1. To guard, defend, or inoculate against someone or something. This vaccine protects against the flu for the entire winter. We need to invest in top-of-the-line security software to protect against cyber attacks. The walls of the encampment were fortified and lined with spears to protect against invaders.

2. To guard, defend, or inoculate someone, something, or oneself against someone or something else. In this usage, a noun or pronoun is used between "protect" and "against." I know you want to protect your kids against injury, but you can't keep them cut off from the real world either. Tourists are being advised to protect themselves against pickpockets in the city center.