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A newly published seroprevalence study out of Norway that looks for cross reactive antibodies (suggestive of potential immunity) against the H3N2v swine flu strain has produced some surprising results.
The H3N2v (variant) strain began showing up in the United States in the middle of 2011, and since that time has been isolated in 13 people across 5 states.
Some, but not all, of the victims had exposure to swine, leading investigators to suspect limited person-to-person transmission might be occurring.
It is too soon to know if this strain has `legs’, and will continue to circulate in humans and perhaps eventually pose a more serious public health threat.
For more background on this emergent flu strain you may wish to revisit (H3N2v: Three’s A Crowd).
Given the possibility that this new strain could gain a foothold in the human population, the CDC has been working on a seed vaccine strain, and investigators around the world have been testing to see what age groups might be most susceptible to this virus.
Early indications have been that those under the age of 12 are at greatest risk, as 12 of the 13 known cases have involved young children.
Antigenically similar H3 viruses circulated through the mid 1990s, which might have left behind some degree of immunity to those exposed. Those born after 1999 would not be expected to carry those antibodies.
In April of 2012 we saw a study (see MMWR: Antibodies Cross-Reactive to Influenza A (H3N2) Variant Virus) that found that children under the age of 10 had few or no cross-reactive antibodies to the H3N2v strain, suggesting they would be the most vulnerable age group to the virus.
Based on the small sampling of tests conducted, approximately 1/3rd of those aged 10–49 years displayed cross-reactive antibodies that might provide some protection against infection from the H3N2v virus.
Seroprevalence testing on those aged 50 to 65 was not included in this particular study. Another study, published in January, found a similar lack of cross-reactive antibodies among children.
All of which serves as prelude to today’s study that appears in this week’s Eurosurveillance Journal.
Eurosurveillance, Volume 17, Issue 19, 10 May 2012
Rapid communications
K Waalen , A Kilander, S G Dudman, R Ramos-Ocao, O Hungnes
Antibody cross-reactivity to the influenza A(H3N2) variant virus recently reported in the United States, was investigated in Norwegian sera. Seroprevalence was 40% overall, and 71% in people born between 1977 and 1993.
The most susceptible age groups were children and people aged around 50 years. The high immunity in young adults is likely to be due to strong priming infection with similar viruses in the 1990s. More research is needed to explain the poor immunity in 45–54 year-olds.
While their data is in pretty good agreement with the two other studies mentioned above, quite unexpectedly they found that levels of seroprotective antibodies to the H3N2v virus were substantially lower among those aged 45 to 54 years than in other adult age brackets.
The authors have no straightforward explanation for this `gap’ in immunity among those born between the late 1950s and early 1960s, and call for more research into this surprising finding.
As for the pandemic potential of this virus, they offer this hopeful assessment in their conclusion:
The considerable prevalence of cross-reactive antibodies suggests that there may be a limit to the epidemic potential of these viruses in their current form.
Of course, the standard caveats apply.
Cross reactive antibodies are suggestive of possible immunity, but don’t always correlate to actual protection against infection or illness. And the H3N2v virus – like all flu viruses – is constantly changing and evolving.
For more background on variant flu viruses, you may wish to visit the following CDC webpage:
Key Facts about Human Infections with Variant Viruses (Swine Origin Influenza Viruses in Humans)
Questions & Answers
On this Page
- What is swine influenza?
- What is a variant influenza virus?
- Can humans be infected with swine influenza viruses?
- How common is it for humans to be infected with influenza viruses that normally spread in swine?
- Why are human infections with variant viruses of concern?
- What symptoms do people have when they are infected with variant viruses?
- Can people catch swine flu/variant flu from eating pork?
- How are variant influenza viruses spread?
- How can human infections with variant influenza viruses be diagnosed?
- What medications are available to treat variant flu infections in humans?
- What other examples of outbreaks with influenza viruses that normally circulate in swine are there?
- Was the “2009 H1N1” pandemic virus the same as previously circulating human H1N1 viruses?
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