rent</em> reproduction number (R) depends not just on the <em>basic</em> reproduction number (R<sub>0</sub>), but <em>also</em> on how many people are no longer <icon s></icon> Susceptible. (For example, by recovering &amp; getting natural immunity.)</p>

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	<p>When enough people have immunity, R &lt; 1, and the virus is contained! This is called <strong>herd immunity</strong>. For flus, herd im