I have written several articles on the coronavirus and on masks and healthcare issues. A series of links have been provided at the bottom of this article for your convenience. This article will, however address a different aspect of the virus or on healthcare issues in general.
With the rising number of cases of COVID-19 around the world, health officials continue to work to find the best way to protect the public from the disease. You may have heard health officials mention herd immunity as a possible way to contain the spread of COVID-19.
Herd immunity, or community immunity, is when a large part of the population of an area is immune to a specific disease. If enough people are resistant to the cause of a disease, such as a virus or bacteria, it has nowhere to go.
While not every single individual may be immune, the group as a whole has protection. This is because there are fewer high-risk people overall. The infection rates drop, and the disease peters out.
Herd immunity protects at-risk populations. These include babies and those whose immune systems are weak and can’t get resistance on their own.
How Do You Achieve Herd Immunity?
There are two ways this can happen.
You can develop resistance naturally. When your body is exposed to a virus or bacteria, it makes antibodies to fight off the infection. When you recover, your body keeps these antibodies. Your body will defend against another infection. This is what stopped the Zika virus outbreak in Brazil. Two years after the outbreak began, 63% of the population had had exposure to the virus. Researchers think the community reached the right level for herd immunity.
Vaccines can also build resistance. They make your body think a virus or bacteria has infected it. You don’t get sick, but your immune system still makes protective antibodies. The next time your body meets that bacteria or virus, it’s ready to fight it off. This is what stopped polio in the United States.
When does a community reach herd immunity? It depends on the reproduction number, or R0. The R0 tells you the average number of people that a single person with the virus can infect if those people aren’t already immune. The higher the R0, the more people need to be resistant to reach herd immunity.
Researchers think that the R0 for COVID-19 is between 2 and 3. This means that one person can infect two to three other people. It also means 50% to 67% of the population would need to be resistant before herd immunity kicks in and the infection rates start to go down.
What Are the Challenges to Developing Herd Immunity to COVID-19?
The main obstacle to herd immunity to COVID-19 right now is that the virus that causes the disease is “novel,” or new. That means that it hasn’t infected humans before and everyone is at risk of infection. There’s no existing immunity to build on.
Another potential barrier is that we don’t know how strong the immune protection is or how long it will last in people who’ve had COVID-19. Early research on monkeys showed that they made antibodies to the virus that protected them from a second infection a month later. If the coronavirus is like the flu, we can expect a few months of protection.
While there are now vaccines to protect against COVID-19, It will be months before enough people can receive them. it is hoped, that the vaccines will eventually help bring the spread under control. Researchers estimate that 75-80% of the population would need to be vaccinated before we can have herd immunity.
Why is herd immunity important?
Herd immunity occurs when a large portion 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.
Often, a percentage of the population must be capable of getting a disease in order for it to spread. This is called a threshold proportion. If the proportion of the population that is immune to the disease is greater than this threshold, the spread of the disease will decline. This is known as the herd immunity threshold.
What percentage of a community needs to be immune in order to achieve herd immunity? It varies from disease to disease. The more contagious a disease is, the greater the proportion of the population that needs to be immune to the disease to stop its spread. For example, the measles is a highly contagious illness. It’s estimated that 94% of the population must be immune to interrupt the chain of transmission.
How is herd immunity achieved?
There are two paths to herd immunity for COVID-19 — vaccines and infection.
A vaccine for the virus that causes COVID-19 is an ideal approach to achieving herd immunity. Vaccines create immunity without causing illness or resulting complications. Herd immunity makes it possible to protect the population from a disease, including those who can’t be vaccinated, such as newborns or those who have compromised immune systems. Using the concept of herd immunity, vaccines have successfully controlled deadly contagious diseases such as smallpox, polio, diphtheria, rubella and many others.
Reaching herd immunity through vaccination sometimes has drawbacks, though. Protection from some vaccines can wane over time, requiring revaccination. Sometimes people don’t get all of the shots that they need to be completely protected from a disease.
