Other Unknowns And A Tentative Bottom Line
While these are the most important sources of uncertainty about herd immunity and the existing data on the existing vaccines for SARS-CoV-2, other factors could affect herd immunity:
- It is possible that the virus could evolve to be more transmissible , increasing R0 and thus f*.
- It is possible that the virus could evolve to partially or fully escape immunity from the vaccine, reducing x and thus increasing f*.
- It is possible that lasting change in our behavior could reduce our degree of contact, making a permanent change in transmission conditions such that R < R0 even without vaccine, which would reduce f*.
- It is possible that our patterns of vaccination will be nonrandom, as has been proposed by most health authorities. Vaccinating those who are not particularly important in transmission could increase the proportion who need to be vaccinated, while vaccinating key transmitters could reduce it. Most prioritization schemes include individuals in both groups .
- Immunity from natural infection, to the extent it is protective against infection and transmission , will supplement that from vaccination, though to a declining degree in a well-vaccinated population . This will at least temporarily reduce the requirement for vaccination.
Think Global Act Local
Dr. Juthani says she believes herd immunity can be achieved, but that it may take several years.
We need to get the rest of the world vaccinated, and that is happening more slowly than we would like, she says. The current outbreak in India is also putting a wrench in global vaccination plans since many vaccines are mass produced in India.
While the vaccination pace in the U.S. has been swift, that is not the case worldwide. As of May 2, 43.9% of the U.S. population have had at least one vaccine dose and 30.9% are fully vaccinated. Worldwide, only 3.5% are fully vaccinated.
There are several reasons to be concerned about COVID in other countries, Dr. Omer says, even if we think it doesnt directly affect us.
There is a responsibility that comes with the resources that each country has. We will be judged by our children and grandchildren in terms of how we behaved in this pandemic, he says, noting that what is happening in India and in other places, is happening on our watch. We are privileged to live in a country where these vaccines are available. Everyone should go out and get it.
And, there is also the case of enlightened self-interest, Dr. Omer adds.
Right now, the dam is holding, he says. But why test the dam by tempting it with these large tidal waves coming through?
Vaccination Alone Wont Get Us Herd Immunity
Some vaccination programs alone can achieve herd immunity, or resilience, meaning the virus wont spread easily and exponentially, in the absence of masks, contact tracing and the other measures we have used during the pandemic.
But given the Delta variant means an average infected person infects five others without any other measures in place, and given vaccines are not perfect, Australia would need 90 percent of adults and children vaccinated to achieve herd immunity . This is unlikely.
There will be some waning in vaccine immunity over time, and new variants for which Pfizer and AstraZeneca are less effective. However, even with 100 percent of the population vaccinated, herd immunity may not be achieved by vaccination alone until booster vaccines become available.
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Read The Latest On Covid
Get reliable information on developments in the authorization, distribution and administration of COVID-19 vaccines.
Herd immunity occurs when a significant portion of a population becomes immune to an infectious disease, limiting further disease spread. For those who are not immune, they are indirectly protected because the ongoing disease spread is small. Learn more from this JAMA Patient Page, What is Herd Immunity?
Some epidemiologists have estimated that 70% of the worldwide population would need to be vaccinated against COVID-19 to establish herd immunity against the disease thats already killed over 620,000 Americans. But, with the Delta variant driving up cases among the unvaccinated as well as causing breakthrough COVID-19 infections, the threshold for herd immunity now seems to be higher.
Two AMA members took the time to discuss what patients need to know about herd immunity to clear up some of that misinformation. They are:
- Peter Hotez, MD, PhD, dean of the National School of Tropical Medicine and professor of pediatrics and molecular and virology and microbiology at Baylor College of Medicine and Texas Childrens Hospital in Houston.
- Thompson W. Liddell, MD, an infectious disease specialist at Hattiesburg Clinica member of the AMA Health System Program.
Uncertainty In X Part 2
The efficacy of vaccines against SARS-CoV-2 has been measured very rapidly, with the consequence that most of the data come from a period very shortly post vaccination. It is completely normal for vaccination to produce antibody concentrations that decline with time, rapidly over months and then more slowly over years , sometimes with a concomitant decline in protection. In other vaccines, the antibody concentration needed to protect in the nasopharynx is higher than that needed to prevent disease . So it is possible that all of the vaccine efficacy measures, including those against transmission, will decline with time since vaccination.
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Q: How Much Of The Population Will Need To Be Vaccinated Against Covid
A: A vaccinated person acts as a barrier to slow and prevent the virus from continuing to spread. The ultimate end goal is to get as many people vaccinated as possible so that more people are protected. The entire population benefits from a high vaccination coverage, especially those who are most at risk and vulnerable.
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Once a population reaches a point of collective immunity where the disease is no longer likely to spread, it reaches the herd immunity threshold. The estimate for COVID-19 is that roughly 70% to 85% of the population will need to be vaccinated to reach the herd immunity threshold, although this will likely be a moving target as we move into different stages of the pandemic.
