CDC study assessed influenza vaccine efficacy in 2007-2008 season in specific population

Influenza was detected in 191 patients; 75% of cases were type A.

Issue: June 2008

Overall influenza vaccine efficacy for the 2007-2008 season was 44% despite a mismatch of primary circulating strains, according to a CDC report.

In a case-control study, CDC investigators researched the effectiveness of trivalent inactivated influenza vaccine for prevention of medically attended, laboratory-confirmed influenza during the 2007-2008 influenza season. The researchers published their findings in the Morbidity and Mortality Weekly Report.

Eligible participants (n=616) included those living in areas surrounding Marshfield, Wis. Nearly all participants receive inpatient or outpatient care from Marshfield Clinic health care providers.

Study enrollment was based on laboratory evidence of influenza circulation from both Marshfield Clinic laboratories and the Wisconsin State Laboratory of Hygiene.

Nasal or nasopharyngeal swabs were obtained with patients’ consent and were tested for influenza A or B infection by reverse transcription–polymerase chain reaction (RT-PCR).

Viral culture

The researchers performed viral culture on all samples that were RT-PCR positive to provide virus isolates for antigenic characterization.

Influenza vaccination status was determined through the Regional Early Childhood Immunization Network, a network used by all public and private immunization providers for vaccines administered to adults and children.

According to the researchers, TIV (Fluzone, Sanofi Pasteur) was the only influenza vaccine used by Marshfield Clinic during the 2007-2008 influenza season.

Medically attended influenza was defined as an acute illness in a patient with feverishness, chills or cough and documentation of influenza infection by RT-PCR. Controls were defined as patients with the same symptoms who had a negative RT-PCR test for influenza.

Patients were then categorized as vaccinated if they had received influenza vaccine 14 days or more prior to enrollment. In addition, children aged younger than 9 years were categorized as vaccinated if they had received two doses of influenza vaccine.

The enrolled patients were categorized into two groups:

People for whom influenza vaccine was recommended by the Advisory Committee on Immunization Practices for the 2007-2008 season based on age or an existing chronic medical condition that increased their risk for influenza-related complications.
Healthy children and adults aged 5 to 49 years.

Influenza cases

Influenza was detected in 191 patients; 75% of cases were type A. During this time, most circulating influenza A (H3N2) and B viruses were suboptimally matched to the vaccine strains.

Despite the suboptimal match between two of three vaccine strains and circulating influenza strains, overall vaccine effectiveness in the study population between Jan. 21 and Feb. 8, 2008 was 44%. This number was highest among patients in the healthy group, aged 5 to 49 years (54%).

Effectiveness for prevention of medically attended influenza A infections was 58%; no vaccine effectiveness was observed for prevention of medically attended influenza B infections.

Distribution by sex was similar for patients who tested positive and for patients who tested negative for influenza. However, the median age was higher for patients who tested positive (21 years) than those who tested negative (10 years).

Nineteen percent of patients who tested positive and 39% of those who tested negative had been vaccinated against influenza.

In an accompanying editorial, researchers from the Marshfield Clinic Research Foundation wrote that when vaccine effectiveness is being assessed, laboratory data on antigenic characterization of circulating influenza viruses compared with vaccine strains should be interpreted along with data on the clinical effectiveness of vaccination in preventing illnesses caused by laboratory-confirmed influenza.

“Although two of three vaccine strains were not optimally matched with circulating viruses this season, an interim vaccine effectiveness estimate suggests that vaccination provided substantial protection against medically attended acute respiratory illness in this study population,” the researchers wrote.

In addition, intraseason estimates of vaccine effectiveness might be useful to public health authorities and medical practitioners in their communication about the benefits of vaccination, especially late in the influenza season, according to the researchers.

For more information:

CDC. Interim within-season estimate of the effectiveness of trivalent inactivated influenza vaccine — Marshfield, Wisconsin, 2007-08 influenza season. MMWR. 2008;57:393-397.