Since the outbreak of the coronavirus $($COVID$-19)$ in early January of $2020,$ the $10-$year Treasury note yield dropped precipitously from $2\%$ to $\mathrm{.}5\%($as of March $9^{th},2020).$ What is the link between the onset of Coronavirus epidemic in the US and the sudden decline in the $10-$year yield $($and $30-$year mortgage rate$)?$ Please explain.

However, the $10-$year Treasury yield gradually began its ascent from $\mathrm{.}5\%$ on August $4^{th},2020$ to $1\mathrm{.}74\%$ on March $19$th of $2021.$ What factors do you think have contributed to the increase in the $10-$year Treasury yield?