Towards Herd Immunity: Valuation of COVID-19 Vaccines
February 27, 2021 | Article No. 75
Towards Herd Immunity: Valuation of COVID-19 Vaccines
February 27, 2021 | Article No. 75
Ayesha Siddiqua MSc, PhD
Mohit Bhandari MD, PhD
- As of February 19, 2021, 11 COVID-19 vaccines have been approved by at least 1 country.
- Vaccines can help achieve herd immunity as they prevent a virus from spreading - when a virus keeps encountering individuals who have been already protected against infection, the chain of transmission is broken.
- Phase 3 trials have shown promising results for several vaccines (e.g., the Moderna: mRNA-1273 and Pfizer/BioNTech: BNT162b2 vaccines), while phase 3 results are still impending for few vaccines (e.g., the FBRI: EpiVacCorona and Bharat Biotech: Covaxin vaccines) that have already been approved by some countries.
- COVID-19 vaccines not only have unique production and deployment challenges, but many of them are simply unaffordable for low- and middle-income countries.
- Many high income countries have pre-purchased large proportions of vaccines, which raises serious allocation concerns for the rest of the world.
- Herd immunity will not be achieved by only vaccinating selected populations given the transient nature of the spread of SARS-CoV-2 that knows no borders - a COVID-19 outbreak anywhere is an outbreak everywhere in the world.
- Investment in equitable and sustainable vaccine strategies at the global scale will be the only way to win the battle against this pandemic.
“Billions of individuals around the world might not have access to COVID-19 vaccines in 2021, which could prolong the pandemic and raise the risk of further mutations of the virus emerging, possibly undermining the efficacy of existing vaccines.”
Wouters et al (2021) (1)
After a year of the COVID-19 pandemic being characterized by chaos and uncertainty, there is finally some hope on the horizon with the emergence of some successful vaccines. While safe and effective vaccines do indeed hold much promise for curbing this global health crisis, there are currently many factors that prevent vaccines from reaching their true protective potential. It is not enough to have successful vaccines – people have to be willing to take them and they have to be delivered to people in a timely manner. This has not been an easy goal to achieve as vaccine hesitancy for COVID-19 has been on the rise (see our OE Insight “Vaccine Hesitancy: A Top 10 Threat to Global Health”). Different vaccines using different platforms have been approved in different countries – which come with their own unique manufacturing and distribution challenges and may become available to their respective populations at different times. Furthermore, the lack of equity has been a prominent challenge since the beginning of the pandemic (see our OE Insight “A Vexing Virus and The Valuation of Vaccines” for details) which continues till today – with high income countries securing large proportions of vaccines in advance and leaving a far smaller share for low- and middle-income countries (LMICs). Not only does vaccine availability vary by country, but the rollout strategies are also very different across different settings – leading to different proportions of the populations in these countries receiving the vaccine, with direct implications for achieving herd immunity against COVID-19. As these system level barriers prevail, SARS-CoV-2 continues to run its course in an ever-evolving manner. There are now new SARS-CoV-2 variants of concern, with a lack of conclusive evidence regarding whether currently approved vaccines are indeed effective in providing protection against the new variants (see our OE Original “COVID-19 Variants of Concern: Will There Be A Third Wave?”). Despite this dire context, global travel has not been restricted and SARS-CoV-2 and its emerging regional variants continue to spread across borders, highlighting the truly global nature of this crisis that calls for a comprehensive action plan. Achieving herd immunity for COVID-19 does not only require customized local strategies, but also investment in global strategies that are committed to vaccinating everyone regardless of where they live.
“As long as vaccines are fragile, expensive and difficult to distribute, the pandemic will continue.”
“But by far the most important issue, Altmann (Dr. Danny Altmann, Immunologist, Imperial College London) says, is “durability”: how long people remain immune after vaccination. If a vaccine confers immunity for only a few months rather than many years, lile progress will have been made in six months. By then we could be faced with more virulent forms of the disease swirling around the globe.”
Aymes (2020) (2)
First Things First: Status of Current Vaccines
As of February 19, 2021, there were 11 vaccines approved by at least 1 country, 20 vaccines in phase 3 trials, 39 vaccines in phase 2 trials, and 27 vaccines in phase 1 trial (3). Exhibit 1 summarizes the 11 approved vaccines, emphasizing key characteristics of their production, affordability, allocation, and deployment – all of which have direct implications for how fast vaccines get to people around the world and how fast we achieve herd immunity in different populations. In summary, vaccines that are expensive and require additional infrastructure for deployment will be more easily accessed by high income countries. Many high income countries have already pre-purchased large volumes of vaccines.
