[Effectiveness of an influenza vaccine in a working population in Colombia]

To determine if a vaccine against influenza significantly decreases episodes of acute upper respiratory infection (AURI) and work absenteeism caused by AURI, in healthy adult employees of a banking entity in the city of Medellín, Colombia.

This was a double-blind randomized placebo-controlled study with 493 volunteers. The volunteers were randomly assigned to two groups, an experimental group and a control group, with 247 and 246 employees, respectively. The experimental group participants received a dose of 0.5 mL of an influenza vaccine containing surface antigens of the strains recommended by the World Health Organization for the 1996-1997 period, with subtypes A/Wuhan/359/95 (H3N2), A/Texas/36/91 (H1N1), and B/Beijing/184/93. An illness was considered an episode of AURI when a participant reported having a sore throat, fever, and a cough lasting more than 24 hours. Evaluations were made every 2 weeks over a 6-month period; the severity of the episodes was assessed in terms of lost workdays due to AURI (defined according to the ninth revision of the International Classification of Diseases, or ICD-9), through monthly evaluations of incapacitating work absences certified by the Colombian Social Security system, over the period of a year.

Side effects associated with the vaccine were erythema (relative risk (RR) = 8.0; P = 0.02) and local edema (RR = 4.5; P = 0.03). The proportion of the annual cumulative incidence of episodes of AURI was 78.5% for the vaccinated persons and 91.5% for those in the placebo group, with a reduction of 14%, with values between 7% and 20% (RR = 0.86; 95% confidence interval (CI) = 0.80-0.93). The annual cumulative incidence of incapacitating AURI was 15.8% in those vaccinated, with a reduction of 31% in comparison to the placebo group (22.8%), with values between 0% and 52% (RR = 0.69; 95% CI = 0.48-1.0). These levels of protection, both against more mildly symptomatic forms of AURI and those causing lost workdays, increased significantly (between 62% and 89%) in the months of May and October, when outbreaks caused by the influenza virus were confirmed in Colombia.

The results demonstrated that the influenza vaccination strategy decreased the AURI episodes as well as the number of work absences due to AURI in the healthy adult employees of a banking entity in the city of Medellín, Colombia. The impact that influenza has on this population group is small, and the effect of this vaccination measure is greater then the influenza virus is in circulation.