January 4, 2012 — Taking no chances, the US Centers for Disease Control and Prevention (CDC) is laying the groundwork for a possible vaccine against a novel strain of a swine influenza virus that has surfaced in 5 states and sickened 12 individuals, mostly children, since July 2011, an agency official told Medscape Medical News Tuesday.
The strain, designated A(H3N2)v, is a variant of the A(H3N2) virus that circulates among humans on a seasonal basis. What makes it a variant is a gene from the pandemic 2009 influenza A(H1N1) virus that codes for matrix proteins found in the viral shell.
In Indiana, Pennsylvania, and Maine, the virus appeared to have spread from pigs to humans, according to the CDC. In Iowa and West Virginia, however, the evidence suggests limited human-to-human transmission. In general, the novel virus is no more severe than ordinary seasonal influenza, and all the people infected with it have recovered.
The CDC is encouraging public health agencies and clinicians to collect more nasopharyngeal swabs from patients presenting influenza-like illness for testing to determine whether the novel virus may be spreading on a sustained basis.
CDC spokesperson Thomas Skinner told Medscape Medical News that the agency already has prepared a "seed virus" obtained from A(H3N2)v specimens that drug manufacturers can use to develop a vaccine if needed. Preparing a seed virus involves genetically manipulating specimens to incorporate preferred features.
Because the new virus is different enough from the seasonal viruses now in circulation, the seasonal influenza vaccine for 2011-2012 "is not expected to provide significant protection" against the newcomer, according to a Morbidity and Mortality Weekly Report that the CDC published on December 23. The trivalent seasonal vaccine is formulated to protect against the 2009 pandemic virus, the regular A(H3N2) virus, and an influenza B strain.
CDC Could Be Preparing for 2012-2013 Influenza Season
So far, the evidence does not suggest that the A(H3N2)v strain is spreading freely through communities, which would create the need for a vaccine. The bug could fizzle out, according to Christine Layton, PhD, MPH, a public health researcher and influenza pandemic expert at RTI International in Research Triangle Park, North Carolina.
"There have been past instances where a novel virus comes up and then goes back down," Dr. Layton told Medscape Medical News. "There may be something about the virus that [prevents] sustained human-to-human transmission."
If the A(H3N2)v virus ever takes off, she said, drug manufacturers would have a tough time turning out a corresponding monovalent vaccine "at the drop of a hat." Influenza vaccines are still mass-produced, for the most part, by being grown in chicken eggs. Together with testing and licensing a vaccine, this process easily can last 3 to 4 months.
A more likely scenario, said Dr. Layton, would be incorporating an A(H3N2)v strain in the trivalent seasonal vaccine for 2012-2013, assuming the novel virus becomes that much of a threat. Each February or so, an advisory panel of the US Food and Drug Administration (FDA) recommends 3 strains of influenza that should make up the seasonal vaccine for the coming fall on the basis of what it expects to see circulating then. The CDC then develops the seed viruses for these 3 strains, which the FDA distributes to vaccine makers.