Countries with approval
Efficacy in phase 3 trials†
Estimated production capacity for 2021
Lowest price offered (US$ per course)
% of doses pre-purchased by high income countries for 2021
|Number of doses||
Storage requirement during transport
|FBRI: EpiVacCorona||Protein subunit||1||--||--||--||--||2||--|
|Moderna: mRNA-1273||mRNA||38||94%||1 bn||$31||97%||2||–20°C|
|Pfizer/BioNTech: BNT162b2||mRNA||60||95%||2 bn||$14||77%||2||–70°C|
|CanSino: Ad5-nCoV||Non replicating viral vector||3||65.7%||320 m||--||0%||1||2–8°C|
|Gamaleya: Sputnik V||Non replicating viral vector||26||92%||1 bn||$6||0.4%||2||–18°C|
|Oxford/AstraZeneca: AZD1222||Non replicating viral vector||48||62%*||3 bn||$5||27%||2||2–8°C|
|Serum Institute of India: Covishield||Non replicating viral vector||11||--||--||--||--||2||--|
|Bharat Biotech: Covaxin||Inactivated||1||--||700 m||$6||0%||2||2–8°C|
|Sinopharm (Beijing): BBIBP-CorV||Inactivated||14||79%**||1 bn||$62||8%||2||2–8°C|
|Sinopharm (Wuhan): Inactivated||Inactivated||2||--||600 m||$62||8%||2||2–8°C|
|Sinovac: CoronaVac||Inactivated||7||50-91%**††||1 bn||$21||18%||2||Room temperature|
“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.”
World Health Organization (2020) (5)
“Herd immunity happens when a virus can't spread because it keeps encountering people who are protected against infection.” “High levels of vaccination-induced immunity in the population benefits those who can't receive or sufficiently respond to a vaccine, such as people with compromised immune systems. Many medical professionals hate the term herd immunity, and prefer to call it “herd protection”… That's because the phenomenon doesn't actually confer immunity to the virus itself — it only reduces the risk that vulnerable people will come into contact with the pathogen.”
“Estimates of the (herd immunity) 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…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” (Dr. Samuel Scarpino, Network Scientist, Northeastern University). 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” (Dr. Scarpino)”.
Aschwanden (2020) (6)
“If vaccines were to prove notably less effective against a (new SARS-CoV2) variant, it would require vaccinating an even greater portion of the population or updating existing vaccines to make them more effective…scientists are still studying the situation, and worry about further mutations.”
Choi & Ghosal (2021) (7)
A Primer on Herd Immunity: Looking at the Nuances
Before we can fully appreciate the complex process for achieving herd immunity for COVID-19, it is important for us to review some basic concepts about this phenomenon. A detailed discussion on herd immunity, as well as the advantages and disadvantages of achieving herd immunity using vaccines or through natural disease transmission has been discussed in a previous OE Insight. Below is an overview of the basic concepts about herd immunity:
- World Health Organization (WHO) definition: “The indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection”.
- Herd immunity occurs when those who are vaccinated or those who have developed immunity to a disease through exposure extend an “umbrella of immunity” to those who have not.
- When a substantial proportion of a population achieves immunity, disease transmission is limited to a small proportion of the population.
- Herd immunity threshold: The proportion of the population that must be vaccinated to achieve herd immunity.
- Factors influencing herd immunity threshold:
- Reproduction number of viruses (R0 or R-naught): The higher the reproduction number, the higher the herd immunity threshold.
- Effectiveness of vaccines: The higher the effectiveness of vaccines, the lower the herd immunity threshold.
- Factors influencing herd immunity threshold:
In reality, calculating herd immunity is not as easy as plugging in numbers in a formula and deriving a threshold that can be expected to remain constant over time. Instead, epidemiologists recommend that it is more appropriate to think of herd immunity as a gradient rather than a steady state (6). This is precisely due to the fact that R0 assumes that everyone is susceptible to the virus – but this changes through the course of an epidemic, where some people become infected and develop immunity (6).
There has been some research on the herd immunity threshold for COVID-19, but there is still no consensus on this number (8,9). One of the biggest limitations of this research is that the estimates of this threshold are built on assumptions that may not reflect real life (6). For example, these models do not account for interventions or behavioural changes that can alter the spread of the virus (6). Most of these models show COVID-19 herd immunity threshold to be closer to 60-70%, however, given the highly infectious nature of SARS-CoV-2, it has been also proposed that the required herd immunity threshold could be much higher (10).
“It is also conceivable that “the best” COVID-19 vaccine could not exist at all, as vaccines developed on distinct technological platforms could induce different forms of immunity, each of them appropriate in different environmental and human contexts. The polio vaccine provides an interesting example. The injectable killed Salk vaccine is turning out to be appropriate for the industrialized world: it is safe and effective in areas of the world where polio no longer exists. By contrast, the aenuated Sabin vaccine, more effective and easier to be administered orally, is appropriate for the developing world where the wild virus is still circulating.”
