A meta-analysis of swine flu studies from 19 countries found that between 20% and 27% of the population showed antibodies consistent with swine flu infection within a year of the start of the pandemic in 2009.
The results varied significantly by age. The infection rates were highest in children aged 5–19 years (39% to 55%) and 0–4 years (30% to 43%), while 8% to 24% of people older than 65 had some preexisting immunity.