When will the COVID-19 pandemic end?

Our November 23 update takes on the questions raised by recent news: When will vaccines be available? And is the end of COVID-19 nearer?

November 23, 2020

Since we published our first outlook, on September 21st, the COVID-19 pandemic has raged on, with more than 25 million additional cases and more than 400,000 additional deaths. While the situation looks somewhat better in parts of the Southern Hemisphere, much of Europe and North America is in the midst of a “fall wave,” with the prospect of a difficult winter ahead. Yet the past two weeks have brought renewed hope, headlined by final data from the Pfizer/BioNTech 1 vaccine trial and interim data from the Moderna trial, both showing efficacy of approximately 95 percent 2 ; and progress on therapeutics. Is an earlier end to the pandemic now more likely?

The short answer is that the latest developments serve mainly to reduce the uncertainty of the timeline (Exhibit 1). The positive readouts from the vaccine trials mean that the United States will most likely reach an epidemiological end to the pandemic (herd immunity) in Q3 or Q4 2021. An earlier timeline to reach herd immunity—for example, Q1/Q2 of 2021—is now less likely, as is a later timeline (2022). If we are able to pair these vaccines with more effective implementation of public-health measures and effective scale-up of new treatments and diagnostics, alongside the benefits of seasonality, we may also be able to reduce mortality enough in Q2 to enable the United States to transition toward normalcy. (See sidebar “Two endpoints” for our definitions.)

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Sidebar

A secondary effect of the recent vaccine trials is to make Q3 2021 more likely for herd immunity than Q4. That said, major questions are still outstanding, even about vaccines, such as long-term safety, timely and effective distribution, and vaccine acceptance by the population, to say nothing of lingering epidemiological questions such as the duration of immunity.

These are estimates for the United States, which is likely to have fast and ready access to vaccines. We will consider timelines for other countries in forthcoming updates; they will vary based on the timing of access and distribution of vaccines and other factors. In this update, we review the most recent findings, look deeper at five implications of the ongoing scientific research, and discuss why our timeline estimates have not shifted meaningfully.

Revelations from vaccine and antibody trials

The world has cheered announcements over the past two weeks by Pfizer and its partner BioNTech, and from Moderna. Their COVID-19 vaccine candidates are showing efficacy rates that are higher than many dared hope for. One is a final result, and the other is an initial result whose sample size is large enough to give reasonable confidence in the data. At about 95 percent, efficacy is higher than expected by most experts. 3 It exceeds the optimistic case that we included in our September article. Higher efficacy provides greater benefit to any vaccinated individual and may help to encourage uptake among some segments of the population. It also reduces the fraction of the population required to reach herd immunity. Moderna also announced that its vaccine is more shelf-stable than expected and would need only refrigeration to keep it stable for 30 days—another piece of good news. Finally, there are a number of other vaccines in late-stage trials from which data is expected in the coming months.

Caution is still warranted. The safety records of the Pfizer and Moderna vaccines appear promising so far (no serious side effects reported), but the coming months will provide a fuller picture as the sample size grows. We don’t yet know how long the protection the vaccines offer will last. The Pfizer trial has enrolled some children (ages 12 and older), but efficacy in those under 18 remains unclear.

Beyond vaccines, science is also progressing in therapeutics for COVID-19. For example, Eli Lilly’s antibody bamlanivimab was granted Emergency Use Authorization (EUA) by the US Food and Drug Administration on November 9, 4 and Regeneron’s EUA for its antibody cocktail REGN-COV2 for EUA was approved on November 22. Emerging data on these antibodies suggest that they can reduce the need for hospitalization of high-risk patients, and hold potential for post-exposure prophylaxis. 5 While they are not recommended for use in hospitalized patients, these antibodies add to the growing armamentarium of treatments and protocols for COVID-19, where every incremental advance could help to reduce mortality. Collectively, these treatments and changes in clinical practice have lowered mortality for those hospitalized by 18 percent or more. 6

Looking deep into the data

Research and findings of the past two months have shed light on a number of uncertainties and in some cases have raised new questions. Here we review five implications; each has helped refine our probability estimates for the COVID-19 pandemic timeline.

Vaccine age restrictions elevate coverage requirements to reach herd immunity

It appears that the two vaccines mentioned will be indicated first for use in adults. 7 It’s not clear when use in children will be indicated. One consequence is that the vaccines’ contribution to population-wide herd immunity will depend on adults, at least until vaccines are approved for use in younger populations. If vaccines are efficacious, safe, and distributed to all ages, vaccine coverage rates of about 45 to 65 percent—in combination with projected levels of natural immunity—could achieve herd immunity (Exhibit 2).

