Influenza Epidemiology and Vaccine Effectiveness among Patients with Influenza-Like Illness, Viral Watch Sentinel Sites, South Africa, 2005–2009

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.

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“…The seven studies in healthy adults comprised one randomized controlled trial, and three cohorts and case–control studies each. Seasonal influenza vaccination provided 50–59% protection in healthy adults 18, 44, 45, 46. The 12 studies in children comprised nine randomized controlled trials, one cohort and two case–control studies.…”

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“…32 The only other VE study conducted in Africa, a 5-year study in a mostly adult population in temperate South Africa, found that VE ranged from −14.2% to 67.4% annually. 33 In our study, influenza vaccine was moderately effective in preventing laboratory-confirmed influenza illness, despite the generally good match between circulating strains and vaccine strains. Similar findings have been reported previously and may be related to differences in strains that reflect mutations that are not routinely detected.…”

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“…Due to technical difficulties, the vaccine was only available from late April onwards which may have compounded the low influenza vaccine coverage of 3%, although overall vaccine coverage in previous years in the same population has ranged from 1.8% in 2012 to 5.1% in 2006 2, 3, 4. In the early part of the season, four patients had to be excluded from the analysis due to having received influenza vaccine <14 days prior to onset of symptoms.…”

“…Sites are mainly general practitioners in the private healthcare setting, who submit the majority of specimens during the influenza season. Since 2005, it has also been used to estimate influenza vaccine effectiveness (VE) 1, 2, 3, 4. Recommendations for the use of influenza vaccine are published annually in South Africa 5.…”