In addition, some people may object to vaccines because of religious objections, fears about the possible risks or skepticism about the benefits. People who object to vaccines often live in the same neighborhoods or attend the same religious services or schools. If the proportion of vaccinated people in a community falls below the herd immunity threshold, exposure to a contagious disease could result in the disease quickly spreading. Measles has recently resurged in several parts of the world with relatively low vaccination rates, including the United States. Opposition to vaccines can pose a real challenge to herd immunity.
Herd immunity can also be reached when a sufficient number of people in the population have recovered from a disease and have developed antibodies against future infection. For example, those who survived the 1918 flu (influenza) pandemic were later immune to infection with the H1N1 flu, a subtype of influenza A. During the 2009-10 flu season, H1N1 caused the respiratory infection in humans that was commonly referred to as swine flu.
However, there are some major problems with relying on community infection to create herd immunity to the virus that causes COVID-19. First, it isn’t yet clear if infection with the COVID-19 virus makes a person immune to future infection.
Research suggests that after infection with some coronaviruses, reinfection with the same virus — though usually mild and only happening in a fraction of people — is possible after a period of months or years. Further research is needed to determine the protective effect of antibodies to the virus in those who have been infected.
Even if infection with the COVID-19 virus creates long-lasting immunity, a large number of people would have to become infected to reach the herd immunity threshold. Experts estimate that in the U.S., 70% of the population — more than 200 million people — would have to recover from COVID-19 to halt the epidemic. If many people become sick with COVID-19 at once, the health care system could quickly become overwhelmed. This amount of infection could also lead to serious complications and millions of deaths, especially among older people and those who have chronic conditions.
How can you slow the transmission of COVID-19?
It’s crucial to slow the spread of the COVID-19 virus and protect individuals at increased risk of severe illness, including older adults and people of any age with underlying health conditions. When possible, get a COVID-19 vaccine. Also, take steps to reduce the risk of infection:
- Avoid large events and mass gatherings.
- Avoid close contact (within about 6 feet, or 2 meters) with anyone who is sick or has symptoms.
- Stay home as much as possible and keep distance between yourself and others (within about 6 feet, or 2 meters) if COVID-19 is spreading in your community, especially if you have a higher risk of serious illness. Keep in mind some people may have the COVID-19 virus and spread it to others, even if they don’t have symptoms or don’t know they have COVID-19.
- Wash your hands often with soap and water for at least 20 seconds, or use an alcohol-based hand sanitizer that contains at least 60% alcohol.
- Wear a cloth face covering in public spaces, such as the grocery store, where it’s difficult to avoid close contact with others, especially if you’re in an area with ongoing community spread. Only use nonmedical cloth masks — surgical masks and N95 respirators should be reserved for health care providers.
- Cover your mouth and nose with your elbow or a tissue when you cough or sneeze. Throw away the used tissue.
- Avoid touching your eyes, nose and mouth.
- Avoid sharing dishes, glasses, bedding and other household items if you’re sick.
- Clean and disinfect high-touch surfaces, such as doorknobs, light switches, electronics and counters, daily.
- Stay home from work, school and public areas if you’re sick, unless you’re going to get medical care. Avoid public transportation, taxis and ride-sharing if you’re sick.
When will the US reach herd immunity and what will it look like?
Herd immunity threshold is likely months away
The Centers for Disease Control and Prevention estimates that more than 83 million people in the United States had Covid-19 by the end of 2020, putting the nation about a third of the way to herd immunity, the point at which enough people are protected against a disease so that it cannot spread through the population. If the pace of vaccinations continues at the current rate, the country could approach herd immunity through a combination of natural immunity and vaccination around June.
Herd immunity at the current pace of vaccinations
The Gallup estimate is based on a survey of 3,104 adults (Dec. 15, 2020-Jan. 3, 2021). Margin of sampling error: ±2% pts. Herd immunity threshold based on February 17 remarks by National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci.
