Vitamin K2, also known as menaquinone, is one of three types of vitamin K, the other two being vitamin K1 (phylloquinone) and K3 (menadione).

There are nine different molecular variants of vitamin K2, determined by number of isoprenyl units in their side chains. The most common in the human diet is the short-chain, water-soluble menatetrenone (MK-4), which is usually produced by tissue and/or bacterial conversion of vitamin K1, and is commonly found in animal products. It is known that production of MK-4 from dietary plant vitamin K1 can be accomplished by animal tissues alone, as it proceeds in germ-free rodents.

Menaquinones longer than MK-4, including MK-7, MK-8 and MK-9 are more predominant in fermented foods such as natto. Longer-chain menaquinones (MK-10 to MK-13) are produced by anaerobic bacteria in the colon, but they are not well absorbed at this level and have little physiological impact.

MK-7 has been found to reduce the severity of covid-19, probably due to MK-7's role in maintaining the structural integrity of arterial & lung tissues. The regular consumption of natto (soybeans fermented with the bacterium Bacillus subtilis, which produces MK-7) might be one of the key reasons Japan has relatively few fatalities from covid-19 even among its large elderly population, despite the high population density. MK-7 being an elusive form of K2, your best bet if you don't eat natto regularly would be to simply take a supplement that contains it. It's harvested from the same bacterium. Humans used to get it from eating raw (and dirty) wild vegetables and from hunting grass-fed wild animals. Since civilization, deficiencies are now common.