How Herd Immunity Works — And What Stands In Its Way

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How Herd Immunity Works — And What Stands In Its Way

Post by trader32176 »

How Herd Immunity Works — And What Stands In Its Way

2/18/21 ... in-its-way

(please refer to linked article for illustrations)

What will it take to finally halt the spread of the coronavirus in the U.S.? To answer that question we've created a simulation of a mock disease we're calling SIMVID-19.

When you click "Run Simulation" above, you are witnessing how a disease can spread through a population and how increased levels of vaccination can stop it in its tracks.

We're chosen to simulate a fake disease since there are too many unknowns to simulate the course of COVID-19. There are common features in how any infection spreads. When enough people are immune — through vaccination or natural immunity — a population achieves herd immunity. The disease stops spreading efficiently and starts to fade away.

How quickly a community gets to herd immunity depends on a number of factors. We've illustrated some major ones in the scenarios that follow.

1) A more infectious variant takes over

What happens if a more infectious strain of SIMVID-19 predominates in a community? You can see in the simulation that even at a 75% vaccination rate, many people in our imaginary community of 400 will still get infected.

It looks like it will take higher vaccination rates to achieve herd immunity against the more infectious strain of SIMVID-19. This is a real concern with emerging variants of the coronavirus, including the strain first detected in the U.K., which is now spreading rapidly in the U.S.

There's always an element of chance in how diseases spread, as you can see each time you re-run these simulations.

2) A population is already heavily exposed

The pandemic has hit some communities harder than others, leaving in its wake some degree of natural immunity. This scenario assumes a lot of people are already immune to SIMVID-19, through a previous infection.

For this illustration, we've decided that a SIMVID-19 infection will persist for the duration of our simulation. You can see how higher levels of immunity — along with a high vaccination rate — would help a community reach herd immunity. For the coronavirus, we don't know what percentage of people who have been infected are immune, or how long that immunity lasts.

3) A population has low levels of initial immunity

You can see in this scenario how SIMVID-19 can spread more easily in a population where few people are immune at the start of an outbreak. It is harder to achieve herd immunity in this scenario, and increased levels of vaccination are key. When it comes to coronavirus, some communities have lower level of preexisting immunity and scientists think that the immunity people do have may fade over time.

Takeaways for COVID-19

In our SIMVID-19 scenarios, 75% vaccination rates were able to kick the imaginary disease in all but one scenario, the more infectious variant. For the U.S. to tame its coronavirus outbreak through herd immunity, scientists expect that somewhere between 70% and 85% of the population must be immune.

Unfortunately, we can't predict exactly what vaccination rate will be needed because there are many questions about the coronavirus we simply can't answer.

For example, people who are vaccinated may still pose some risk of spreading the virus, even though the vaccines protect them from serious illness. And no vaccine is 100% effective, so some vaccinated people will still get ill and could still pass on the disease. You can see this in our model, as dark-gray, vaccinated individuals turn infected pink, from time to time.

And though we do know that perhaps 120 million Americans have already been infected, we don't know how long immunity following infection will last. Finally, there is a concern that some variants now spreading around the world will render vaccines less effective.

One thing that's evident from this simulation (and real life) is that the faster the population is protected by vaccination the better.
Posts: 1779
Joined: Fri Jun 26, 2020 5:22 am

Re: How Herd Immunity Works — And What Stands In Its Way

Post by trader32176 »

The U.S. May Never Reach Full Herd Immunity Against Coronavirus, Former FDA Head Says

2/21/21 ... db65af4b46

While the strong decline in new coronavirus infections across the country this month is an optimistic sign he expects to continue, former Food and Drug Administration commissioner Dr. Scott Gottlieb said the U.S. may never reach herd immunity against the virus.

Key Facts

“I don’t really think we’re ever going to reach full herd immunity,” Gottlieb told Margaret Brennan on CBS’ Face The Nation Sunday, when asked about vaccinating children, though he didn’t elaborate on why. “This isn’t going to be like measles or smallpox, which just sort of goes away. Covid is going to continue to circulate at a low level.”

While estimates vary for how many Americans need to receive a vaccine or have been previously infected for the country to reach herd immunity, the country’s top infectious disease expert Dr. Anthony Fauci said in December it could be as high as 90% of the population.

Other studies have found herd immunity in the U.S. could be reached as early as May if vaccinations continue to pick up.

Gottlieb said he’s still “optimistic” about the immediate future, saying that rising vaccination rates and the fact that a third of the American public has already been infected with the coronavirus likely provides enough protection to continue to decrease the number of new infections, even if the population doesn’t reach full immunity against the virus.

On Thursday, the seven-day rolling average for new coronavirus infections had fallen by a staggering 43% in just two weeks.

“This has taken a tragic toll on the United States, but I think we should be optimistic in my view,” Gottlieb said.

Key Background

Some experts predict that if the U.S. never achieves full immunity, Covid-19 may become a seasonal virus, as reinfection of coronaviruses is very common. The number of new daily coronavirus infections and coronavirus patients in hospitals began to slump this month after the country recorded rising cases that peaked in January, a phenomenon some experts have blamed on people traveling and meeting with groups outside of their households for Thanksgiving, Christmas and New Year’s Eve. According to Johns Hopkins University, more than 28 million people in the U.S. have tested positive for coronavirus, making up roughly a quarter of the world’s confirmed cases.
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Re: How Herd Immunity Works — And What Stands In Its Way

Post by trader32176 »

Will Population (Herd) Immunity To Covid-19 Be Permanent Or Seasonal?

