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. 2014 Apr 15;9(4):e94681.
doi: 10.1371/journal.pone.0094681. eCollection 2014.

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

Genevie M Ntshoe  1 , Johanna M McAnerney  2 , Stefano Tempia  3 , Lucille Blumberg  1 , Jocelyn Moyes  2 , Amelia Buys  2 , Dhamari Naidoo  4 , Marietjie Venter  2 , Terry Besselaar  4 , Barry D Schoub  5 , Bernice N Harris  6 , Cheryl Cohen  2
Affiliations

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

Genevie M Ntshoe et al. PLoS One. .

Abstract

Background: There 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.

Methods: Epidemiological 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.

Results: From 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%).

Conclusion: Influenza 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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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Number of specimens testing positive for influenza and detection rates by year, Viral Watch, South Africa, 2005–2009.
Figure 2
Figure 2. Influenza detections and detection rates for seasonal influenza (N = 2490) and influenza A(H1N1)pdm09 (N = 496) by age group, Viral Watch, South Africa: 2005–2009.
Figure 3
Figure 3. Percentage of influenza positives by age group and virus type, Viral Watch, South Africa; 2005–2009.
Figure 4
Figure 4. Number of influenza positives by virus type, subtype and detection rate by week and year, Viral Watch, South Africa, 2005–2009.
Figure 5
Figure 5. The vaccine coverage and detection rate during influenza by year and age group, Viral Watch, South Africa, 2005–2009.
See this image and copyright information in PMC

References

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    1. Statistics South Africa. Mortality and causes of death in South Africa, 2005: Findings from death notification. Available: http://www.statssa.gov.za/publications/P03093/P030932005.pdf. Accessed 2011 Dec 19.
    1. Statistics South Africa. Mortality and causes of death in South Africa, 2006: Findings from death notification. Available: http://www.statssa.gov.za/publications/P03093/P030932006.pdf. Accessed 2011 Dec 19.

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