The epidemic in Nanjing is spreading rapidly. One picture will tell you how fierce "Delta" is?

Recently, the Nanjing epidemic was imported from abroad and spread within the country. The transmission was hidden and wide, and the culprit was the mutated strain of the new coronavirus "Delta".

Where did the "Delta" strain come from? How fierce is it? Is the vaccine still effective? This issue of "One Picture to Understand" will help you understand this aggressive new coronavirus variant.