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Will herd immunity end the coronavirus?

Some researchers suggest the threshold for herd immunity is lower than we thought, but ending social distancing measures prematurely would be a mistake

Crowds visit a beach in Rio de Janeiro, Brazil, on Sept. 6 AP Photo/Bruna Prado

Will herd immunity end the coronavirus?
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In the COVID-19 debates, one topic manages simultaneously to be the most and least controversial: “herd immunity.” The concept of herd immunity is uncontroversial, and simply states that when enough members of a population develop immunity to a virus, the bug will fizzle out for lack of people to infect. The controversy, though, echoes every child’s favorite road-trip question: Are we almost there yet?

Two developments have raised the question with new vigor. One is in Washington, where Dr. Scott Atlas has become the newest White House adviser on the coronavirus task force. Atlas advocated for herd immunity in his May Senate testimony, saying, “If infection is still prevalent, socializing among these low-risk groups [younger, healthy adults] presents the opportunity for developing widespread immunity and eradicating the threat.” (His recommendations assume viral spread among low-risk groups will remain largely confined to those groups and won’t have serious long-term health consequences.)

The second development was summarized by a recent Washington Post article: Researchers are now questioning where the threshold for coronavirus herd immunity actually lies. So at what point can we argue that we’ve reached herd immunity?

Viruses spread at different rates in different populations. In a March letter to the editor in the Journal of Infection, a Hong Kong–based team attempted to calculate the coronavirus herd immunity threshold for various countries. The authors started with the basic idea that the more easily the virus spreads within a given population, the further that population must be from the immunity threshold. They based this on estimates of Rt, a figure measuring the “effective reproductive number.” (Think of weeds in gardens: They spread freely in general, but they may spread even better—or worse—depending on their circumstances. An Rt value below 1.0 would mean weeds are decreasing.)

The Hong Kong team estimated the U.S. herd immunity threshold was 69.6 percent, assuming no masks or social distancing—meaning about 7 out of 10 Americans would need to get the coronavirus (and survive) in order to stop its spread.

However, a Brazilian team’s new study (not yet peer reviewed) argues that individual variation in susceptibility or exposure could lower the threshold for herd immunity. That’s an interesting hypothesis, and the logic behind it makes sense up to a point. Clearly people who spend much of their time in crowded indoor places, such as the unfortunate folks who “get every cold that goes around,” are at higher risk than others. But—there’s always a “but,” isn’t there?—the Rt already takes that into account: It’s a population-level figure, and it considers those susceptible people along with the happy souls whose immune systems (or introverted lifestyles) mean they seemingly haven’t caught a virus in years.

The Brazilian team argues that under optimal circumstances, herd immunity could require as little as 20 percent of the population to be immune. Wonderful news if true—but is it? And how close can we get to those optimal circumstances? The fact is, we can do things to change effective reproductive number: America’s calculated Rt value, for example, went from an extreme high of 3.8 in some states down to slightly above 1.0 with social distancing, masking, contact tracing, and full-then-partial shutdowns.

We thus end up in another good news, bad news situation. The good news is that being able to change Rt means we can also temporarily, by all working together, get the benefits of herd immunity—reduced case numbers, reduced deaths, sustained reopening—even without having enough people contract SARS-CoV-2 to obtain true herd immunity. This is how distancing, hand-washing, mask-wearing, contact tracing, and staying home have held Taiwan’s population of 23 million people to a total of seven COVID-19 deaths. The Taiwanese are not unusually immune to the coronavirus. They simply worked together to pull Rt down to a level where the virus couldn’t spread.

The bad news comes if we mistake the good news for a get-out-of-jail-free card. The virus hasn’t changed, and the outbreaks at college campuses show just how easily it still spreads. A vaccine should be mass-produced soon—God willing, within the next six months. Observing that Rt is currently low, or that the infection and death figures are more reassuring now, should encourage us to stay the course with measures like masking and social distancing, not abandon them prematurely.

Charles Horton, M.D. Charles is WORLD's medical correspondent. He is a World Journalism Institute graduate and a physician. Charles resides near Pittsburgh with his wife and four children.


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Gregory P

As far as I know, there are no conclusive scientific studies to show the effectiveness of general wearing of face masks to slow the spread of COVID-19 (or any other coronavirus).  I think there are statements to this effect as late as April by WHO and June by CDC.  WHO has even recommended against them while outside exercising, but I still see people outside walking or bicycling wearing them.  I question the statement, "The virus hasn't changed."  I have seen several statements from "experts" that it has.  I appreciate Dr. Horton's reporting on different perspectives on herd immunity.  It it interesting that some doctors think that to achieve herd immunity we will need to have a sufficent number of people take the vaccine (and they may have to take it more than once.)  And also interesting to note that a few weeks ago the WHO was celebrating the eradication of (wild) polio in Africa.  But there have been cases of people getting polio from the polio vaccine.  With the percentages of growth of cases and growth in number of deaths from COVID-19 going down, and overall percentages of reported deaths of reported cases approaching 1% or lower, how many people will risk taking a vaccine which may carry a higher risk than natural exposure to the virus?   After so many months, even with whatever precautions, it is hard for me to believe that most people, perhaps almost all people except for those especially isolated, in the United States have not been exposed multiple times so the virus.  I also think President Trump has been wise to emphasize therapeutics.  Understanding the stages of the disease may help.  Some medications or nutrients may be more effective at fighting the infection of the virus at early onset, and other medications such as immunodepressors at later stages to combat an overactive and debilitating or lethal immune response.  And why do we not emphasize taking simple measures that would boost the body's immune system, such as with nutrients and exercise?  That so many can fight off this disease should be a reminder that we are fearfully and wonderfully made (Psalm 139:14).  Christians and churches should be practicing God's instructions (James 5:13-16) before someone goes to the hospital and he can't be approached by even his own immediate family. 