More than 66 million shots have been administered, according to the latest federal data, with nearly 8% of the US population fully vaccinated. Promises from manufacturers indicate that the US should have enough vaccine supply to cover everyone by June. More than a quarter of the population may already have natural immunity after previous infection — and that number may be much higher than official counts show. However, some new variants threaten progress, potentially lessening protection offered by vaccines and skirting some degree of natural immunity. Vaccine hesitancy may also create some limitations.
To understand how these factors may play into the future timeline of the pandemic, five experts were consulted: Dr. Arturo Casadevall, chair of molecular microbiology and immunology at Johns Hopkins University; Justin Lessler, associate professor of epidemiology at Johns Hopkins University; Jessica Malaty Rivera, science communications lead at the COVID Tracking Project; Dr. Aneesh Mehta, of the Emory Vaccine Center; and Dr. Christopher Murray, director of the Institute for Health Metrics and Evaluation.
Their responses have been edited below for length and clarity.
There’s still so much to learn about Covid-19. But if you had to put a number on it, what level of population protection is required to reach herd immunity against Covid-19?
Casadevall: I am in the 65-80% range. We all expect that this virus is not different from other viruses and that we will reach a point that a sufficient number of people are immune so that the virus cannot jump any more. It reaches a point that there are so few hosts, so few people that it can jump to, that the epidemic crashes. The number of cases is the threat, and in the declining curve, we see that the number of people it can jump to is dropping.
Lessler: Community immunity is a continuum. As more immunity builds, the disease becomes easier to control. Right now, control measures in place — such as masking and limiting capacity — are offering some protection, which is contributing to the drop in cases. But to get to the magic number where you can drop most of those measures and not worry about large outbreaks — there will still be some — is somewhere between two-thirds and 80%. But additional community immunity gives real benefits even when right at that threshold.
Malaty Rivera: We really need to be vaccinating at least 70% of the population. That seems potentially possible by the end of the year if there are no significant bottlenecks in production or delivery.
Mehta: I’ve seen estimates as low as 65% and as high as 95%. What I’d really like is to make sure we get to at least 75%.
Murray: Respiratory viruses are very seasonal, so the level of immunity required in the summer is much lower than in winter. Covid is less dramatically seasonal than other respiratory viruses, but as we’ve learned, it still is. I would give a summer herd immunity threshold of 65% and for winter 85%.
Individuals who have already had Covid-19 may have some natural immunity after infection. Can we count those people toward herd immunity?
Casadevall: Yes. The number of people known to have reinfection is very low. There have definitely been some cases, and they’ve been documented. But despite all this virus still floating around, people are not getting sick again, and to me that’s really encouraging
Lessler: In the short term, over the next six months or so, I would count everybody who’s been infected and most people who have been vaccinated as immune. There are some cases of reinfection, but they’re mostly among people who had a pretty mild disease in the first infection.
Malaty Rivera: No. Herd immunity can only be discussed in the context of mass vaccination. I’m on the more conservative side and very hesitant to claim that natural immunity is causing a meaningful difference in these numbers.
Mehta: Maybe. The US has had many, many infections, but they’ve been spread out over time. It’s hard to know exactly how many people have been infected, and it appears that certain people lose their immunity some time after infection. The goal should be to protect the population through vaccination.
Murray: I normally would say yes, but some evidence around the variant first discovered in South Africa brings that into question. Also, anything we know about waning natural immunity is purely speculative.
New variants potentially threaten levels of protection, both in terms of vaccine efficacy and possible reinfection. How much does this threat cut into measures of progress to herd immunity?
Casadevall: Variants are the big threat on the horizon. They have the potential to derail things, but I stress potential. We know variants are out there, but there are still very few cases of documented reinfection. What that suggests to me is that variants are not evading immune defense. The most encouraging thing to me is how rapidly that curve is coming down. It gives me hope that we’re going to crash the curve before variants become a threat.
