Contagiousness And Vaccine Effectiveness Drive The Equation
Taken from the go-to physicians’ manual for shots, Plotkin’s Vaccines, the equation is simple, and driven by two factors. One is the contagiousness of a disease. This is what scientists refer to as R0 and it measures how quickly the disease can spread.
Because the Delta variant is about twice as contagious as other versions of the virus, it more easily spreads from person to person. So, R0 is up.
Vaccine expert Paul Offit from Children’s Hospital of Philadelphia, a co-author of the Plotkin’s vaccine handbook, uses an R0 somewhere between five and seven for Delta, similar to Centers for Disease Control and Prevention estimates.
The second factor to consider in our immunity equation is how well a given immunization works at beating the virus.
“Obviously, the more effective the vaccine, the fewer people you would need to vaccinate,” Offit said.
That’s part of the reason disease experts were thrilled when some of the first vaccines authorized during the pandemic turned out to be around 95% effective at preventing illness. But those blockbuster vaccine effectiveness numbers have started to come down, in part because vaccine-induced immunity wanes over time, but mainly because more evasive variants like Delta arrived before enough people were vaccinated to stop them, just as everyone started taking off their masks.
Offit says a rough and “generous” estimate for our current vaccine effectiveness, given these recent developments, is around 90%.
Uncertainty In X Part 1
The primary endpoint in all randomized clinical trials of SARS-CoV-2 vaccines to date has been reduction in COVID-19 symptomatic disease caused by SARS-CoV-2. This is the endpoint on which the estimates of ~95% have been obtained for the two mRNA vaccines. Vaccines can have three different kinds of beneficial effects . They can reduce:
- Susceptibility to infection. This means that a vaccinated person exposed to the virus will remain uninfected with greater probability than if they had been unvaccinated. The efficacy component that reflects this activity is called vaccine efficacy for susceptibility, or VES.
- Progression to symptoms. This means that a vaccinated person who does become infected is less likely to experience symptoms than if they had been unvaccinated and become infected. This component is known as vaccine efficacy against progression, VEP.
- Infectiousness to others. This means that a vaccinated person who becomes infected will infect fewer other people than if they had been unvaccinated and become infected. This component is known as vaccine efficacy against infectiousness, VEI.
The primary endpoint doesnt tell us anything about VEI and while it gives us some information about likely values of VES, it doesnt rule out any of them .
Why That Calculation May Be Too Simple
That calculation may be too simple because its assumptions about the inputs x and R0 may be wrong, and in particular, overly optimistic. It may also be too simple because while it assumes x and R0 are fixed numbers, they may actually vary. In particular, the protection against infection may x decline over time, and R0 likely varies from location to location. There are also some factors that could help, reducing the proportion we have to vaccinate to achieve Rvac < 1.
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Its Unclear Whether Vaccines Prevent Transmission
The key to herd immunity is that, even if a person becomes infected, there are too few susceptible hosts around to maintain transmission those who have been vaccinated or have already had the infection cannot contract and spread the virus. The COVID-19 vaccines developed by Moderna and PfizerBioNTech, for example, are extremely effective at preventing symptomatic disease, but it is still unclear whether they protect people from becoming infected, or from spreading the virus to others. That poses a problem for herd immunity.
Herd immunity is only relevant if we have a transmission-blocking vaccine. If we dont, then the only way to get herd immunity in the population is to give everyone the vaccine, says Shweta Bansal, a mathematical biologist at Georgetown University in Washington DC. Vaccine effectiveness for halting transmission needs to be pretty darn high for herd immunity to matter, she says, and at the moment, the data arent conclusive. The Moderna and Pfizer data look quite encouraging, she says, but exactly how well these and other vaccines stop people from transmitting the virus will have big implications.
At Least 85% Need To Be Vaccinated
What we need to do is we need to get 85% of the U.S. population vaccinatedwe need all of the adolescents and all of the adults, said Dr. Hotez, noting that except for a few areas in the Northeast and maybe Pacific Northwest, it’s not happening.
In fact, the opposite’s happening in the South and that’s why we’re seeing Delta rage through the South right now, he added. Its going to be really problematic to get through a school year safely in many parts of the South unless we can fix these vaccination rates.
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Herd Immunity Can Help Prevent New Dangerous Variants
Bhayani cautioned that new variants mean the bar to reach herd immunity could get even higher, and its critical that more people get vaccinated.
If everyone does their part in getting the vaccine, and boosters once available, we can strive to reach herd immunity, said Bhayani.
Grosso emphasized that this is the only way out of the pandemic, and the longer it takes to reach herd immunity through vaccination, the greater risk of dangerous variants emerging.
