Children and middle-aged adults have little or no immunity to swine-origin variant H3N2 influenza virus, according to a new study.
Half of adolescents and young adults, though, have some degree of immunity as measured by antibody levels, according to CIDRAP News
The report, published in the Journal of Infectious Diseases, also found that seasonal flu vaccines used for the last two years did not improve a patient's immunity to H3N2v.
Danuta M. Sowronski of the British Columbia Centre for Disease Control, led the study and was the first author, CIDRAP News
The study concluded that a specific flu vaccine would be needed in the event of an H3N2v epidemic spread. The virus emerged last summer in the United States, causing a dozen cases. The H3N2v illness has spread rapidly in the last few weeks, though, with 150 cases reported, mostly in children who came into contact with pigs at county fairs.
The U.S. Centers for Disease Control and Prevention agrees with the study's findings and is preparing a candidate vaccine for the novel virus with clinical trials expected to begin this fall.
The researchers conducting the study used serum samples from people of all ages in order to look for cross-reactive antibodies to H3N2v. They also took sera from children, adults and the elderly who received the seasonal flu vaccine years prior, testing them against those that had not.
Hemagglutination inhibition and microneutralization were used to assess antibodies to H3N2, according to CIDRAP News
The study found that 25 percent of the 1,116 participants had an HI antibody titer of at least 40 to the H3N2v strain, which they said offers some degree of protection from infection. Less than 20 percent of children 14 or younger or over the age of 40 had this seroprotective level of antibody. No children younger than five years of age had the antibody.
Subjects in their 20s had antibody protection at a rate of approximately 60 percent, which dropped to approximately one-third for adults in their 30s. Less than 10 percent of adults between 40 and 69 had protection and approximately 20 percent of those aged 69 or older had seroprotection.
Researchers also looked at cross-reactivity to an H3N2 strain that circulated in the 1990s called A/Sydney. The strain is the most closely related human ancestor strain to H3N2v. They found that 61 percent of subjects had a seroprotective level of antibodies to A/Sydney, with 90 percent of teens and adults in their 20s having seroprotection, according to CIDRAP News
The study found that those who had received previous seasonal flu vaccines did not have enhanced protection against H3N2v.
"Our serologic findings suggest substantial protection against A(H3N2)v in late adolescence and young adulthood, but broad susceptibility in children and older adults," the authors said, CIDRAP News