Exhibit 2

On the other hand, if vaccines are efficacious but distributed only to adults, who comprise only 76 percent of the US population, 8 then higher vaccine coverage rates—approximately 60 to 85 percent—could be required to achieve herd immunity.

Another consequence is that older children, who have twice the COVID-19 incidence of younger children and who have higher viral loads (and therefore greater potential contagiousness) than adults 9 may not have immediate access to vaccines.

We recognize that calculating herd immunity thresholds is complex. Basic formulas fail to account for variations in the way populations interact in different places. 10 For this reason we include relatively wide ranges.

Unclear impact of vaccines on transmission could raise the bar on coverage

Vaccine trials and regulatory approval will be based on safety and efficacy in reducing virologically confirmed, symptomatic disease among individuals. 11 That’s not the same as reducing transmission. This distinction will have much to say about whether the United States reaches normalcy in Q2 or Q3 of 2021. In practice, we have data on whether people who are vaccinated are less likely to get sick with COVID-19 (and less likely to get severe disease), but we won’t have data on how likely they are to transmit to others. It’s an important distinction because what will drive herd immunity is reduction in transmission. If vaccines are only 75 percent effective at reducing transmission, then coverage of about 60 to 80 percent of the population will be needed for herd immunity. And if a vaccine is only 50 percent effective at reducing transmission, coverage of over 90 percent would be required (Exhibit 3).

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Wide variations in local seroprevalence suggest heterogeneous paths to herd immunity

Improved estimates of seroprevalence are increasingly available for many regions. They vary widely, from as low as 1 to 2 percent in some states like Colorado and Kansas to 14 to 20 percent in New Jersey and New York. 12 Because achieving herd immunity relies in part on a population’s natural immunity, it appears that some locations are closer to herd immunity than others (and have likely also experienced worse impact on public health to date.) Based on a range of likely vaccine scenarios and the fact that those with prior exposure to SARS-CoV-2 will still be eligible for vaccination, every ten percentage-point increase in seroprevalence could roughly translate into a one-month acceleration of the timeline to the epidemiological endpoint.

However, it is possible that areas with higher seroprevalence may also have higher thresholds for herd immunity, because their populations may mix more, 13 which could have contributed to higher seroprevalence to begin with. If that’s true, then while they are further along, they may also have further to go. Well-executed distribution of effective vaccines will still be paramount.

Potentially shorter duration of immunity could prolong the path to the ‘end’

Earlier in the pandemic, it was unclear how long immunity after COVID-19 infection would last. Duration of immunity matters, obviously; for instance, our modeling suggests that if natural immunity to COVID-19 lasts six to nine months, as opposed to multiple years (like tetanus) or lifelong (like measles), herd immunity is unlikely to be achieved unless adult vaccination rates approach 85 percent. While COVID-19 reinfection is documented but rare, there are now population-level studies that question the durability of immunity. Antibody levels may wane after just two months, according to some studies, while a United Kingdom population-monitoring effort reported that antibody prevalence fell by 26 percent over three months. 14 The relationship between waning antibodies and reinfection risk remains unclear. Other research suggests that even with waning levels of COVID-19 antibodies, the immune system may still be able to mount a response through other specific B-cell and T-cell immune pathways, where emerging evidence shows much greater durability after six months. 15

Manufacturing and supply issues are clearer, but have not vanished

If the initial efficacy data from the Pfizer and Moderna vaccine trials hold up, and if no significant safety issues emerge, then initial demand is likely to be high. Two promising candidates are better than one, but supplies will undoubtedly be constrained in the months following EUA and approval. The situation may be dynamic as vaccines are approved at different times, each with its own considerations in manufacturing and distribution. For example, current data suggest that Moderna’s vaccine is stable at refrigerated temperatures (2 to 8 degrees Celsius) for 30 days and six months at –20 degrees Celsius. Pfizer’s vaccine can be stored in conventional freezers for up to five days, or in its custom shipping coolers for up to 15 days with appropriate handling. Longer-term storage requires freezing at –70 degrees Celsius, requiring special equipment. 16 Both Pfizer’s and Moderna’s would be two-dose vaccines, necessitating rigorous follow-up for series completion. These and other complexities create risk of delay. Timelines to reach the desired coverage threshold will be affected by health systems’ abilities to adapt to changing needs and updated information.

The pandemic’s end is more certain, and may be a little nearer

Given all of these variables, where do we net out?

While the winter of 2020/2021 in the Northern Hemisphere will be challenging, we are likely to see mortality rates fall in Q2 (or possibly late Q1) of 2021. Seasonality and associated changes in behavior will begin to work again in our favor in the spring, and the combination of early doses of vaccines targeted to those at highest risk (and the benefits of the Pfizer and Moderna vaccines in reducing severe disease), advances in treatment, expanded use of diagnostics, and better implementation of public-health measures should serve to significantly reduce deaths from COVID-19 in the second quarter. At this stage, when monthly mortality from COVID-19 may start to resemble that of flu in an average year, we may see a transition toward normalcy, albeit with public-health measures still in place.