The longer this takes, the likelier it is that new variants will continue to emerge, some of which may be more transmissible or more resistant to the current vaccines or both, he warned. If we cant get more people immunized, it is possible that this vicious cycle will continue for quite a while.
How Did We Get To 90 Percent
The Delta variant has an R0 of about 5.0 under pre-COVID-19 ways of living. This is twice that of the original Wuhan virus which had an R0 about 2.5.
For an R0 of 5.0, theoretically, 80 percent of the population have to be immune not just vaccinated for virus transmission not to take off.
Picture Credit: Business Standard
But the actual vaccine coverage required is higher, as the vaccines arent 100 percent effective at stopping any infection. And even though a person has been vaccinated, it doesnt mean theyre always immune from the virus, as the person may not develop a strong immune response.
Vaccination with two doses of Pfizer among adults aged 16-60 years old is likely about 80 percent successful at stopping any Delta infection. For those 60 or older, the effectiveness of AstraZeneca at reducing the risk of any infection is less: about 60 percent against the Delta virus.
Over half, 58 percent, of the population are aged 16-59, and 23 percent of the population are aged 60-plus. So for 80 percent vaccination coverage of adults, the estimated percentage of the population who are immune is 48 percent well short of the 80 percent herd immunity threshold.
For those still at risk of getting infected after either Pfizer or AstraZeneca vaccination, they are 50 percent less likely to transmit it. So the 16 percent of the population who were vaccinated but can still get infected are half as likely to pass it on.
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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?
When Doesnt Herd Immunity Work
Herd immunity doesnt work once the percentage of immunised people in a population drops below the herd immunity threshold, for example, if less than 95% of people in the community are immunised against the measles. In such a situation, exposure to a contagious disease could spread very quickly throughout the community.
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Widespread Vaccination Is Key
Vaccination is important in halting vaccine preventable diseases, said Dr. Hotez. The most dramatic example was, from my perspective, Haemophilus influenzae type BHib meningitis.
When I was a house officer in Boston in the eighties, I was admitting a very sick child with bacterial meningitis to my service every couple of weeks, he added. And over a period of a couple of years, the disease more or less disappeared because of widespread vaccination.
That means that vaccinating large cohorts works. It really can halt transmission, said Dr. Hotez. By the time I was an attending physician I would practically teach about Hib meningitis for mostly historic interest.
The goal of vaccinating is not really herd immunity. That ultimately is a good goal, but the goal of vaccinating right now with COVID is to protect the people around you and yourself, said Dr. Liddell. If we really could vaccinate on a large scale, then yes we would have that barrier and we would be able to have herd immunity and we wouldnt be as sick from this.
If it passes through everybody, then it is going to impact a larger amount of people, but if we can keep it controlled, the amount of people who get sick, the amount of people who die is much less, he said, adding whether or not we achieve herd immunity, every time someone gets the vaccine everyone’s safer.
How Have We Been Tracking
We are close to meeting our aspirational target of 95% for all age groups, having almost achieved 95% coverage for one year olds and five year olds. We are currently tracking at 92.60% coverage for all two year olds, a slight increase on the last quarterly report.
Our successes so far:
- 96.83% of Aboriginal and Torres Strait Islander five year olds are covered.
- Coverage rates for all one year olds are at 94.61%.
- We have almost reached the aspirational 95% for all 5 year olds.
For detailed data, take a look at:
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People Who Depend On Herd Immunity
Some people in the community rely on herd immunity to protect them. These groups are particularly vulnerable to disease, but often cannot safely receive vaccines:
- People without a fully-working immune system, including those without a working spleen
- People on chemotherapy treatment whose immune system is weakened
- People with HIV
- Newborn babies who are too young to be vaccinated
- Elderly people
- Many of those who are very ill in hospital
For these people, herd immunity is a vital way of protecting them against life-threatening disease. See this article about herd immunity written by a parent of four boys who have primary immune disease , which begins: Herd immunity or, as I much prefer, community immunity is not just a vague idea for my family: it is literally what keeps my kids from getting sick…”
Why Is Herd Immunity Important
While it may be unrealistic to assume every individual in a population is able to get vaccinated, those outstanding individuals will then rely on herd immunity to avoid contracting a virus.
Reasons for not receiving a vaccination include:
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People with weak immune systems
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People that are going through chemotherapy treatments
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Newborn babies and elderly people
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People living with HIV
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Those who refuse vaccination
In every community, you will find individuals who fall under the above categories, making herd immunity that much more important. These people then depend on others getting vaccinated in order to be indirectly protected by them.
