CDC says influenza B strain doesn't match vaccine

Feb 8, 2008 (CIDRAP News) – The US Centers for Disease Control and Prevention (CDC) said today that most circulating influenza B viruses tested so far this season don't match this year's vaccine, signaling that two of the three vaccine components are off-target.

Joe Bresee, MD, chief of epidemiology and prevention in the CDC's Influenza Division, told reporters at a briefing today that 93% of the circulating influenza B viruses analyzed so far belong to the Yamagata lineage, which doesn't match the influenza B component of this year's vaccine. The B component is a B/Malaysia/2506/2004-like virus, which belongs to the Victoria lineage.

Because the Yamagata and Victoria lineages are distinct, the vaccine provides little cross-protection for influenza B, he said.

The news of the mismatch between the influenza B strains closely follows recent indications that a circulating influenza A/H3N2 subtype, A/Brisbane/10/2007-like, doesn't match the vaccine. However, the CDC said the Brisbane strain is a recent antigenic variant of the A/Wisconsin/67/2005-like virus, a component of this year's flu vaccine. Consequently, CDC director Julie Gerberding said this year's vaccine should provide some cross-protection against the Brisbane strain, according to a Feb 2 Associated Press report.

"Even when there is less than an ideal match, there is still some cross-protection," Bresee said. "This is especially true for high-risk populations. Getting a flu vaccine may reduce the risk of death or hospitalization from the disease."

According to the CDC's most recent weekly flu report, for the week ending Jan 26, US laboratories have characterized 197 flu viruses so far this season. Of 43 influenza B viruses, 40 (93%) belonged to the Yamagata lineage. Three (7%) belonged to the Victoria lineage.

Of the other viruses analyzed so far, 53 were A/H3N2 and 101 were A/H1N1. H1N1 is the third subtype included in the seasonal vaccine. So far, the H1N1 component in this year's vaccine seems well-matched to circulating H1N1 viruses, health agencies have reported.

Bresee said it's difficult for global health officials to predict what strains will be prevalent in the next influenza season, because the selection must allow enough time for vaccine production. World Health Organization (WHO) officials meet next week to advise what three components should be included in the northern hemisphere's 2008-09 seasonal flu vaccine.

Despite the difficulties, he said the seasonal flu vaccine has been well matched to the circulating strains in 16 of the last 19 seasons.

The US flu season started out slowly, but activity has increased sharply, which is typical, Bresee told reporters. All states are now reporting influenza activity, and 31 are reporting widespread outbreaks.

So far, one flu-related death in a child has been recorded, but it is likely that five additional deaths will be added to the CDC tally within the next few days, Bresee said. He said the CDC doesn't know yet if any of the children who died of flu this year were coinfected with methicillin-resistant Staphylococcus aureus (MRSA). The CDC recently urged clinicians to be alert for coinfections, particularly with MRSA. In the last flu season the number of pediatric flu deaths was moderately higher than the previous season; 15 of 73 deaths involved MRSA coinfections.

"This reminds us all that influenza is a common disease and can be a severe disease," he said.

The news about the influenza B mismatch is the latest in a string of unusual findings about circulating flu viruses. In late January, the European Centre for Disease Prevention and Control sounded a warning that some H1N1 influenza viruses in Europe were showing signs of oseltamivir (Tamiflu) resistance. Yesterday the WHO reported that resistance has been found in 14 countries, 10 of them in Europe.

According to a WHO table updated yesterday, 8.4% (15 of 179) of the H1N1 viruses analyzed in the United States so far had the resistance mutation, and 6.3% (8 of 128) Canadian isolates showed signs of oseltamivir resistance.

Bresee said the CDC is watching the oseltamivir-resistant viruses very closely and that investigators still don't know what caused the mutation.

See also:

CDC's weekly flu overview

Feb 7 CIDRAP News story "Europe reports further increase in Tamiflu resistance"

Feb 6 CIDRAP News story "US seeing flu strain not matched by vaccine"

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