Lessler: There are some variants that appear to be escaping that immunity and changing the equation. In a completely susceptible population, the average person with the original strain would spread the virus to three people, so you need two people to be immune to start decreasing the spread. But with some variants, the average person may spread the disease to around five people, so you need four people to be immune to cut the spread.
Malaty Rivera: The conversation around variants really needs to be around preventing infection. If we keep transmission low, we can keep variants low. As the vaccines roll out, one of the many reasons we’re still wearing masks and continuing to practice mitigation strategies is to buy ourselves time.
Mehta: We think vaccines will provide good immunity to most variants out there, but there are some — particularly the variant first identified in South Africa — that appear to have the ability to get around that and may alter our level of protection. That’s why it’s so important to vaccinate as quickly as we can. The quicker we get to higher levels of protection in our community, the chances for new variants to spread and develop diminishes.
Murray: There isn’t a very clear route to herd immunity if those variants spread, and it’s a very tricky business trying to predict at what pace they’ll spread. But cases are coming down quickly, perhaps even more than we had expected.
Some surveys still show a significant amount of vaccine hesitancy. How will this affect the path to herd immunity?
Casadevall: The number of cases is the threat, and a lot depends on what the vaccine uptake is by next year. The more cases you have, the more virus replication and the more likelihood that a person can be infected. If we keep going the way we are and crash the curve, the likelihood of a bad scenario is reduced.
Malaty Rivera: Anti-vax people are a loud minority. They don’t represent a large group of people in this country, and I don’t think they’ll have tremendous victory when it comes to things like herd immunity. Usually, the impediment to people not taking a vaccine is understanding, but we’ve had major wins with campaigns to address this.
Mehta: It’s really understandable why some people in our communities may have hesitation around the vaccine. What we need to do is continue to do a good job caring for them and being role models, not only by sharing knowledge and taking the vaccine ourselves, but continuing to take precautionary measures such as wearing a mask and practicing good hand hygiene.
Murray: If we can move from 25% of the population not being vaccinated to 10%, that really boosts herd immunity way up. Even if variants aren’t as bad as we fear, it’s still going to be pretty close and vaccinating over the tipping point would make a huge difference.
What’s the bottom line? Generally, when can we expect a return to normal?
Casadevall: No one in this world can tell you what percent of protection is needed or what date it will happen by. My gut is positive, and I do think 2021 is going to be a better year than 2020. Assuming we don’t have a variant that forces new lockdowns, I think the second half of this year will look different. We may lift restrictions in the fall, or maybe a little earlier if the rapid decline continues and the curve stays down.
Lessler: When we get to very, very low numbers of new cases, that’s going to be the sign that things can be fine. As we step back control measures, we should do so slowly and deliberately and watch for any resurgence. Some of this comes down to deciding what is normal, because the fact of the matter is we need to accept the fact that we will be living with this virus forever. It will come back again and again like the flu, but that doesn’t mean that it will always be the scourge that it is now.
Malaty Rivera: Vaccinating 70% of the population is going to be an absolute game changer for us in terms of getting back to what we could do similar to pre-Covid days. I’m optimistic that this fall is going to look very different, and hope that means things like travel and mask-free socializing with loved ones. We’re on the road to getting there if we keep the trends down.
Mehta: Thanksgiving is my favorite holiday, when everyone in the family comes together at my mother’s house. My hope — both personally and professionally — is that it happens before Thanksgiving.
Murray: I really do think it will be a quiet summer. But whether it will come back is an open question, that we won’t know the answer to until maybe December. Two powerful forces — seasonality and the scale of vaccination — combined are going to bring things down pretty steadily, but we’ll have to wait and see.
