A case-control study of influenza vaccine effectiveness among Malaysian pilgrims attending the Haj in Saudi Arabia.

OBJECTIVES: To determine influenza vaccine effectiveness against clinically defined influenza-like illness among Malaysian pilgrims attending the Haj in Saudi Arabia. METHODS: During February and March 2000, the authors conducted an unmatched case-control study. Case patients were identified at one of five hotel clinics, while controls were residents of these hotels who had not attended a clinic. Results: Among 820 case patients--84% of whom had received antibiotics--and 600 controls, the adjusted vaccine effectiveness against clinic visits for influenza-like illness was 77% (95% confidence interval: 69, 83), and that against receipt of antibiotics was 66% (95% confidence interval, 54, 75). The vaccine did not prevent clinic visits for non-influenza-like upper respiratory tract illness (adjusted vaccine effectiveness, 20%; 95% confidence interval: -24, 49). CONCLUSIONS: Influenza vaccine was effective in preventing clinic visits for influenza-like illness and antibiotic use. Pilgrims traveling to the Haj in Saudi Arabia should consider influenza vaccination use.