too terribly much of this, as taking a little off the table would have made sense, if it happened later in 2016, after Kenilworth's run-up in NYSE price.
For the past four years (since 2013 -- with a chart looking back every year, to 2008), BlackRock has held between 7.0 per cent and 6.6 per cent of all of Merck's outstandings. [Analogous prior year's post, of mine -- from four years past, here.]
Given that Merck issues additional shares under options and restricted stock grants to executives every year, it is probably best to focus on each institutional investor's holdings level, as a percentage of all outstandings -- not just the absolute number of shares held or controlled.
. . . .183,718,408 shares. . . 6.7 per cent. . . .
Now you know. Onward. Smile.