>> And I could not exactly figure out what the meaning of titrate is……
> Neither could I.
Titration is a chemical procedure in which you measure the concentration of a solution by adding something else until an indicator chemical, also in the fluid, changes color, and then use the amount added to calculate the original solution's concentration. A common example is determining the pH of a solution. It's more loosely used to discuss adding two things together to get a mixture at a given ratio of the inputs, and even more loosely used in conversation among us nerds if the two things aren't even chemicals in an experiment, such as titrating some experienced dancers into your newbie workshop to get a 1:1 ratio. (NOT advocating that we pick up this particular usage in contra!)
Perhaps the poster intended that the use of masks allowed the adjustment of risk to a tolerable level, though that's a rather coarse analogy to actual titration, unless somehow a variable percentage of masks is in play. Or maybe they meant "mitigate", as you titrate the inputs, not the output.
--jh--