Suppose that a vaccine is developed for a highly contagious strain of flu.The likelihood that anyone will get this flu decreases as more people receive the vaccine.One of the demand curves in the diagram reflects private benefits and the other reflects social benefits. 
Refer to the figure above.The socially optimal number of vaccines is _______.
Definitions:
Marks & Spencer
A major British multinational retailer that specializes in selling clothing, home products, and food items, known for its commitment to quality and sustainability.
Place Utility
The value added to products by having them available at a location where they are needed or desired by consumers.
Place Utility
The value added to products by having them in a location where they are needed or desired by consumers.
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