VOA慢速英语2021--我们怎么知道什么时候达到群体免疫?（在线收听）

How Will We Know When We Reach Herd Immunity?

How will we know when we reach herd immunity?

That is the question people are asking as governments race to vaccinate people around the world to stop the spread of COVID-19.

Herd immunity is when a large number of people in a community will either have recovered from infection or been vaccinated. Herd immunity does not mean everyone is protected from the virus. It means that the coronavirus can no longer spread easily, helping to protect those who are at risk.

Once people are sick or receive the vaccine, they start to develop antibodies. Experts think the number of people with antibodies needs to reach 70 percent or more for herd immunity.

However, the virus is always changing and developing into new variants. Health officials are now concerned with variants first discovered in Britain, South Africa and Brazil spreading around the world.

How do you calculate the percentage of people who need to be immune?

Finding how many people, on average, catch the virus from one infected person is important to understanding herd immunity.

Dr. Walter Orenstein is an infectious disease expert at Emory University. He said the percentage of people who need to be immune depends on a number of things. The number could be different depending on where you live. But it is not an exact number.

"It's not 64.9 is terrible and 70.1 is fantastic," he said.

How do we know when we have reached herd immunity?

Experts say there will not be a big announcement.

The evidence will come when there is a large decrease in new infections and fewer people need to go to the hospital for treatment.

The numbers will be different depending on what part of the world you are in.

For example, in India, scientists believe more people will need to be protected from the virus in cities than those who live in rural areas. The virus spreads easier and faster in densely populated cities.

India is working to find out how many of its 1.4 billion people already have recovered from the virus. And if current vaccines work well against the virus, fewer people will need to be vaccinated to reach herd immunity.

How will the virus variants affect herd immunity?

If vaccines or antibodies from past infections protect people from getting sick again, the fast-spreading variants are not a big concern.

However, if current vaccines are less effective against the variants, drug makers will have to update the shots to make them more effective. And even more people will need to be vaccinated.

Scientists are also worried about vaccinating people as quickly as possible to prevent new variants from developing.

Does herd immunity need to be global?

Yes, it is important to reach herd immunity around the world. But some parts of the world will struggle with the coronavirus longer than in others.

The World Health Organization said herd immunity will probably not happen around the world this year. That is because rich nations have reserved most of the vaccines that will be produced in 2021. Poor countries will have to wait longer.

Experts say the virus will never completely come to an end because different countries will have different vaccination levels.

Can herd immunity wear off?

Experts think immunity from earlier infection or a vaccine should last several months. But it is likely we will need follow-up shots, known as boosters, in the future.

The seasonal flu viruses are changing all the time. That is why people need a flu shot each year. The coronavirus is also changing but not as easily. So it is possible people will probably not need a COVID-19 shot every year.

What if the vaccines do not keep people from getting sick?

The current vaccines, from Pfizer-BioNTech and Moderna, seem to work very well. They do prevent people from getting sick.

It is not clear, however, how well they stop the virus from moving from person to person. But experts say they should help to reduce the spread of the virus.

Deborah Fuller is a vaccine expert at the University of Washington. She said the vaccines should limit the amount of time people can spread the virus to others if they do get sick.

Words in This Story

variant - n. something that is different in some way from others of the same kind

immunity - n. the power to keep yourself from being affected by a disease

antibody - n. a substance produced by the body to fight disease

update - v. to change (something) by including the most recent information

flu - n. a common disease that is caused by a virus and that causes fever, weakness, body aches, and breathing problems