The concept stems from the fact that people have social networks, and those networks tend to intersect. But the importance of this “small world” phenomenon extends beyond whose email address you have on your Blackberry. Now scientists think they can take advantage of these networks to design more efficient vaccination programs.

According to an international team of physicists, the most effective way to protect a population from disease is to immunize people who have the largest networks. If you can keep the most highly connected people from infecting their friends, neighbors, co-workers, you can halt an epidemic. And you may only have to vaccinate half the number of people you would normally. The findings will appear in an upcoming issue of Physical Review Letters. The biggest problem with such a program—people are bound to feel left out when they’re told that they don’t need a shot.

—Karen Hopkin 

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