Uptake and effectiveness of monovalent influenza A (H1N1) pandemic 2009 vaccine among healthcare personnel in Kenya, 2010

Introduction: During April-June 2010, the Kenya Ministry of Public Health and Sanitation distributed free monovalent influenza A(H1N1)pdm09 vaccines to health care personnel (HCP) and other vulnerable groups. We conducted a prospective, cohort study among HCP to characterize influenza A(H1N1)pdm09 vaccine uptake, and to assess influenza A(H1N1)pdm09 vaccine effectiveness. Methods: We enrolled HCP from 5 hospitals and followed them for 6 months. At enrollment, we asked HCP if they had received the influenza A(H1N1)pdm09 vaccine and their reasons for their decision. We administered weekly questionnaires to participants about respiratory symptoms suffered during the previous week. Participants who had acute respiratory illness were asked to contact our surveillance clinician, and nasopharyngeal and oropharyngeal specimens were collected and later tested for influenza by real-time reverse-transcriptase polymerase-chain-reaction. Vaccine effectiveness was estimated by comparing the incidence of acute respiratory illness, absenteeism from work due to respiratory illness and laboratory-confirmed influenza among vaccinated and unvaccinated HCP. Results: We enrolled 3803 HCP from the five hospitals; 64% received influenza vaccine. Vaccinated HCP were more likely to develop acute respiratory illness (ARI) and more likely to report missed days of work due to respiratory illness compared to non-vaccinated HCP (adjusted incidence rate ratio (aIRR) 1.50, 95% confidence intervals (CI): 1.33-1.70) and (aIRR 2.02, 95% CI: 1.41-2.88), respectively. Of 531 samples collected from vaccinated and non-vaccinated HCP, 30 were influenza A and 3 were influenza B. Two influenza A(H1N1)pdm09 subtypes were isolated; one from vaccinated and the other from non-vaccinated HCP. Discussion and conclusions: A majority of Kenyan HCP surveyed reported receiving the influenza A(H1N1)pdm09 vaccine. Because of low circulation of influenza A(H1N1)pdm09 virus during the study period, vaccine effectiveness could not be determined. The findings of increased ARI events and missed days of work among vaccinated HCP were likely confounded by vaccine-seeking behavioral factors.