For this article I have researched extensively the topic herd immunity. When I started this article, I had no hidden agenda, I simply wanted to discover the facts. I want to clarify that I am not a medical doctor, but I do have extensive training and knowledge in the medical field. I have written several articles on the coronavirus, including my flagship article which I keep updated with new information as it becomes available, “The Corona Virus Exposed.” While this virus is a fairly new virus for us, it still has to follow the same rules as any other virus does. Without vaccines, polio, small pox, measles, chicken pox, mumps and many other viruses would still be ravaging our planet. So, do I believe in vaccines, the answer is yes. If we allowed these viruses to follow their normal pattern, millions of people would have died in order for us to build up an immunity to them. Maybe we would never build up an immunity. Look at the flu virus, it is easily spread and mutates frequently, so much so that we need new vaccines every year. I believe if every body received a flu vaccination, I believe that we could eventually eradicate it as well. However there are too many people spreading false rumors on vaccinations, so millions of people refuse to receive the vaccination every year. Many also refuse to get the vaccine for pneumonia, so as a result 100,000s of people die around the world from pneumonia and the flu.
One clarification that needs to be made, if you have been infected by the virus, you don’t need the vaccine. So far we have been lucky with the virus, in that it is has not mutated enough to make the current vaccine ineffective. However, if we don’t eliminate this virus, eventually it will mutate enough that we will eventually need a 2nd generation of vaccines. It is only a matter of time. So get of your asses and get the vaccine as soon as it is available.
webmd.com, “What Is Herd Immunity?” By Arefa Cassoobhoy, MD; mayoclinic.org, “Herd immunity and COVID-19 (coronavirus): What you need to know;” cnn.com, “When will the US reach herd immunity and what will it look like?” By Deidre McPhillips; nature.com, ” The false promise of herd immunity for COVID-19,” By Christie Aschwanden;
How high is the threshold for SARS-CoV-2?
Reaching herd immunity depends in part on what’s happening in the population. Calculations of the threshold are very sensitive to the values of R, Kwok says. In June, he and his colleagues published a letter to the editor in the Journal of Infection that demonstrates this. Kwok and his team estimated the Rt in more than 30 countries, using data on the daily number of new COVID-19 cases from March. They then used these values to calculate a threshold for herd immunity in each country’s population. The numbers ranged from as high as 85% in Bahrain, with its then-Rt of 6.64, to as low as 5.66% in Kuwait, where the Rt was 1.06. Kuwait’s low numbers reflected the fact that it was putting in place lots of measures to control the virus, such as establishing local curfews and banning commercial flights from many countries. If the country stopped those measures, Kwok says, the herd-immunity threshold would go up.
Herd-immunity calculations such as the ones in Kwok’s example are built on assumptions that might not reflect real life, says Samuel Scarpino, a network scientist who studies infectious disease at Northeastern University in Boston, Massachusetts. “Most of the herd-immunity calculations don’t have anything to say about behaviour at all. They assume there’s no interventions, no behavioural changes or anything like that,” he says. This means that if a transient change in people’s behaviour (such as physical distancing) drives the Rt down, then “as soon as that behaviour goes back to normal, the herd-immunity threshold will change.”
Estimates of the threshold for SARS-CoV-2 range from 10% to 70% or even more. But models that calculate numbers at the lower end of that range rely on assumptions about how people interact in social networks that might not hold true, Scarpino says. Low-end estimates imagine that people with many contacts will get infected first, and that because they have a large number of contacts, they will spread the virus to more people. As these ‘superspreaders’ gain immunity to the virus, the transmission chains among those who are still susceptible are greatly reduced. And “as a result of that, you very quickly get to the herd-immunity threshold”, Scarpino says. But if it turns out that anybody could become a superspreader, then “those assumptions that people are relying on to get the estimates down to around 20% or 30% are just not accurate”, Scarpino explains. The result is that the herd-immunity threshold will be closer to 60–70%, which is what most models show.
