietjie Venter

doi:10.1371/journal.pone.0094681

Abstract

BackgroundThere is limited data on the epidemiology of influenza and few published estimates of influenza vaccine effectiveness (VE) from Africa. In April 2009, a new influenza virus strain infecting humans was identified and rapidly spread globally. We compared the characteristics of patients ill with influenza A(H1N1)pdm09 virus to those ill with seasonal influenza and estimated influenza vaccine effectiveness during five influenza seasons (2005–2009) in South Africa.MethodsEpidemiological data and throat and/or nasal swabs were collected from patients with influenza-like illness (ILI) at sentinel sites. Samples were tested for seasonal influenza viruses using culture, haemagglutination inhibition tests and/or polymerase chain reaction (PCR) and for influenza A(H1N1)pdm09 by real-time PCR. For the vaccine effectiveness (VE) analysis we considered patients testing positive for influenza A and/or B as cases and those testing negative for influenza as controls. Age-adjusted VE was calculated as 1-odds ratio for influenza in vaccinated and non-vaccinated individuals.ResultsFrom 2005 through 2009 we identified 3,717 influenza case-patients. The median age was significantly lower among patients infected with influenza A(H1N1)pdm09 virus than those with seasonal influenza, 17 and 27 years respectively (p<0.001). The vaccine coverage during the influenza season ranged from 3.4% in 2009 to 5.1% in 2006 and was higher in the ≥50 years (range 6.9% in 2008 to 13.2% in 2006) than in the <50 years age group (range 2.2% in 2007 to 3.7% in 2006). The age-adjusted VE estimates for seasonal influenza were 48.6% (4.9%, 73.2%); −14.2% (−9.7%, 34.8%); 12.0% (−70.4%, 55.4%); 67.4% (12.4%, 90.3%) and 29.6% (−21.5%, 60.1%) from 2005 to 2009 respectively. For the A(H1N1)pdm09 season, the efficacy of seasonal vaccine was −6.4% (−93.5%, 43.3%).ConclusionInfluenza vaccine demonstrated a significant protective effect in two of the five years evaluated. Low vaccine coverage may have reduced power to estimate vaccine effectiveness.

Highlights

Influenza is an acute viral infection characterized by rapid spread, regular winter epidemics in temperate countries and yearround circulation in the tropical regions [1,2]
In South Africa, influenza vaccination is provided at no charge at public health facilities for people who are at risk of severe disease and is available at a fee in the private sector [13]
Ethics Statement The National Institute for Communicable Diseases (NICD) has ethics clearance for essential communicable disease surveillance activities of public health importance in South Africa granted by the Human Medical Research Ethics Committee of the University of the Witwatersrand, Johannesburg

Summary

Introduction

Influenza is an acute viral infection characterized by rapid spread, regular winter epidemics in temperate countries and yearround circulation in the tropical regions [1,2]. It is highly infectious and associated with significant morbidity and mortality in high-risk individuals worldwide [3]. The WHO recommends the content of the influenza vaccine for the forthcoming influenza season [9] These recommendations are based on data submitted by its global influenza surveillance network [9]. We compared the characteristics of patients ill with influenza A(H1N1)pdm virus to those ill with seasonal influenza and estimated influenza vaccine effectiveness during five influenza seasons (2005–2009) in South Africa

Methods

Results

Conclusion