Is there pre-existing cross-reactivity to influenza A (H1N1)2009 in a tropical population?

Results 12.6% had detectable antibody titres against A/Auckland/1/2009(h1N1)v. Of these, only 2.9% had titres ≥ 40; such titres were most frequently seen in persons aged at least 70 years (9.4%; 3 of 32), followed by those aged 20 – 29 years (6.5%; 2 of 31) and 30 – 39 years (5.9%; 2 of 34). The peak titre was 160, in 2 individuals aged 91 and 30 years. None from the other age groups showed any sero-reactivity. Comparatively, antibody reactive to A/Brisbane/59/ 2002(H1N1) was detectable in 52.7% of all individuals and 20.8% of these had titres ≥ 40. Those aged 10 – 19 years, followed by 20 – 29 years, had the highest percentage of titres ≥ 40 of 51.5% and 45.2%, respectively. The peak titre detected was ≥ 1280.