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How Many People Does South Africa Need To Vaccinate
Lets get into the specifics of how different vaccines would impact national roll-out plans.
South Africa has secured doses of the Johnson & Johnson vaccine along with the Pfizer jab, which recently received emergency use approval from the South African Health Products Regulatory Authority .
The Johnson & Johnson vaccine initially announced an efficacy of 57% in South Africa, where the majority of cases were as a result of the 501Y.V2 variant. A new analysis shared by the United States Food and Drug Administration on 26 February, shows that the jab actually has a higher efficacy of 64% in South Africa.
So with South Africas plan to immunise 67% of the population, the Johnson & Johnson vaccine would give you a population immunity level of 43%.
The Pfizer vaccine, which will also be used for South Africas roll-out, has a 95% efficacy, as previously mentioned. The Pfizer jab would therefore provide roughly 64% immunity with our current target of vaccinating two-thirds of the population but we dont yet have trial data on how the vaccine performs against the new variant identified in the country. Lab studies, however, indicate that the 501Y.V2 variant could lower the Pfizer jabs efficacy , which means the immunity provided on a population level is likely to be lower than 64% in South Africa.
Vaccination alone wont guarantee the end of the pandemic but we have the ability to reduce the spread of the virus locally, explains Schoub.
Herd Immunity Can Be Confusing
Technically what herd immunity means is, depending on the level of transmissibility of an infectious agent, if enough people become infected and recoverand therefore are partially immuneor if they get vaccinated, it means that that will diminish, reduce or even halt virus transmission in the community, Dr. Hotez explained.
Herd immunity is a confusing topic because its different for different things, said Dr. Liddell. But the general idea of herd immunity is that you have protection in a community.
Even though a person may not have immunity for whatever reasonmaybe they don’t have a strong immune system, they’re unable to get the vaccine or children are the obvious example right nowwhat we do is we surround them, he said. We insulate them from the virus by having everybody around them have immunity.
The best example is measles. Measles is one of the most transmissible agents, said Dr. Hotez. We know it has a reproductive number of between 12 and 18, and what that means is if you have over 90% of the population immuneeither because of infection or vaccinationit can halt transmission. This has been more or less the situation for the last two decades in the U.S, until measles transmission resumed in 2019 due to local declines in immunization rates due to antivaccine targeting of parents.
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Vaccine Herd Immunity Explained: How Are Vaccinations Related To Herd Immunity
Herd immunity is reliant on immunity from a disease. Immunity can be achieved in one of two ways: prior infection or vaccination. In many cases, relying on prior infection is an unrealisticand even dangerousway to reach herd immunity.
Thats why vaccines are essential in the fight to establish herd immunity. Illnesses like polio and chickenpox are now very rare due to the impact of vaccine herd immunity.
Wide-spread vaccination makes large swaths of the population immune to a given disease, dramatically reducing the potential to transmit it to others, and ultimately preventing an outbreak from happening.
In cases where vaccinations cannot get to every person in the population, herd immunity makes it difficult for the disease to spread from person to person, protecting those that cannot access vaccinations.
The specifics of herd immunity will often depend on the nature of the disease or virus in circulation. Certain diseases such as Ebola, influenza, and COVID-19 spread easily and swiftly. Other diseases require more unique circumstances for contraction.
Depending on the nature and level of contagion of a given disease, public health professionals will determine the total percentage of people in a population that must receive the vaccine to help facilitate herd immunity.
Coronavirus Disease : Herd Immunity Lockdowns And Covid
‘Herd immunity’, also known as ‘population immunity’, is the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection. WHO supports achieving ‘herd immunity’ through vaccination, not by allowing a disease to spread through any segment of the population, as this would result in unnecessary cases and deaths.
Herd immunity against COVID-19 should be achieved by protecting people through vaccination, not by exposing them to the pathogen that causes the disease. Read the Director-Generals 12 October media briefing speech for more detail.
Vaccines train our immune systems to create proteins that fight disease, known as antibodies, just as would happen when we are exposed to a disease but crucially vaccines work without making us sick. Vaccinated people are protected from getting the disease in question and passing on the pathogen, breaking any chains of transmission. Visit our webpage on COVID-19 and vaccines for more detail.
To safely achieve herd immunity against COVID-19, a substantial proportion of a population would need to be vaccinated, lowering the overall amount of virus able to spread in the whole population. One of the aims with working towards herd immunity is to keep vulnerable groups who cannot get vaccinated safe and protected from the disease. Read our Q& A on vaccines and immunization for more information.
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