2007 Flu vaccine: Pain, and gain?

The influenza vaccine contains three influenza virus strains – A (H1N1), A (H3N2), and B. The effectiveness of the vaccine depends on the match between the strains in the vaccine and those that are circulating in the community.

In years when vaccine and circulating strains were poorly matched, the vaccine can be ineffective (Bridges 2000). (Surprisingly, this study also concluded that even in a year when they were well matched, the cost outweighed benefit for people under 65.) In contrast, other studies have demonstrated some benefit even when the vaccine and circulating strains were not well matched. During the 2003-04 season, for example, vaccine effectiveness among people 50-64 years old was over 50% for protecting against contracting flu and 90% against hospitalization (Herrera 2006). The 2003-04 vaccine was similarly effective in children (Ritzwoller 2005). Children are responsible for the most transmission yet they are vaccinated at a lower rate than older adults.

For the current flu season, the Advisory Committee on Immunization Practices (ACIP) recommended vaccination with the strains A/Solomon Islands/3/2006 (H1N1)-like, A/Wisconsin/67/2005 (H3N2)-like, and B/Malaysia/2506/2004. The first detailed analysis of the current flu season was posted last week by the CDC. Between September 30 and December 1, 2007, World Health Organization and National Respiratory and Enteric Virus Surveillance System tested 24,897 respiratory specimens for influenza viruses; 559 (2.2%) were positive. Of these, 92% were influenza A viruses, and 8% were influenza B viruses. One hundred thirty-five of the influenza A viruses were subtyped; 83% of these were influenza A (H1) viruses, and 17% were influenza A (H3) viruses. CDC characterized 27 isolates to date (see table): 19 were influenza A (H1) isolates, 5 were influenza A (H3) isolates, and 3 were influenza B isolates. All 19 A/H1 viruses were Solomon Islands/3/2006-like. Two A/H3 isolates were Wisconsin/67/2005-like. Three A/H3 isolates were similar to Brisbane/10/2007, a strain recommended in the vaccines for the Southern Hemisphere. The three influenza B viruses characterized all belong to the Yamagata/16/88 lineage, whereas the Malaysia strain in the vaccine belongs to the Victoria lineage.

The fact that all H1 isolates are in the vaccine indicates that ACIP accurately predicted the rise of this strain. Although the efficacy of the vaccine is not established, its good match with the circulating strains suggests it will be beneficial. However, the H3 component matches only a minority of the circulating strains and the B component match none. Fortunately, these strains are less prevalent than the H1 strains.