BETHESDA, Md. - The Food and Drug Administration (FDA) Vaccines and Related Biological Products Advisory Committee recently made recommendations for the 1998-99 influenza vaccine.
In three unanimous votes, the committee recommended the strain representing the influenza B component, but deferred recommendation of the influenza A(H3N2) and influenza A(H1N1) components.
The committee recommended retaining the influenza B/Harbin/07/94-like strain. The committee also recommended an influenza A/Sydney/05/97-like strain for the H3N2 component if data still being collected do not suggest otherwise. The advisory committee will recommend the remaining influenza virus vaccine strains at a later date.
"The 1997-98 influenza season got off to a slow start in the United States and in Europe," explained Nancy J. Cox, PhD, chief of the Influenza Branch at the Centers for Disease Control and Prevention (CDC). "We will have quite a bit of additional data generated over the next few weeks."
Based on worldwide surveillance, influenza A(H1N1), influenza A(H3N2) and influenza B strains have again been circulating, and therefore, a trivalent influenza vaccine is needed, said Roland A. Levandowski, MD, supervisory medical officer, FDA division of viral products.
Influenza B isolates obtained from the Americas, Europe, Asia, Africa and Australia are similar to the B/Harbin/07/94 strain in the 1997-98 vaccine. Strains of the B/Victoria/02/87 lineage continue to appear in Asia, but have circulated there for the past several years without spread to other areas, Levandowski explained.
"Most of the strains worldwide are similar to the current vaccine strain; the vaccines being used are immunogenic," Levandowski said. "The Harbin strains circulating are well inhibited by the vaccine."
Surveillance during the influenza season identified a new influenza A(H3N2) strain not well inhibited by the current vaccine. Therefore, the committee agreed a strain antigenically similar to the newly isolated Sydney/05/97 strain should be used for the H3N2 component of the vaccine and replace the A/Nanchang/933/95 strain from last year. However, if new data emerges, the committee agreed to again review choices for the influenza A(H3N2) strain.
Cox stressed that influenza A/Sydney/05/97-like strains have been isolated worldwide and have resulted in outbreaks among populations with typically high vaccination rates, including nursing homes. This strain is the most prevalent H3N2 strain in the current U.S. epidemic.
Committee consultant Robert B. Couch, MD, mentioned that the 1997-98 influenza season has resulted in an unusually high number of calls and complaints by patients claiming the flu is worse than previous years. He also said the vaccine doesn't appear as effective as in previous years and attributed this to the newly isolated Sydney strain and the weakness of the Nanchang vaccine strain.
Couch is a professor of medicine, microbiology and immunology, Baylor College of Medicine, Houston, Texas.
The most recently isolated influenza A(H3N2) strains are poorly inhibited by the current vaccine strain of A/Nanchang/933/95 and a change would achieve a better antigenic match with circulating strains. Strains suitable for production are available now, he added.
Cox mentioned that a few new influenza A(H3N2) strains, intermediate between the Wuhan and the Sydney viruses, recently have been identified. The CDC is reviewing these strains from Asia to determine whether they represent an emerging group of viruses.
"The H1N1 component is a more complex issue," said committee chairwoman Patricia L. Ferrieri, MD, professor in the departments of laboratory medicine and pathology, and pediatrics, University of Minnesota Medical School, Minneapolis, Minn.
The most frequently isolated influenza A(H1N1) strains circulating this season are A/Bayern/07/95-like strains, which is currently represented in the vaccine. However, strains similar to the A/Beijing/262/95, known as the H1 deletion mutant, have now appeared outside Asia and are not well inhibited by the current vaccine. Strains have been isolated in west and south Africa, which suggests potential activity elsewhere.
"Data suggest that the current vaccine isn't as protective against the H1 deletion mutant," Levandowski said. More epidemiologic and serologic data will be available in the upcoming weeks, and will help clarify the strain selection issues.
Approximately 80 million doses of the trivalent influenza vaccine were manufactured for the 1997-1998 U.S. influenza season. Manufacturers stressed to the committee that influenza strains with good growth potential are important to make large amounts of influenza vaccine. Because every strain isn't suitable for vaccine manufacturing, reassortants are produced regularly using the influenza A/Puerto Rico/08/34 strain as a high-growth egg-adapted donor.
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