[ti:How Many People Need Vaccines to Reach Herd Immunity?]
[by:www.21voa.com]
[00:00.00]更多听力请访问21VOA.COM
[00:00.04]Governments and officials around the world
[00:04.72]are hoping COVID-19 vaccines can create "herd immunity"
[00:10.20]to protect large populations.
[00:13.60]Some experts predict that giving a vaccine to just
[00:18.24]two-thirds of a population could protect whole communities or nations.
[00:24.56]But the effectiveness of such efforts is highly dependent
[00:29.80]on what the vaccines will be able to prevent.
[00:33.68]Herd immunity happens when a large percentage "
[00:37.68]of a community (the herd) becomes immune to a disease,
[00:42.96]making the spread of disease from person to person unlikely.
[00:48.16]As a result, the whole community becomes protected
[00:52.48]— not just those who are immune," the Mayo Clinic notes on its website.
[00:59.16]But there are still many open questions about herd immunity
[01:04.24]and COVID-19 vaccines.
[01:07.52]For example, will the first vaccines be able to stop the spread of the virus,
[01:14.24]or just stop people from getting sick?
[01:17.88]How many people in a population will accept a vaccine?
[01:22.92]Will vaccines offer the same protection to everyone?
[01:27.16]Josep Jansa is an expert at the Stockholm-based
[01:32.68]European Centre for Disease Prevention and Control (ECDC).
[01:37.56]"Herd immunity is sometimes wrongly understood as individual protection,"
[01:43.84]Jansa told the Reuters news agency.
[01:47.48]He added, "Herd immunity refers to community protection,
[01:52.92]not to how an individual is protected."
[01:56.76]One of the central questions about herd immunity and COVID-19
[02:02.76]is the reproduction rate, or R value, of the virus that causes it.
[02:09.72]R value is a measure of how many people get the virus
[02:15.00]from an average infected individual
[02:18.48]in environments where no restrictions are in place.
[02:23.44]For example, an R value of 2 means an infected person
[02:29.04]would be expected to pass the virus on to 2 other people.
[02:34.28]Winfried Pickl is a professor at the Medical University of Vienna.
[02:40.64]"The problem is that for now we don't know exactly
[02:44.96]how fast the virus spreads without any precautions
[02:49.40]and with the normal travel and social activities
[02:53.36]we had a year ago," Pickl told Reuters.
[02:57.68]Health experts have noted that anything less than 100 percent vaccine efficacy
[03:04.92]would require an increase in vaccinations to reach herd immunity.
[03:11.44]Early data on the leading COVID-19 vaccines
[03:15.88]– from Pfizer-BioNTech and Moderna
[03:19.72]- suggests only about 90 percent efficacy.
[03:24.08]Amesh Adalja is with the Johns Hopkins Center for Health Security.
[03:30.40]He said a good target for immunity in the United States
[03:35.36]would be to have more than 70 percent of the population vaccinated.
[03:40.84]But the number could go up if vaccines are less effective, he added.
[03:47.16]The ECDC uses an estimated herd immunity level of 67 percent for its models.
[03:57.00]In Germany, Chancellor Angela Merkel said this month
[04:02.40]that COVID-19 restrictions there could be lifted
[04:07.00]once 60 to 70 percent of the population reached immunity.
[04:13.00]World Health Organization experts have also suggested
[04:17.60]a 65 to 70 percent vaccine coverage rate
[04:22.24]as a way to reach population immunity through vaccination.
[04:28.08]Experts say another important question
[04:32.00]is whether vaccines can stop the spread of the coronavirus,
[04:37.00]which causes the disease COVID-19.
[04:41.56]Evidence so far suggests the first COVID-19 vaccines
[04:47.00]will at least stop people from developing the disease.
[04:51.80]But it cannot be ruled out that people will still catch the virus
[04:57.16]and pass it on to others.
[05:00.48]Bodo Plachter is a professor and deputy director
[05:05.00]of the Institute of Virology at Germany's Mainz University teaching hospital.
[05:11.88]Plachter said that respiratory infections
[05:15.84]can be hard to block completely with vaccines
[05:19.68]- although the shots are expected to reduce the amount of virus
[05:24.40]that is moving around.
[05:26.44]"It may well be that vaccinated people will shed fewer viruses," he said.
[05:33.04]"But it would be a mistake," he added,
[05:36.32]to think that "vaccination alone can suppress a pandemic."
[05:42.08]I'm John Russell. 更多听力请访问21VOA.COM
END OF TRACK. "END OF TRACK." The two men bowed. "Whoever was that person you were talking to?" she enquired, as soon as they stood together. The took of triumph faded from her eyes, she had grown worn and weary. The roses were wilting on the walls, the lights were mostly down now. Hetty, looking in to see if anything was wanted, found herself driven away almost fiercely. I only saw Master Jervie once when he called at tea time, The year 1747 was opened by measures of restriction. The House of Lords, offended at the publication of the proceedings of the trial of Lord Lovat, summoned the parties to their bar, committed them to prison, and refused to liberate them till they had pledged themselves not to repeat the offence, and had paid very heavy fees. The consequence of this was that the transactions of the Peers were almost entirely suppressed for nearly thirty years from this time, and we draw our knowledge of them chiefly from notes taken by Horace Walpole and Lord Chancellor Hardwicke. What is still more remarkable, the reports of the House of Commons, being taken by stealth, and on the merest sufferance, are of the most meagre kind, sometimes altogether wanting, and the speeches are given uniformly under fictitious names; for to have attributed to Pitt or Pelham their[112] speeches by name would have brought down on the printers the summary vengeance of the House. Many of the members complained bitterly of this breach of the privileges of Parliament, and of "being put into print by low fellows"; but Pelham had the sense to tolerate them, saying, "Let them alone; they make better speeches for us than we can make for ourselves." Altogether, the House of Commons exhibited the most deplorable aspect that can be conceived. The Ministry had pursued Walpole's system of buying up opponents by place, or pension, or secret service money, till there was no life left in the House. Ministers passed their measures without troubling themselves to say much in their behalf; and the opposition dwindled to Sir John Hinde Cotton, now dismissed from office, and a feeble remnant of Jacobites raised but miserable resistance. In vain the Prince of Wales and the secret instigations of Bolingbroke and Doddington stimulated the spirit of discontent; both Houses had degenerated into most silent and insignificant arenas of very commonplace business. "It certainly will be. Miss Widgeon," answered Maria, with strictly "company manners." "One who has never had a brother exposed to the constant dangers of army life can hardly understand how glad we all feel to have Si snatched from the very jaws of death and brung back to us." "Just plug at 'em as you would at a crow, and then go on your way whistlin'?" persisted Harry. "Hurroo!" echoed Hennessey; "that's the ticket." "Come forward, keeper," continued the baron, "and state how these arrows came into your hands!" "Yes." HoMEJULIA京香2018下载
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