“Thus, the evolution of the pandemic could make some vaccines more appropriate in different geographic contexts or for different clusters (e.g., infants, elderly) of the human population. It is quite possible that the sequential arrival of subsequent wages of vaccines could increase and make more appropriate the protection initially induced by the first vaccines.”
Forni & Mantovani 2021 (11)
“Dr. Walter Orenstein (Professor and Associate Director, Emory Vaccine Center) notes vaccination levels and other factors that affect spread could differ even within a city. In India, for instance, scientists believe that more people will need to be protected in densely populated cities, where the virus spreads faster, than in its vast countryside.”
Choi C & Ghosal 2021 (7)
Multidimensional Threats to Herd Immunity
Beyond characteristics of the virus itself and the vaccines for COVID-19, many other factors have been identified as important determinants of herd immunity for this disease. It has been hypothesized that COVID-19 herd immunity will also vary by the differences between communities, populations prioritized for vaccines, in addition to a wide range of system level barriers (5). Exhibit 2 provides a detailed summary of all these factors, illustrating the many pieces of the puzzle that must come together in order to achieve herd immunity for this new disease. One key takeaway message from our analysis of these factors is that given the different efficacies of vaccines from different companies and their varying expected availability and acceptability in the respective countries where these vaccines have been approved, the process of achieving herd immunity will be very different depending on where we are looking in the world.
|Virus||New SARS-CoV-2 variants of concern||
|Public health interventions||
“Dr. Maria Van Kerkhove (WHO's technical lead of COVID-19) noted that during the summer, COVID-19 cases were down to single digits in most countries across Europe. “We lost the bale because we changed our mixing paerns over the summer, into the fall and especially around Christmas and the new year,” she said, explaining that many people had multiple contacts with family and friends over the holidays. “That has had a direct impact on the exponential growth that you have seen in many countries,” she said, describing the case count increase in some places as “vertical.”
Cheng & Keaton 2021 (10)
“Differences in vaccination levels among countries are also why many experts believe the virus will never be completely stamped out.”
Choi & Ghosal 2021 (7)
“However, because it is unclear which vaccines will be distributed to which countries at what time, it is challenging for governments reliant on COVAX to plan vaccination programmes. Similarly, uncertainty about COVAX supply complicates governments' decisions about how to acquire the best vaccine portfolios for their populations, including doses beyond those covered by COVAX.”
“The success and approval of the first COVID-19 vaccines should not detract from the enthusiasm and practical possibility of planning new studies leading to the development and production of second and thirdgeneration vaccines as well as the design of different types of clinical trials. Indeed, COVID-19 eradication is going to be a long and winding road that will not finish once we have the first vaccine available.”
Forni & Mantovani 2021 (11)
2020 and Beyond
Even after significant investment in developing vaccines and the necessary infrastructure to produce and deploy them around the world, one of the most difficult realities to accept is that COVID-19 is here to stay for the long haul. Since the beginning of the pandemic, epidemiologists have been developing many models based on different assumptions to make short- and longterm projections regarding the spread of SARS-CoV-2. Despite the differences between these models, epidemiologists agree that the future of the pandemic depends on many unknowns – which includes whether people will develop long-term immunity to SARS-CoV-2, whether seasonality will affect its spread overtime, the timing of the introduction a new virus, the crossreactions between the coronaviruses themselves, as well as the decisions made by governments and individuals (13,14). Vaccines can indeed be the magic bullet for eradicating the pandemic by achieving herd immunity globally. However, when vaccines are administered under circumstances, which influence their effectiveness, that are constantly in flux, herd immunity becomes an extremely difficult goal to achieve.
Finish Line is Much Further than We Think
Reflecting on the current state of the pandemic and vaccines, we have identified some key challenges for achieving herd immunity (Exhibit 3), which requires many factors to operate effectively in tandem.
It is undeniable that each country will have their own set of challenges to tackle to disseminate the vaccines, however, LMICs will require a significant amount of support to protect their populations. There may be some unknowns about the current vaccines and there will likely be the need for new vaccines as we move through the future. Yet, the first step towards achieving herd immunity would be administering the vaccines in the first place, and subsequently avoid unnecessary illness, hospitalizations, and deaths. In the words of Dr. Zain Chagla, an infectious disease physician from Hamilton, Canada, “the best vaccine is the one that's administered” – reminding us that a pragmatic approach may be the best one to combat one of the biggest pandemics of our time (15).
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