Infection rates across the U.S have increased by over 300 percent in under a month.

increase by 增加了

eg. This year's production was increased by 20 percent.

今年的产量增加了20%。

【相关短语】 increase to 增加到

eg. In 2004, the European Union increased to 25 members.

2004年，欧盟扩展到25个国家。

in under a month 在不到一个月的时间里

under在这里的意思是“少于”

eg. The number of students in each class must not be under twenty.

每班的学生不得少于20名。

【相似表达】 within a month 一个月内

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