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What is herd immunity and is it the solution to the coronavirus pandemic?

The more infectious a disease is the more people need to be immune to achieve herd immunity. But because COVID-19 is so new, it comes with a lot of questions

ST. LOUIS — Sometimes an infection gets under control because of something called "herd immunity. So, how does that come into play with the coronavirus? Can it help us avoid another societal shut-down?

Herd immunity is achieved when enough people are immune to a disease that it no longer causes mass outbreaks: it’s like flattening the curve, limiting spread because most people aren’t at risk of catching it and passing it along. 

The more infectious a disease is — or the more easily it spreads — the more people need to be immune to achieve herd immunity.

The common flu, for example, needs about 50% of the population to be immune in order to achieve herd immunity. Measles, which is highly contagious, needs about 95% population immunity to be kept under control.

Researchers expect 60 to 80% of the population, four out of five, needs to be immune to get to herd immunity with COVID-19.

“It's still a lot of people that need to be immune, either from vaccination or from being infected,” said Dr. David Warrenan infection disease specialist at Washington University and Barnes-Jewish Hospital.

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However, because it’s so new, herd immunity with the coronavirus comes with a lot of questions.

“We don't know how long people are immune. Once they get infected, how long that immunity lasts. And what sort of immunity they have,” said Dr. Warren.

Since we don’t know when we’re getting a vaccine, what if we just accept a high infection rate and get to herd immunity that way? Doctors caution: that’s an extremely risky approach.

“One of the problems, the concerns with that is that this virus can cause really serious illness and can be fatal, and especially in people that are older and have problems with our immune system or other health conditions,” said Dr. Warren.

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He cited findings in New York City that showed about one in four people have COVID-19 antibodies, a sign they’d been exposed to the virus. While that’s a lot more than the number who’d gotten sick, it hasn't been enough to prevent one of the deadliest outbreaks in the country.

“What that tells us is that we still have, we would have a long way to go in that to achieve herd immunity and that we still need to take precautions,” he said.

And herd immunity requires upkeep. Both the flu and measles, for example, are considered viruses to which we can achieve herd immunity due to widely vaccinating against them. It will be the same case, experts anticipate, with the coronavirus. They say social distancing and hygiene methods must work to keep as much of that herd as possible until a vaccine is developed.

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