Since 2003, there has been a world-wide shortage in armored vests and high-tech ballistic material (as in the stuff used for the bullet proof plates that make American protective vests so effective.) The war in Iraq, and the determination of the Americans to deal with the Sunni Arab terrorists, while rebuilding Iraq, created a huge demand for the materials used protective vests and lightweight vehicle armor. It wasnt just the troops who needed the new vests, and replacement parts, but also the foreign civilians who came to Iraq, and were equal in number to the American troops there. The U.S. Department of Defense had the authority to get priority on new production of these bullet proof materials, and they have used it. 

The shortage, and the war, has caused manufacturers to increase their research into new and more effective materials.

There are currently three types of material. 

Most common are the aramid fibers, of which Kevlar is the most common type. Multiple layers of this stuff will stop low velocity bullets (from pistols) and most fragments from shells, grenades and bombs. The war on terror has prompted a lot of more research on aramid fibers, and the Kevlar brand is now available in a new version that is 25 percent lighter and more flexible (thus more comfortable to wear.) One manufacturer of aramid fibers had quality control problems recently, with its material degrading more quickly than expected to heat, humidity and rough handling. This caused massive recalls of vests, and added to the shortage. 

A competitor for aramid fibers is Dyneema, which is 40 percent stronger than aramid fibers, floats, and is more resistant to sunlight, moisture and chemicals. It is also a lot more expensive. Most protective vests still use Kevlar. 

Then there are the bullet proof plates that will stop high powered rifle projectiles. These plates tend to be either ceramics, or composed of multiple layers. Much research is going into making these plates lighter, less brittle and cheaper.

A final factor is the way these fabrics and plates are put together. The design of the protective vests can make a big difference in how effective provided is. This has produced many new products that are lighter, more effective and more comfortable to wear.