Impact of transmissibility of the oncoming reassortant strain and cross-immunity induced by seasonal flu vaccination on the annual emergence probability of pandemic strain via reassortment.

<p>The duration of both infection and vaccination-induced immunity is two years and the values of other model parameters are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0114637#pone-0114637-t001" target="_blank">Table 1</a>. Here we consider a situation of cross-immunity structure where the cross-immunity conferred by vaccination are less than or equal to that by natural infection: <i>φ</i>:<i>ψ</i>:<i>φ</i>_V:<i>ψ</i>_V<i> = </i>0.5:0.5:0.1:0.1 (red), and 0.5:0.5:0.5:0.1 (yellow), 0.5:0.5:0.5:0.5 (green). For comparison, the situations without vaccination (blue) are also shown. As the basic reproduction number (<i>R</i>₀^r) of reassortant strain reduces, the emergence probability of pandemic decreases rapidly. For example, when <i>R</i>₀^r decreases to 1.4, the annual probability of pandemic emergence reduces to below 10⁻⁶ (data not shown).