Its a safer place to live when you have a high vaccination rate and we hope that everyone will take the vaccine when theyre eligible to receive it.
Serpil Erzurum, MD, Chair of the Lerner Research Institute.
Will It Ever End
Many scientists have been pessimistic from the start about the chances of us reaching the herd immunity threshold and eradicating Covid-19, even with the swift development of effective vaccines.
Professor Paul Hunter from the University of East Anglia said: Our grandchildrens grandchildren will be getting the infection. Looking at the other coronaviruses, we can expect to get repeat infections every four to six years, probably more frequently this decade.
But dont despair. We know that second infections and infections after immunization are usually less severe than first infections, so the vast majority of these infections will be asymptomatic or mild common cold.
Professor Willem van Schaik from Birmingham University told FactCheck: The focus on herd immunity has been somewhat confused from the start of the pandemic as the underlying assumption of herd immunity claims was that those that had antibodies to SARS-CoV-2, either through vaccination or prior infection, would be entirely protected from re-infection and would no longer transmit the virus.
This is true for a number of infectious diseases but SARS-CoV-2 is a different type of virus that can still infect people that have antibodies. Importantly, upon infection symptoms will be generally mild in these individuals but they will still be able to transmit the virus.
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Reaching Herd Immunity Will Be Difficult But There Is Hope
Lauren Ancel Meyers, who manages the Covid-19 Modeling Consortium at the University of Texas, listed off the reasons to CNN why getting to herd immunity will be so difficult. Due to the scope of the contagion and the lack of vaccines to inoculate the world population there will be reservoirs of the disease. In those areas, if the virus is free to run amuck without control mutations will develop which in turn could lessen the efficacy of the vaccines.
Even in the US which is now awash in vaccine doses these pockets can exist in the population that refuses to get vaccinated. Currently, about a quarter of the population is hesitant or unwilling to get vaccinated. And relying on getting infected and riding it out to get immunization isnt a guarantee. As with the vaccine, the length of time after the primary infection that a person will have immunity against reinfection is not fully known. Researchers are studying booster shots for the covid-19 vaccines.
New CDC report: Coronavirus infections could be under control this summer in the U.S. if the vast majority of people get vaccinated and continue with precautions against viral transmission.
Kyle Griffin May 6, 2021
What Is An Example Of Herd Immunity
Measles provides a good example. It is a highly contagious infectious disease for which we have very effective vaccines. Public health experts say if 93-95% of the population is immune to measles, a feasible target with the available vaccine, then that will protect the entire population.
This herd or community immunity acts as a barrier against the disease, with the immune people breaking the potential chain of transmission so those vulnerable populations are unlikely to get it.
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If I Already Had Covid
The short answer is: yes. It is rare to be re-infected by COVID-19 but because the CDC does not yet know how long a person who was infected with COVID-19 will stay immune, they suggest getting the vaccine anyway.
But, if you were treated for COVID-19 using monoclonal antibodies or convalescent plasma, you should wait 90 days before getting the COVID-19 vaccine, the CDC says.
How Is The Vaccine And Booster Rollout Going
So far, more than 51 million people have had a first vaccine dose – some 90% of over-12s. More than 47 million – 82% of over-12s – have had both doses.
While uptake of first and second doses has dropped off, there has been a steep rise in people having boosters.
About 32 million booster doses have been administered across the UK, with a daily average of almost 890,000 jabs.
Vaccination rates have now levelled off in every age group in England apart from the youngest bands, as the chart below shows.
The highest rates of vaccination can be seen in the oldest age groups – among the first to be vaccinated.
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Vaccines Are Still Working
Scientists say its important to understand that vaccination is still helping to slow the spread of Covid.
Dr Louise Dyson from the University of Warwick told FactCheck: The herd immunity threshold is the point at which there is enough immunity in the population that we could lift all measures and still have cases falling so R is less than one.
That threshold is very difficult to know precisely, and I would urge caution about quoting a given percentage of the population as a herd immunity threshold.
However, even without reaching the herd immunity threshold, the amount of immunity in the population still has an effect. The more immunity there is, the fewer infections we will see at the same level of measures.
Professor Graham Medley from the London School of Hygiene & Tropical Medicine, the Chair of the SPI-M group of expert advisors to the government, said: Its not an on/off process herd immunity kicks in as immunity increases.
People in England are approximately making contact with about half the people that we were pre-pandemic.
We estimate that at full contact rates and no immunity the R for Delta would be about 7. At half the contact rates, we would expect R to be about 3.5.
But we are actually seeing R about 1.4, and the difference is immunity so immunity is reducing the R value by about 60 per cent. Without immunity we would be having a huge epidemic.
Why We Need Vaccines & How Chickenpox Is Different From Covid
Vaccines are typically required to achieve herd immunity. Childhood illnesses such as measles, mumps, diphtheria, polio, chickenpox, etc., all reached herd immunity through this approach. We have seen measles outbreaks in communities who have lost herd immunity due to anti-vaccine movements.