2/1/21 ... dd47046fb1

In just three weeks, the number of Covid-19 cases in the United States has plummeted by 35 percent. Death rates have yet to follow suit, but they have leveled out, with hospitalization rates on the decline, too. Given that the vaccine rollout is still proceeding slowly, the ebb is the glimmer of hope we need as we start to emerge from a long, dark winter. But is this really the light at the end of the tunnel, or just a period of calm before yet another storm?

There are two possible expirations for the sudden decrease. Americans, whether chastened by the holiday surges or influenced by the new administration, may be embracing safety measures like mask-wearing and social distancing. But it could be that population immunity, also known as herd immunity, is on the horizon. While confirmed case counts hover around 26 million, a study recently published in JAMA suggests the actual number is likely four times as high, around 100 million.

Between the 100 million Americans who have contracted the virus and the 24 million vaccinated so far, we may be in the early phase of a steady decline. Similarly, India reached peak infection in mid-September 2020, a time when an estimated one third of the population has been infected. Since then new cases have been on a steady decline from a high of 100,000 per day to roughly 12,000.

By many estimates population immunity requires the majority of a population, around 60 to 70 percent, to have some level of immunity to the virus, offsetting its ability to move freely amongst naive hosts. Last summer, there was discussion that herd immunity—achieved not with vaccines, but through rampant, unchecked infection—might be a potential strategy for ending the pandemic. In the writings I published in response, I argued that this was not just a fallacy, but a recipe for further carnage and tragedy. With deaths in the US alone projected to surpass half a million by the end of February, this is regrettably now our reality.

But now that case counts are down and mass vaccinations underway, population immunity has re-entered the realm of possibility cast in a much different light. In today’s discussions, population immunity isn’t a strategy in and of itself, but a byproduct of unevenly implemented public health interventions and a year’s worth of transmission. Some now argue that infection of only third of the population is enough to spark a downward trend.

How might the pandemic play out over the next 11 months of 2021? Will the rate of infection drop and remain low, or will we face yet another wave of increased infections and deaths come late fall and early winter? In other words, are we entering a period of prolonged or of seasonal population immunity? In considering these two possibilities, I look to three factors: the immune response to infection, the history of coronaviruses that cause colds each winter, and the emergent shape-changing proclivities of SARS-CoV-2, the virus that calls SARS-CoV-2. Our collective experience with both cold viruses and influenza acquaints us all with the concept of immunity that lasts but a season.

Infection by SARS-CoV-2 triggers the production of IgM, IgA, and IgG antibodies that recognize the virus. IgM and IgA antibodies protect us against infection at our mucosal surfaces, but they are effervescent, lasting no more than a few weeks. IgG antibodies are thus the longest-acting of the three and carry the greatest potential for preventing infection, though reports of just how long neutralizing antibodies last in people infected by SARS-CoV-2 have so far been mixed. In some studies, including a recent one from China, neutralizing activity decreased as much as tenfold between February and October 2020. Overall both the magnitude and duration antibody response may depend upon the severity of the disease.

We all know influenza has the propensity to return to plague us each winter in a new guise, one that slips by our natural and vaccine-induced defenses. That is why we receive annual vaccines— to protect us from the latest influenza variants. The capacity of influenza to change seems unlimited. For as long as I can remember, each new winter is heralded by a new flu variant.

Until very recently the annual return of cold-causing coronaviruses was more mysterious, as these viruses were thought to be stable, with no significant variations reported. As it happens that is because no one looked until now. Surprise! Influenza now has a changing cousin. The one cold-causing coronavirus strain studied, known as 229E, not only changes, but evolves over time to evade the prevailing immune responses to its predecessors. The seasonal influenza's winter peaks and those of the cold causing coronaviruses appear to be due to the same two factors, waning protective immunity and immune evasion.

It is this context that makes the recent discovery of new SARS-CoV-2 coronavirus variants so troubling. The variants share several features of concern. They are more easily transmitted than their predecessors, more resistant to neutralization by the antibodies in the blood of patients infected with early strains of the virus (and in some cases to the antibodies in those immunized with anti-Covid-19 vaccines), and are found in more abundance in nasal secretions of those infected. Specifically, one recent study found that whereas most of the sera collected in October from 20 people infected in January was able to neutralize the original, so-called Wuhan strain, only two of the 20 inactivated B.1.351, the so-called South Africa strain. Meanwhile another study conducted in South Africa found definitive evidence that B.1.351 infects those who’ve recovered from prior infection by the original infecting variant. The South Africa variant is also far more resistant to the Novavax vaccine, evading both natural and vaccine induced immunity.

My conclusion is that we may very well face a renewed winter wave of Covid-19 this year, driven both by aging immunity and virus variation. We must heed the lessons of influenza, as well as those of other coronaviruses. What we may be entering now may be only a respite, best described as seasonal immunity.

We are not helpless. First and foremost, we must do all we can to eradicate Covid-19 as quickly and as thoroughly as possible within our respective borders. Today the US accounts for almost one quarter of all infections. We should beware not only imported variants but homegrown ones as well, some of which, like the California and Ohio strains, may have been spotted already. The current vaccines will speed us on our way. New vaccines adapted to the variants as they arise will also help. But I am not alone in warning that vaccines alone are not the entire solution.

We must also do what is needed to prevent transmission by following the guidance of our public health experts. Others have kept their countries free of Covid-19, save for intermittent imported outbreaks. Identification of those infected, followed by isolation of those infected and exposed, is critical. Today the US has fallen far short of this goal. Vaccines will make the job of eliminating Covid-19 easier, but cannot do it alone. Failing Covid-19 eradication, I fear we are in for another dark winter—perhaps not as dark as today, but one we all must do our best to avoid.
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