How Many People Need Vaccines to Reach Herd Immunity?
2020-12-07
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1Governments and officials around the world are hoping COVID-19 vaccines can create "herd immunity" to protect large populations.
2Some experts predict that giving a vaccine to just two-thirds of a population could protect whole communities or nations.
3But the effectiveness of such efforts is highly dependent on what the vaccines will be able to prevent.
4Herd immunity happens when a large percentage "of a community (the herd) becomes immune to a disease, making the spread of disease from person to person unlikely.
5As a result, the whole community becomes protected - not just those who are immune," the Mayo Clinic notes on its website.
6But there are still many open questions about herd immunity and COVID-19 vaccines.
7For example, will the first vaccines be able to stop the spread of the virus, or just stop people from getting sick?
8How many people in a population will accept a vaccine?
9Will vaccines offer the same protection to everyone?
10Josep Jansa is an expert at the Stockholm-based European Centre for Disease Prevention and Control (ECDC).
11"Herd immunity is sometimes wrongly understood as individual protection," Jansa told the Reuters news agency.
12He added, "Herd immunity refers to community protection, not to how an individual is protected."
13One of the central questions about herd immunity and COVID-19 is the reproduction rate, or R value, of the virus that causes it.
14R value is a measure of how many people get the virus from an average infected individual in environments where no restrictions are in place.
15For example, an R value of 2 means an infected person would be expected to pass the virus on to 2 other people.
16Winfried Pickl is a professor at the Medical University of Vienna.
17"The problem is that for now we don't know exactly how fast the virus spreads without any precautions and with the normal travel and social activities we had a year ago," Pickl told Reuters.
18Health experts have noted that anything less than 100 percent vaccine efficacy would require an increase in vaccinations to reach herd immunity.
19Early data on the leading COVID-19 vaccines - from Pfizer-BioNTech and Moderna - suggests only about 90 percent efficacy.
20Amesh Adalja is with the Johns Hopkins Center for Health Security.
21He said a good target for immunity in the United States would be to have more than 70 percent of the population vaccinated.
22But the number could go up if vaccines are less effective, he added.
23The ECDC uses an estimated herd immunity level of 67 percent for its models.
24In Germany, Chancellor Angela Merkel said this month that COVID-19 restrictions there could be lifted once 60 to 70 percent of the population reached immunity.
25World Health Organization experts have also suggested a 65 to 70 percent vaccine coverage rate as a way to reach population immunity through vaccination.
26Experts say another important question is whether vaccines can stop the spread of the coronavirus, which causes the disease COVID-19.
27Evidence so far suggests the first COVID-19 vaccines will at least stop people from developing the disease.
28But it cannot be ruled out that people will still catch the virus and pass it on to others.
29Bodo Plachter is a professor and deputy director of the Institute of Virology at Germany's Mainz University teaching hospital.
30Plachter said that respiratory infections can be hard to block completely with vaccines - although the shots are expected to reduce the amount of virus that is moving around.
31"It may well be that vaccinated people will shed fewer viruses," he said.
32"But it would be a mistake," he added, to think that "vaccination alone can suppress a pandemic."
33I'm John Russell.
1Governments and officials around the world are hoping COVID-19 vaccines can create "herd immunity" to protect large populations. 2Some experts predict that giving a vaccine to just two-thirds of a population could protect whole communities or nations. But the effectiveness of such efforts is highly dependent on what the vaccines will be able to prevent. 3What is herd immunity? 4Herd immunity happens when a large percentage "of a community (the herd) becomes immune to a disease, making the spread of disease from person to person unlikely. As a result, the whole community becomes protected - not just those who are immune," the Mayo Clinic notes on its website. 5But there are still many open questions about herd immunity and COVID-19 vaccines. 6For example, will the first vaccines be able to stop the spread of the virus, or just stop people from getting sick? How many people in a population will accept a vaccine? Will vaccines offer the same protection to everyone? 7Josep Jansa is an expert at the Stockholm-based European Centre for Disease Prevention and Control (ECDC). 8"Herd immunity is sometimes wrongly understood as individual protection," Jansa told the Reuters news agency. He added, "Herd immunity refers to community protection, not to how an individual is protected." 9R Value and disease spread 10One of the central questions about herd immunity and COVID-19 is the reproduction rate, or R value, of the virus that causes it. R value is a measure of how many people get the virus from an average infected individual in environments where no restrictions are in place. For example, an R value of 2 means an infected person would be expected to pass the virus on to 2 other people. 11Winfried Pickl is a professor at the Medical University of Vienna. 12"The problem is that for now we don't know exactly how fast the virus spreads without any precautions and with the normal travel and social activities we had a year ago," Pickl told Reuters. 13Health experts have noted that anything less than 100 percent vaccine efficacy would require an increase in vaccinations to reach herd immunity. Early data on the leading COVID-19 vaccines - from Pfizer-BioNTech and Moderna - suggests only about 90 percent efficacy. 14Amesh Adalja is with the Johns Hopkins Center for Health Security. He said a good target for immunity in the United States would be to have more than 70 percent of the population vaccinated. But the number could go up if vaccines are less effective, he added. 15The ECDC uses an estimated herd immunity level of 67 percent for its models. In Germany, Chancellor Angela Merkel said this month that COVID-19 restrictions there could be lifted once 60 to 70 percent of the population reached immunity. 16World Health Organization experts have also suggested a 65 to 70 percent vaccine coverage rate as a way to reach population immunity through vaccination. 17Vaccines and virus transmission 18Experts say another important question is whether vaccines can stop the spread of the coronavirus, which causes the disease COVID-19. 19Evidence so far suggests the first COVID-19 vaccines will at least stop people from developing the disease. But it cannot be ruled out that people will still catch the virus and pass it on to others. 20Bodo Plachter is a professor and deputy director of the Institute of Virology at Germany's Mainz University teaching hospital. Plachter said that respiratory infections can be hard to block completely with vaccines - although the shots are expected to reduce the amount of virus that is moving around. 21"It may well be that vaccinated people will shed fewer viruses," he said. "But it would be a mistake," he added, to think that "vaccination alone can suppress a pandemic." 22I'm John Russell. 23Kate Kelland and Ludwig Burger reported on this story for Reuters. John Russell adapted it for Learning English. Bryan Lynn was the editor. 24________________________________________________________________ 25Words in This Story 26immune - adj. having special protection from something; of or relating to the body's immune system 27refer - v. to talk or write about someone or something 28precaution - n. something that is done to prevent possible harm or trouble from happening in the future 29efficacy - n. the power to produce a desired result or effect 30respiratory - adj. medical: of or relating to breathing or the organs of the body that are used in breathing 31shed - v. to get rid of (something); to lose (leaves, skin, fur, etc.) naturally 32suppress - v. to slow or stop the growth, development, or normal functioning of (something)