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http://www.promedmail.org/direct.php?id ... 11.1130181

Date: Thu 10 May 2012
Source: Eurosurveillance edition 2012; 17(19) Rapid Communication [summ., edited]
http://www.eurosurveillance.org/ViewArt ... leId=20170

Age-dependent prevalence of antibodies cross-reactive to the influenza A(H3N2) variant virus in sera collected in Norway in 2011
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[Authors: Waalen K, Kilander A, Dudman SG, Ramos-Ocao R, Hungnes O. Department of Virology, Norwegian Institute of Public Health, Oslo, Norway]

Summary
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Antibody cross-reactivity to the influenza A(H3N2) variant [A/H3N2v] virus recently reported in the United States, was investigated in Norwegian sera. Seroprevalence was 40 percent overall, and 71 percent 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.

Introduction
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From August 2011 to April 2012, 13 cases of human infection were identified in the United States (US) with a variant of influenza A(H3N2) virus that had been circulating in pigs in North America. The variant has been designated as A(H3N2)v by the World Health Organization (WHO) [1]. Almost all cases have been in children, some of them with no recognised exposure to pigs, and limited human-to-human transmission appears to have occurred [2-5]. These viruses have not been shown to circulate in European swine, and until now no human influenza A(H3N2)v cases have been reported in Europe.

The haemagglutinin of these H3N2v viruses is descended from H3N2 viruses that were circulating worldwide in humans in the mid-1990s [6], with A/Wuhan/359/1995(H3N2)-like viruses the most similar vaccine strain [7].

In order to assess the risk and possible impact of further spread of influenza A(H3N2)v viruses in the human population, we need to clarify whether prior exposure to earlier antigenic variants of human H3N2 viruses, either through infection or through vaccination, may have resulted in persisting immunity that could protect segments of the population today against the current H3N2v virus.

Although the genetic similarities to previously circulating viruses suggest that pre-existing immunity may exist, it is important to corroborate this with seroepidemiological evidence. This study presents a first analysis of antibodies reactive to the H3N2v virus in a panel of human sera collected in Norway in August 2011.

more at link ...

Results
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A considerable overall proportion, 40 percent, of the analysed sera contained antibody to the H3N2v virus with HI titres correlating with protection (HI titre 40 or higher) (data tabulated in original text).

A distinctive age-related pattern was observed. Very high proportions of [protection] of approximately 71 percent were seen in people born between the late 1970s and the early 1990s. High proportions of 40 to 50 percent presumably seroprotective antibodies were also seen in the age group born between 1967 and 1976 as well as in persons born in the mid-1950s or earlier. In particular, in children born in 1999 or later, no protective HI titres to the H3N2v virus were seen. Children born in the latter part of the 1990s showed a seropositivity rate of 16 percent.

Remarkably, the prevalence of seroprotective antibodies to the H3N2v virus was low, with 14 percent, in people born in the last part of the 1950s and the 1st part of the 1960s. This low seroprevalence was significantly different from other adult age groups. The seroprevalence results were in general also reflected by the pattern of geometric mean titres in the respective age groups. Statistical significance was reached for many of the differences between age groups in seroprevalence and antibody levels (data tabulated in the original text).

Discussion
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We have investigated the occurrence of antibodies reactive to the influenza A(H3N2)v virus in a serum panel representing all age groups from 0 to 97 years. The finding of a considerable antibody prevalence in persons who were young in the 1990s, that is, those between 18 and 34 years old, is in good agreement with the fact that the haemagglutinin gene of the H3N2v viruses is descended from a human H3N2 antigenic variant that was circulating in the mid-1990s [7], represented by the vaccine virus A/Wuhan/359/1995.

see link

Odd that there's a hole in the age groups with antibodies. So this was around in the early fifties?

Just to clarify. H3N2v has a HUMAN H3 and a HUMAN N2. The 1968 pandemic was caused by H3N2, so most developed immunity. Those who were older in 1968 may not have good immunity, which is why most (over 90%) fatal seasonal H3N2 cases are over 65.

H3N2v has a human H3 that traces back to the mid 1990's, so those born AFTER the 1990's would have less H3 immunity becuase seasonal H3N2 continues to evolve AWAY from the H3N2 in the mid 1990's.

H3N2v has an N2 which traces back to 2003, so those born AFTER 2003 would have less N2 immunity.

As a result, those with the lowest immunity are the very young.

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