Study: Variant H3N2 risk greatest in kids under 10

Apr 12, 2012 (CIDRAP News) – The US Centers for Disease Control and Prevention (CDC) today released more details about a new H3N2 variant (H3N2v) case in Utah, along with results of a serum study that suggests children younger than 10 are likely to be at greatest risk for contracting the novel virus.

Yesterday, officials with Utah's Weber-Morgan Health Department announced the confirmation of a new H3N2v case, and today the CDC said the patient is a child who had exposure to swine, pushing the nation's number of similar cases since 2011 to 13 and the number affected states to six.

The CDC said more investigations are under way to identify the source of the infection and to determine if there are more human cases. So far 12 of the 13 H3N2v infections involved children, and about half of the cases had a history of swine exposure. However, because others did not, the CDC has said the cases probably reflect limited human-to-human exposure.

The latest case was detected during routine surveillance. The patient had sought medical care for a fever, was treated with oseltamivir (Tamiflu), and recovered at home. The CDC said none of the viruses in the 13 cases had genetic markers of resistance to neuraminidase inhibitors, the most commonly used class of antiviral drugs against flu.

The triple-reassortant swine-origin H3N2v strain includes the M (matrix) gene from the pandemic 2009 H1N1 (pH1N1) virus.

Earlier this year, a research group from the CDC and Harvard tested four H3N2v viruses that have been linked to human cases in the United States since 2009 to gauge how well they spread in ferrets, which is a measure of possible pandemic potential.

The viruses were similar to seasonal H3N2 viruses: They spread by respiratory droplets, causing disease that wasn't lethal. The group also found that the virus replicated well in human bronchial cells.

The group concluded that the virus had pandemic potential, but more information was needed to determine what cross-reactive immunity the general population has to the new viruses.

Risk in children
In today's MMWR report, CDC researchers fleshed out answers to some of the questions about cross-protection against the H3N2v strain responsible for the 12 cases reported last year. They used serum samples from a 2010-11 trivalent inactivated vaccine (TIV) study and samples from a 2007-08 National Health and Nutrition Examination Survey (NHNES) study. They performed hemagglutination inhibition (HI) and microneutralization tests using a 2010 H3N2v sample from Minnesota and two seasonal H3N2 samples.

They found no evidence of antibodies to H3N2v in 20 children ages 6 to 35 months, either before or after 2010-11 seasonal flu vaccination. In younger and older adults, seasonal flu vaccination boosted antibodies to H3N2v, but the level wasn't as high as for the seasonal H3N2 vaccine component.

The TIV study samples didn't include children ages 4 to 17 years, so the researchers used the NHANES samples to study cross-reactive antibodies to H3N2v in two groups: younger children and older ones. They detected higher antibody levels in children age 10 and older.

Overall, they found that about one third of people ages 10 to 49 had cross-reactive antibodies that might provide some protection against H3N2v and that those younger than 10 were likely to be most susceptible to H3N2v infection.

The group also concluded that the 2010-11 seasonal flu vaccine, which contains the same strains as the 2011-12 vaccine, can be expected to provide no cross-protection in young children and limited cross-protection in adults.

They said the study was based on a small number of subjects in each age-group and that another study using a larger number of samples is under way.

If sustained human-to-human H3N2v infection occurs, an H3N2v vaccine would provide optimal protection, the group wrote.

Earlier this year, a Canadian research team that explored cross-protection against swine-origin H3N2 viruses reported that young adults likely have some antibodies against the viruses, but the levels waned in middle-aged people. They said this finding raises concerns for seniors, who are more vulnerable to severe H3N2 infections.

The CDC said in its statement today it has produced a candidate H3N2v vaccine virus, and that a vaccine—in case it is needed—is being developed and will likely be ready for clinical trials in the coming months.

See also:

Apr 12 CDC statement

Apr 12 MMWR report

Feb 21 CIDRAP News Scan "Study: Swine-origin H3N2 viruses transmit well in ferrets"

Jan 27 CIDRAP News story "Study suggests older adults may have little defense against swine H3N2 viruses"

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