Emergence of 'new gene constellation' for influenza A (H3N2)
CDC reports 12 cases of human infection with influenza A (H3N2) variant viruses involving genes from avian, swine, and human viruses that had acquired the M gene from the influenza A (H1N1)pdm09 virus.
CDC reported the emergence of a "new gene constellation" for influenza A (H3N2) and its temporal association with 12 human infections of A (H3N2)v, leading the agency to state that a better understanding is needed of the ability of these viruses to cause human infection and the degree to which current seasonal influenza vaccines might generate cross-reactive antibodies to them.
In the April 12 Morbidity and Mortality Weekly Report (MMWR), CDC reported that since August 2011, it had gained knowledge of 12 cases of human infection with influenza A (H3N2) variant viruses involving genes from avian, swine, and human viruses (i.e., A [H3N2]v) that had acquired the M gene from the influenza A (H1N1)pdm09 virus. Of the cases, 11 were among children younger than 10 years of age and, for 6, the affected patients were not reported to have had recent exposure to swine, indicating that human transmission had occurred.
CDC analyzed presence of serum cross-reactive antibody in various age groups that were or were not vaccinated with the 2010–11 seasonal trivalent influenza vaccine (TIV). According to patient age, the following results were reported by the agency:
<10 years: Little or no cross-reactive antibody to A (H3N2)v
<3 years: No effect on cross-reactive antibody levels
≥10 years: Cross-reactive antibody detected in 20% to 30%
Adults: Modest increase in level of cross-reactive A (H3N2)v antibodies
Hemagglutination inhibition (HI) and microneutralization (MN) assays were performed. HI assays detect antibodies that slow the binding of virus to receptors on red blood cells, while MN assays quantify antibodies that neutralize and prevent infection, explained CDC. Serum HI titers of 40 or greater are related to lowered risk for influenza infection among adults. The 50% protective titer for the MN assay is not known; however, research regarding antibody response in patients infected with influenza A (H1N1)pdm09 virus showed that the MN titer was generally twofold higher than the HI titer when the HI titer was 160 or lower. Following this rationale, CDC presented titer achievements of 80 or more for the MN assay.
The agency stated that no evidence of antibodies to A (H3N2)v was found among 20 children aged 6 to 35 months, either before or after receiving the 2010–11 TIV. However, 40% or 45% of children demonstrated seroconversion (i.e., increase in antibody titer of a fourfold or more) to the seasonal A (H3N2) virus contained in the vaccine according to HI and MN assays.
This was in contrast to the results for 30 patients aged 18 to 49 years, in whom somewhat higher levels of prevaccination antibody to A (H3N2)v were observed. Among this age group, 33% attained HI titers of 40 or more and 43% MN titers of 80 or more. Following immunization with TIV, the percentage of patients aged 18 to 49 years with cross-reactive HI and MN antibody to A (H3N2)v increased to 50% and 63%, respectively. Consistent with CDC expectations, following vaccination, 80% of adults in this age group achieved HI titers of 40 or higher and 70% achieved MN titers of 80 or higher to the seasonal A (H3N2) vaccine component.
Prevaccination antibody to A (H3N2)v also was seen among older adults (aged ≥65 years), with 17% of 30 patients achieving HI titers of 40 or more and 30% achieving MN titers of 80 ore more. According to either assay, this increased to 40% postvaccination. As a comparison, for the seasonal A (H3N2) vaccine component, CDC reported that 67% and 90% of patients 65 years or older exhibited postvaccination HI titers of 40 or greater or MN titers of 80 or greater, respectively.
The results for these two adult populations, said CDC, indicated that "receipt of TIV boosted the levels of antibodies to A (H3N2)v, but to a lesser extent than the antibody response to the A (H3N2) vaccine component."
The agency further reported that a vaccine virus specific for A (H3N2)v has been developed and, if needed, could be used to produce an H3N2v vaccine. CDC advised that receiving the seasonal influenza vaccine continues to be the best option for protecting against circulating human influenza viruses for all age groups and that the vaccine could provide some protection against A (H3N2)v infection in the adult population.
Serum samples from a 2010–11 TIV study and the 2007–08 National Health and Nutrition Examination Survey (NHANES) were used for analysis. The TIV study included samples from patients aged 6–35 months, 18–49 years, and 65 years or older, and all serum samples were collected in fall 2010 before vaccination and at 3 to 4 weeks postvaccination. CDC noted that samples from children 4 to 17 years of age came from NHANES and were part of a larger set of samples received by the agency that were labeled with age and date of sample collection only.
Posted by Joe Sheffer (email@example.com
April 13, 2012http://www.pharmacist.com/AM/Template.c ... ntID=28288