Looking at known superspreader events in prisons and on cruise ships, it seems clear that COVID-19 spreads widely initially, before slowing down in a captive, unvaccinated population, Andersen says. At San Quentin State Prison in California, more than 60% of the population was ultimately infected before the outbreak was halted, so it wasn’t as if it magically stopped after 30% of people got the virus, Andersen says. “There’s no mysterious dark matter that protects people,” he says.
And although scientists can estimate herd-immunity thresholds, they won’t know the actual numbers in real time, says Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security in Baltimore. Instead, herd immunity is something that can be observed with certainty only by analysing the data in retrospect, maybe as long as ten years afterwards, she says.
Will herd immunity work?
Many researchers say pursuing herd immunity is a bad idea. “Attempting to reach herd immunity via targeted infections is simply ludicrous,” Andersen says. “In the US, probably one to two million people would die.”
In Manaus, mortality rates during the first week of May soared to four-and-a-half times what they had been the preceding year. And despite the subsequent excitement over the August slowdown in cases, numbers seem to be rising again. This surge shows that speculation that the population in Manaus has reached herd immunity “just isn’t true”, Andersen says.
Deaths are only one part of the equation. Individuals who become ill with the disease can experience serious medical and financial consequences, and many people who have recovered from the virus report lingering health effects. More than 58,000 people were infected with SARS-CoV-2 in Manaus, so that translates to a lot of human suffering.
Earlier in the pandemic, media reports claimed that Sweden was pursuing a herd immunity strategy by essentially letting people live their lives as normal, but that idea is a “misunderstanding”, according to the country’s minister for health and social affairs, Lena Hallengren. Herd immunity “is a potential consequence of how the spread of the virus develops, in Sweden or in any other country”, she told Nature in a written statement, but it is “not a part of our strategy”. Sweden’s approach, she said, uses similar tools to most other countries: “Promoting social distancing, protecting vulnerable people, carrying out testing and contact tracing, and reinforcing our health system to cope with the pandemic.” Despite this, Sweden is hardly a model of success — statistics from Johns Hopkins University show the country has seen more than ten times the number of COVID-19 deaths per 100,000 people seen in neighbouring Norway (58.12 per 100,000, compared with 5.23 per 100,000 in Norway). Sweden’s case fatality rate, which is based on the number of known infections, is also at least three times those of Norway and nearby Denmark.
What else stands in the way of herd immunity?
The concept of achieving herd immunity through community spread of a pathogen rests on the unproven assumption that people who survive an infection will become immune. For SARS-CoV-2, some kind of functional immunity seems to follow infection, but “to understand the duration and effects of the immune response we have to follow people longitudinally, and it’s still early days”, Buckee says.
Nor is there yet a foolproof way to measure immunity to the virus, Rivers says. Researchers can test whether people have antibodies that are specific to SARS-CoV-2, but they still don’t know how long any immunity might last. Seasonal coronaviruses that cause common colds provoke a waning immunity that seems to last approximately a year, Buckee says. “It seems reasonable as a hypothesis to assume this one will be similar.”
In recent months, there have been reports of people being reinfected with SARS-CoV-2 after an initial infection, but how frequently these reinfections happen and whether they result in less serious illnesses remain open questions, says Andersen. “If the people who are infected become susceptible again in a year, then basically you’ll never reach herd immunity” through natural transmission, Rivers says.
“There’s no magic wand we can use here,” Andersen says. “We have to face reality — never before have we reached herd immunity via natural infection with a novel virus, and SARS-CoV-2 is unfortunately no different.” Vaccination is the only ethical path to herd immunity, he says. How many people will need to be vaccinated — and how often — will depend on many factors, including how effective the vaccine is and how long its protection lasts.
People are understandably tired and frustrated with imposed measures such as social distancing and shutdowns to control the spread of COVID-19, but until there is a vaccine, these are some of the best tools around. “It is not inevitable that we all have to get this infection,” D’Souza says. “There are a lot of reasons to be very hopeful. If we can continue risk-mitigation approaches until we have an effective vaccine, we can absolutely save lives.”
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