The efficacy of influenza vaccine

Abstract

Inactivated influenza virus vaccine offers 80–90% protection against influenza-like illnesses in young healthy individuals when vaccine and epidemic strains are closely related. In the elderly however, the protection against influenza-like illnesses is only 20–30%. Of greater relevance in elderly people are the reductions in morbidity and mortality associated with vaccine. Meta-analysis of uncontrolled studies revealed 23% reduction in morbidity and 67% reduction in mortality. Recent case-controlled studies have shown vaccination of the elderly to offer around 40% protection against serious influenza-related morbidity including death, and a placebo-controlled study has shown that influenza vaccination may halve the incidence of serological and clinical influenza in the elderly. The immune response to vaccination may be affected by several factors including age, health status of vacinees, and medication. In patients with chronic renal failure the percentage developing 'protective' or significant rise in antibody titres is reduced. In patients infected with the human immunodeficiency virus (HIV), immune responses following vaccination are sub-optimal compared with normal healthy subjects. Only 37–50% of patients receiving cancer chemotherapy seroconvert when vaccine is administered simultaneously with cancer chemotherapy. In contrast, 90% seroconvert when no cytotoxic therapy is administered. Overall, these observations indicate that influenza vaccine is of great benefit, and appears to be cost-effective. However, patients with renal failure, AIDS, ARC and neoplasm undergoing chemotherapy may benefit from prophylaxis with an antiviral (e.g. amantadine) during known outbreaks of influenza A.