Cross‐reacting antibodies against the pandemic (H1N1) 2009 influenza virus in older Australians

Abstract

Objective: To assess background pre‐pandemic cross‐reacting antibodies to the pandemic (H1N1) 2009 virus in older populations in Australia. Design, setting and participants: Data were opportunistically generated from three cross‐sectional pre‐pandemic studies involving people aged 60 years or older: a 3‐year (2006–2008) study of influenza outbreaks in aged care facilities (ACFs) in Sydney; an investigation of a respiratory virus outbreak in an ACF in rural New South Wales in June 2009; and a non‐influenza serosurvey undertaken in NSW in 2007 and 2008. Main outcome measure: Prevalence of pandemic (H1N1) 2009 haemagglutination inhibition (HAI) antibody titres ≥ 1:40 (putative protective level) in pre‐pandemic sera. Results: In total, 259 serum samples from individuals aged 60 years or older (range, 60–101 years) were tested. More than half of the individuals tested were women (151/259; 58.3%). About a third of individuals (37.5%) had cross‐reacting HAI antibody titres ≥ 1:40. The prevalence of cross‐reacting antibodies was highest in the oldest age groups (≥ 85 years), with more than 60% of these people having HAI antibody titres ≥ 1:40. The proportion of subjects with HAI antibody titres ≥ 1:40 decreased significantly and successively in younger groups to only 12% of those aged 60–64 years. Conclusions: Our study suggests a pre‐existing influenza A antibody reserve in most of the oldest group of people that was cross‐reactive to the new pandemic (H1N1) 2009 virus; this is likely to be lifelong and to have provided them with clinical protection against the first wave of the pandemic. Pandemic influenza control measures need to focus more on younger adults naive to the pandemic virus and at increased risk of severe disease.

Pandemic (H1N1) 2009 virus revisited: An evolutionary retrospective

“…High sequence similarity (∼80%) was found between the two pandemic viruses (1918 and 2009), with the differences restricted mainly to the Ca region of HA. This agrees with the findings of Booy et al (2009), Morens et al (2010), and Hancock et al (2009) that cross-reactive antibodies to H1N1pdm do exist in older people.…”

Antibody responses and cross protection against lethal influenza A viruses differ between the sexes in C57BL/6 mice

“…Conversely, cross-reactive total antibodies were present at somewhat higher titers among female than male mice. In humans, women reportedly have a higher frequency of cross-reacting antibodies against pandemic 2009 H1N1 than men, as measured by hemagglutination inhibition [67]. Although hemagglutination inhibition titers correlate well with virus neutralizing titers [68], the strain specificity of the virus neutralization assay may have reduced detection of cross-reacting antibodies against HA in the present study.…”

Searching for Sharp Drops in the Incidence of Pandemic A/H1N1 Influenza by Single Year of Age

“…In New Zealand, 22.6% (95% CI: 15.3–30%) of adults >60 years old (N = 124 samples) had pre-existing protection (determined by HI assay) [29]. In North Queensland, Australia, 19% (95% CI: 4–34%) of adults >65 years old (n = 27) had pre-existing immunity (determined by HI assay) while a larger study of 259 adults >60 years old in Australia found pre-existing immunity in 37.5% (95% CI: 31.6–43.3%) [33], [34]. In the United States, 34% of adults born before 1950 (N = 115) had cross-reactive antibodies to pH1N1 (determined by MN) [9] while in Hong Kong 37% (n = 30) adults >65 years showed seroprotective levels of antibodies to pH1N1 [35].…”