Read this excerpt from "Herd about the Flu.” The flu shot helps prevent huge numbers of infections. Even when it doesn’t fully prevent infection in every person, the shot makes any flu infection less serious. On a global scale, the vaccine can reduce the number of infections only if most people in the community take it. That’s because vaccines need to reach enough people to achieve a "herd immunity,” the level of immunity needed to prevent the virus from spreading. If too few people get the vaccine, the people who receive it may not be well protected against infection.

So the next time you are at the drugstore, and you see No Flu Shots Available, think about the reasons behind it. Drug makers are scared to make too many flu shots because they could lose too much money. They then make fewer flu shots and the effect of the flu shots becomes weaker because not enough people take it. People become more reluctant to take the flu shot since they don’t think it will fully work. The demand goes down further and the flu companies make even fewer shots the next year.

This could continue until the community changes its mind and we all get the flu shot. "That is a community responsibility," says the president of Merck, one of the two remaining flu vaccine manufacturers.

What is the main idea presented in this excerpt?

As fewer people get the flu virus, the flu shot becomes less and less useful.
As fewer people get the flu virus, the flu shot becomes more and more useful.
As fewer people get the flu shot, it becomes more effective, but as more people get it, the flu shot becomes less effective.
As fewer people get the flu shot, it becomes less effective, but as more people get it, the flu shot becomes more effective.