Clinical Effectiveness of Influenza Vaccination in Persons Younger than 65 Years with High-Risk Medical Conditions. Hak E, Buskens E, van Essen GA, de Bakker DH, Grobbee DE, Tacken M, van Hout BA, and Verheij T. Archives of Internal Medicine 2005;165:274-280.
What is the effectiveness of influenza vaccination in reducing serious complications in persons less than 65 years old with high-risk medical conditions during an influenza epidemic?
In the Netherlands, researchers conducted a case-control study to estimate the effectiveness of influenza vaccination in reducing serious complications during the 1999-2000 influenza A epidemic among high-risk children and adolescents younger than 18 years, high-risk adults aged between 18 and 64 years, and elderly persons 65 years or older.
The population studied included 75,227 people. Of these individuals, 8% were high-risk children and adolescents (5,933), 33% were high-risk adults (24,928) and 59% were 65 years of age or older (44,366).
A case was defined as a hospitalization for influenza, other respiratory illness, heart attack, heart failure, stroke, death from any cause, and/or visits to the physician where recurrent asthma or chronic lung disease was treated with corticosteroids or a referral for heart failure was made. For each case identified, 4 control patients were selected from the same population that did not have one of these endpoints. Trivalent inactivated influenza vaccine (TIV) was assumed to have been administered if the physician submitted a bill for payment for vaccine delivery.
The researchers then assessed the TIV vaccination status of the cases and controls, comparing the vaccinated to the unvaccinated subjects in an attempt to estimate the effectiveness of TIV in preventing complications of influenza. They adjusted their analysis for age, sex, prior health care use, medication use, and other medical conditions present at the time. Comparisons were made by dividing the number of cases with complications who had not been vaccinated by the number of cases in the group that had been immunized.
Vaccinated subjects were older and likely were of lower socioeconomic status than the unimmunized patients.
This study estimated that TIV vaccination prevented 43% of physician visits for children and adolescents younger than 18 years of age with high-risk conditions.
Among high-risk adults between 18 and 64 years of age, vaccination prevented 78% of deaths, 87% of hospitalizations and 26% of GP visits. Among persons 65 years of age or older, TIV was estimated to have prevented 50% of deaths and 48% of hospitalizations.
Immunization with TIV reduced hospitalization and mortality rates for persons with high-risk medical conditions of any age.
Despite selection bias against their hypothesis—the individuals who were more likely to have influenza complications were also more likely to have been vaccinated—this study demonstrated that TIV immunization reduces the complications that high-risk individuals have from influenza. These findings occurred despite lower immunization rates than expected. They were unable to demonstrate a benefit for those with diabetes in different age groups, however.