Feb 22, 2008 (CIDRAP News) – With many of the influenza viruses now infecting people not matched well by this year's flu vaccine, a US Food and Drug Administration (FDA) advisory committee recommended yesterday that producers use three new flu strains in next season's vaccine, replacing all three components of this year's version.
FDA officials reported that the agency's Vaccines and Related Biological Products Advisory Committee voted unanimously to follow the lead of the World Health Organization (WHO), which recommended last week that all three strains of this year's vaccine for the northern hemisphere be replaced for next season.
"FDA will now take the recommendation and make a final decision quickly," FDA spokeswoman Peper Long told CIDRAP News.
Changing one or two strains in the vaccine is not unusual, but Dr. Nancy Cox, director of the Centers for Disease Control and Prevention (CDC) influenza division, said changing all three is unprecedented.
"This sounds like a major change, because this really hasn't occurred before," Cox said at a news briefing this afternoon. She said the change may not be as difficult for vaccine manufacturers as it might seem, because they have already had some experience with two of the strains, but how well the strains will grow is unpredictable.
The flu vaccine is reformulated each year to try to keep pace with the fast-evolving viruses. The WHO and FDA recommend the strains for the vaccine in February to give manufacturers time to grow the viruses in chicken eggs and process them into vaccine doses. The choice of strains is an annual gamble, since the predominant viruses may differ from those in the vaccine, but most years the vaccine is reasonably on target.
The three flu virus varieties in the vaccine include a type A/H1N1, type A/H3N2, and a type B. Last week the Centers for Disease Control and Prevention (CDC) said that most of the H3N2 and B viruses it has analyzed this winter differed from the corresponding strains in the vaccine, but most of the H1N1 viruses tested so far continue to match up well with the vaccine.
However, the WHO recommended replacing all three strains for next year's vaccine, saying that the majority of recent H1N1 isolates differ from the H1N1 strain in the vaccine. At today's briefing, Cox explained that in Europe, where H1N1 viruses are predominant this winter, they are "not so well matched with the vaccine."
The three strains recommended by the WHO and now by the FDA committee are:
- For the H1N1 component, a strain similar to A/Brisbane/59/2007, replacing A/Solomon Islands/3/2006
- For H3N2, an A/Brisbane/10/2007-like strain, replacing A/Wisconsin/67/2005
- For the B component, a B/Florida/4/2006-like strain, replacing B/Malaysia 2506/2004
The Brisbane strains of H1N1 and H3N2 will be used in this year's vaccine for the southern hemisphere, where the flu season runs from May through October, according to the WHO.
Cox said that because the Brisbane H1N1 and H3N2 strains were picked for this year's southern hemisphere vaccine, vaccine manufacturers already have had a chance to obtain and work with them, which should help them produce the new northern hemisphere vaccine.
Experts were concerned about the Brisbane H3N2 strain a year ago, but it was not possible to add it to the vaccine at that point, Cox said. "Unfortunately, we did not have a viable egg isolate that could be used by the manufacturers, so it was necessary to continue to use the Wisconsin/67 strain in the vaccine," she said.
An Associated Press (AP) report published yesterday said producers of the southern hemisphere vaccine have found that the Brisbane H3N2 strain doesn't grow very quickly in the laboratory. Cox said scientists are working on possible solutions to the problem of growing the strain, such as using a very similar virus found in Uruguay, the story said.
At today's briefing, Cox said, "One of the great limiting factors [in vaccine production] is how well the virus strains grow, and of course when you change strains, the growth properties are inherently unpredictable."
Of the flu viruses analyzed by the CDC so far this season, about 83% have been type A and about 16% have been type B, which tends to cause milder illness than type A, Cox said.
Of the type A viruses, 63% have been H3N2 and 37% have been H1N1, she reported. H1N1 viruses—which in the United States have been well matched by the vaccine—predominated until early January, but since then the situation has reversed. Seasons in which H3N2 viruses are predominant tend to be more severe than those in which H1N1 strains predominate, experts say.
Feb 14 CIDRAP News story "WHO advises total makeover for 2008-09 flu vaccine"
CDC's weekly flu surveillance report