【正确答案】 1、protect against    

【答案解析】根据语法知识可以判断，本题空格显然是要填不定式，而题干的关键词可以看成为scientists和an anti- AIDS vaccine。由此可寻读到原文最后一段的相关说法Scientists first identified the virus that is believed to cause AIDS in l983．Many of them then thought they could produce an anti-AIDS vaccine．可见，本题要填的是词组 protect against。