Nothing will end the Coronavirus, just as nothing ends the flu.  People will always get sick and die.  There is no cure for death.  A far worse disease is a government which has just tasted absolute power.  The Bill of Rights is becoming a pleasant memory, which we will never get back.  Tyranny may subside somewhat after the election, but it will always be ready to reassert itself for endless lesser causes.


I hear a lot of people comparing the flu to covid. Johns Hopkins estimates a person is up to 10 times more likely to die from COVID-19 than from a seasonal flu. People question the effectiveness of a vaccine if people still die from seasonal flu. My question is, what is the ratio of people dying from seasonal flu who eschewed the vaccine vs those who received the vaccine.

Laypeople are prematurely making up their minds about covid while scientists and medical experts are still making discoveries about it. New research is focusing on debilitating after effects of those "recovering" from covid 19 as well as whether covid antibodies are useful in prevention. Some reinfected patients have milder cases but there is at least one incidence of a more severe case of reinfection. 

Herd immunity is an experiment that results in loss of real life. Our family prefers following the experiment of masks and distancing. It's a minor sacrifice to make and may save lives, perhaps your life. When a reliable vaccine is ready, we will get it, just as we get an annual flu vaccine. 


Half the jobs lost due to Covid-19 have been brought back according to the August jobs report. I am all for opening up as fast as possible.  You are correct, there has been a loss of life due to job loss and isolation. The problem is knowing exactly where that balance point is at. The whole thing is a moving target and the government has to make decisions with a flow of ever changing and conflicting information.  I do not know why we are so critical of our government’s decision-making ability (when has if ever been that good?) when all of us, and even the experts, are struggling and stumbling along trying to figure this Covid-19 thing out. God help us!


We should also consider those who die because of the complications of lockdowns, job losses, and isolation.  That and the lowering severity and death rates is why I think we should err on the side of more opening of the economy and schools.


TinaH, While you make a lot of good points, your argument falls down here, “while targeting for special protection those who probably cannot.”  The word “probably” is the issue. Yes, the death rate is extremely low, but we have had cases of young people who are perfectly healthy die from Covid-19. For most, the odds are great for surviving it, but it can still kill anyone, we just don’t know who, yet.  We may discover that mask wearing was more of a physiological benefit than physical benefit, but still worth it for the mental health advantage. If we did drop some safety acts to hurry it along to herd immunity, but it kills your friend, your spouse, your child, your parent or grand parent what will you say then?  If we only held on a little longer when we learned, x y and z about how to deal better with Covid-19, we could have saved those lives? How many are we willing to sacrifice in that process?


I am also in Oregon, and we certainly aren't maintaining the level of precautions that Taiwan has, but we are doing much better than much of the US. As of today we have not quite hit 500 deaths.  Our death rate is better than all other states besides Maine, Vermont, Wyoming, Hawaii and Alaska.  Our mask mandate is practiced in places of business but we have lots of folks who choose not to use masks and/or physcially distance in other situation, and some who won't follow business mandates either.  (thanks alot QAnon).  And we have folks who refuse to participate contact tracing (which isn't that robust here.)   As we have opened up more categories of businesses over time, transmission has increased. If all Oregonians did take masks and physical distancing more seriously, I imagine our schools (and more businesses) could indeed be open.   Funny how the folks insisting on opening schools and businesses are also the ones who won't take the individual measures to allow that to happen.

We have all understood for quite some time now that fomite transmission is not the primary way that Coronavirus is transmitted, but rather through respiratory droplets - hence the masks.  And of course, handwashing always makes sense.


Oregon has been using all of the same precautions ( "distancing, hand-washing, mask-wearing, contact tracing, and staying home") as Taiwan for 6 months now (universal mask mandate started July 1st, but probably 80% of population were masking for at least a month before the mandate, and had been steadily increasing since at least May) and our infection rate has stayed the same, hospitalization and death rates have increased slightly since July 1st. We are essentially in the same place we were at six months ago, (only less people staying at home 24/7). That tactic doesn't seem to be having much of an effect in Oregon.

I read that Taiwan closed their borders to China really early, before the WHO said there was a concern, and that likely is why their death count is so low.

Funny how "masking" has replaced hand washing and covering your coughs or sneezes now. The evidence for cloth face coverings being effective is inconclusive at best, while hand washing? That's been around since Moses! If masks and social-distancing were that effective, Oregon schools would be opening this fall.