加拿大研究发现，季节性流感疫苗接种增加2009年大流行甲型流感(H1N1)感染的危险性，而澳大利亚研究未能证实这个发现。雪貂实验结果表明，以前的季节性流感感染能防御大流行甲型流感(H1N1)，但以前的季节性流感疫苗接种则不能。模型研究显示， 流感感染可导致对不同亚型的暂时性免疫。这些观察可以解释加拿大和澳大利亚的不一致发现。澳大利亚近期没有季节性流感流行，因而对大流行流感没有暂时性免疫。季节性流感疫苗与大流行流感感染无关。加拿大大流行流感流行前有季节性流感流行，未接种疫苗的人群由于罹患季节性感染而获得对大流行流感的暂时性免疫，但接种疫苗的人群未罹患季节性感染而不能产生对大流行流感的暂时性免疫，因而看来似乎季节性流感疫苗接种增加了2009年大流行甲型流感(H1N1)感染的危险性。

Canadian studies have found that receipt of seasonal influenza vaccine increased the risk of infection with 2009 pandemic influenza A(H1N1), and Australian studies have not confirmed this finding. Ferret experiments have suggested protection against pandemic influenza A (H1N1) from prior seasonal influenza infections but not from prior seasonal vaccination. Modelling studies suggest that influenza infection leads to heterosubtypic temporary immunity. These observations can explain the discrepant findings in Canada and Australia. In Australia there was no recent prior circulation of seasonal influenza and thus no temporary immunity to pandemic influenza. There was no association of seasonal influenza vaccine with pandemic influenza infection. In Canada seasonal influenza preceded circulation of pandemic influenza. An unvaccinated proportion of the population developed temporary immunity to pandemic influenza from seasonal infection but a vaccinated proportion of the population did not get seasonal infection and hence did not develop temporary immunity to pandemic influenza. It may therefore have appeared as if seasonal vaccination increased the risk of infection with 2009 pandemic influenza A(H1N1).