Prior to vaccines those diseases had a level of herd immunity in adult populations, but outbreaks regularly occurred in children and the immunocompromised.
When I was a child, some parents would hold chickenpox parties to achieve immunity. While chicken pox can cause severe disease, the rate of severe illness is much lower than with COVID-19. About 150 people die from chicken pox each year and the virus has also been linked to birth defects if pregnant women get infected. These statistics do not include shingles, which is a reactivation of the chickenpox virus later in life that may occur in adults with compromised immune systems. Luckily, shingles is normally caught and treated early, so it accounts for less than 100 deaths a year. Even with these low rates, the CDC recommends vaccination rather than natural infection.
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% Of Us Need To Be Vaccinated To Kill Off Delta
The Plotkin’s textbook formula for community immunity through vaccination involves a bit of long division:
/ % vaccine efficacy
/90% = 95%
Or, assuming a slightly lower R0 of 5: /90% = 90%
Any way you do this math with Delta, around nine out of ten of us need to get vaccinated in order to achieve a level of widespread disease protection. Other leading experts agree with the calculation, putting the figure for herd immunity well above 80%.
Even though some Americans who aren’t vaccinated have had prior infections, which confers some immunity, “you really still need to have at least another 25% to 30% of people who have not been naturally infected or immunized to be immunized. You need that,” Offit said.
Plus, “there is just a certain group of people who are not going to get vaccinated,” he added. “What do you do then? I think the answer is what we’re doing, which is mandate vaccines.”
If we get somewhere near 90% vaccination in the US, we’ll have the same level of protection from COVID-19 that we already do for many other vaccine-preventable diseases, like measles, polio, chickenpox, and tetanus.
“Then you’ll see, I think, a pretty dramatic decline in the incidence of the disease,” Offit said. “I mean, we’re not going to eliminate it from this country, but we will get to a point where we’re comfortable that the incidence of cases and deaths is low enough that we don’t feel we need to change our life anymore.”
Five Reasons Why Covid Herd Immunity Is Probably Impossible
As COVID-19 vaccination rates pick up around the world, people have reasonably begun to ask: how much longer will this pandemic last? Its an issue surrounded with uncertainties. But the once-popular idea that enough people will eventually gain immunity to SARS-CoV-2 to block most transmission a herd-immunity threshold is starting to look unlikely.
That threshold is generally achievable only with high vaccination rates, and many scientists had thought that once people started being immunized en masse, herd immunity would permit society to return to normal. Most estimates had placed the threshold at 6070% of the population gaining immunity, either through vaccinations or past exposure to the virus. But as the pandemic enters its second year, the thinking has begun to shift. In February, independent data scientist Youyang Gu changed the name of his popular COVID-19 forecasting model from Path to Herd Immunity to Path to Normality. He said that reaching a herd-immunity threshold was looking unlikely because of factors such as vaccine hesitancy, the emergence of new variants and the delayed arrival of vaccinations for children.
Long-term prospects for the pandemic probably include COVID-19 becoming an endemic disease, much like influenza. But in the near term, scientists are contemplating a new normal that does not include herd immunity. Here are some of the reasons behind this mindset, and what they mean for the next year of the pandemic.
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New Variants Change The Herd
Even as vaccine roll-out plans face distribution and allocation hurdles, new variants of SARS-CoV-2 are sprouting up that might be more transmissible and resistant to vaccines. Were in a race with the new variants, says Sara Del Valle, a mathematical and computational epidemiologist at Los Alamos National Laboratory in New Mexico. The longer it takes to stem transmission of the virus, the more time these variants have to emerge and spread, she says.
Brazil began widespread distribution of Sinovac Biotechs CoronaVac vaccine in January.Credit: Rodrigo Paiva/Getty
Theres another problem to contend with as immunity grows in a population, Ferrari says. Higher rates of immunity can create selective pressure, which would favour variants that are able to infect people who have been immunized. Vaccinating quickly and thoroughly can prevent a new variant from gaining a foothold. But again, the unevenness of vaccine roll-outs creates a challenge, Ferrari says. Youve got a fair bit of immunity, but you still have a fair bit of disease, and youre stuck in the middle. Vaccines will almost inevitably create new evolutionary pressures that produce variants, which is a good reason to build infrastructure and processes to monitor for them, he adds.
Good News: Mixing Covid Jabs Has Good Immune Response Study Finds
A UK vaccine trial that studied the use of different combinations of approved COVID vaccines for first and second immunisation doses, has found that mixing brands gives good protection against the virus.
The Oxford Vaccine Groups Com-Cov study followed 830 participants aged over 50. It looked at the efficacy of two doses of the Pfizer-BioNTech vaccine, two of Oxford-AstraZeneca, or of one of them followed by the other the doses were given four weeks apart.
It looked at which combinations gave the best immediate neutralising antibody response and which gave the best T-cell response this is needed for long-lasting protection and allows the immune system to kill the coronavirus should the vaccinated person come into contact with it later on.
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