We are as excited as others about the stunning developments in vaccines. We think Q3 or Q4 of 2021 are even more likely to see herd immunity in the United States. This is based on EUA of one or more high-efficacy vaccines in December 2020 or January 2021, as manufacturers are targeting 17 ; distribution to people at highest risk (healthcare workers, the elderly, and those with comorbidities) in the early months of 2021 18 ;full approval of a vaccine in March or April; and then widespread rollout. Our estimates of three to eight months for manufacturing, distribution, and adoption of sufficient vaccine doses to achieve herd immunity remain unchanged, and suggest that the milestone may be reached between July and December 2021.

Recent developments suggest that herd immunity is less likely to come in early 2021, given that vaccines are arriving roughly on the expected timeline; and the downside scenario stretching into 2022 is also less likely, since efficacy is clearer. The new vaccines may slightly accelerate the timeline—the ongoing surge in cases will likely continue into winter, which would increase natural immunity levels going into Q2. Further, higher-than-expected efficacy may help offset coverage challenges that surveys have suggested. Those two factors could advance the timeline, and make Q3 a little more likely than Q4.

Our estimate is based on the widest possible reading of the current scientific literature and our discussions with public-health experts in the United States and around the world. It’s possible that unforeseen developments such as significantly more infections than expected this winter could lead to earlier herd immunity. And real downside risk remains, especially with respect to duration of immunity and long-term vaccine safety (given the limited data available so far). Herd immunity might not be reached until 2022 or beyond.


Even when herd immunity is achieved, ongoing monitoring, potential revaccination, and treatment of isolated cases will still be needed to control the risk of COVID-19. But these would fall into the category of “normal” infectious disease management—not the society-altering interventions we have all lived through this year. The short term will be hard, but we can reasonably hope for an end to the pandemic in 2021.


Sarun Charumilind and Jessica Lamb are both partners in McKinsey’s Philadelphia office, Matt Craven is a partner in the Silicon Valley office, Adam Sabow is a senior partner in the Chicago office, and Matt Wilson is a senior partner in the New York office.

The authors wish to thank Gaurav Agrawal, Xavier Azcue, Jennifer Heller, Anthony Ramirez, Shubham Singhal, and Rodney Zemmel for their contributions to this article.


This article was edited by Mark Staples, an executive editor in the New York office.




September 21, 2020

Normalcy by spring, and herd immunity by fall? We assess the prospects for an end in 2021.

In 1920, a world wearied by the First World War and sickened by the 1918 flu pandemic desperately sought to move past the struggles and tragedies and start to rebuild lives. People were in search of a “return to normalcy,” as Warren G. Harding put it. Today, nearly every country finds itself in a similar position.

More than eight months and 900,000 deaths into the COVID-19 pandemic, 19 people around the world are longing for an end. In our view, there are two important definitions of “end,” each with a separate timeline:

  • An epidemiological end point when herd immunity is achieved. One end point will occur when the proportion of society immune to COVID-19 is sufficient to prevent widespread, ongoing transmission. Many countries are hoping that a vaccine will do the bulk of the work needed to achieve herd immunity. When this end point is reached, the public-health-emergency interventions deployed in 2020 will no longer be needed. While regular revaccinations may be needed, perhaps similar to annual flu shots, the threat of widespread transmission will be gone.
  • A transition to a form of normalcy. A second (and likely, earlier) end point will occur when almost all aspects of social and economic life can resume without fear of ongoing mortality (when a mortality rate is no longer higher than a country’s historical average) or long-term health consequences related to COVID-19. The process will be enabled by tools such as vaccination of the highest-risk populations; rapid, accurate testing; improved therapeutics; and continued strengthening of public-health responses. The next normal won’t look exactly like the old—it might be different in surprising ways, with unexpected contours, and getting there will be gradual—but the transition will enable many familiar scenes, such as air travel, bustling shops, humming factories, full restaurants, and gyms operating at capacity, to resume.

The two ends are related, of course, but not linearly. At the latest, the transition to normal will come when herd immunity is reached. But in regions with strong public-health responses, normalcy can likely come significantly before the epidemiological end of the pandemic.

The timeline to achieve the ends will vary by location. In this article, we’ll explain the criteria that will be key factors in determining when each is reached. In the United States and most other developed economies, the epidemiological end point is most likely to be achieved in the third or fourth quarter of 2021, with the potential to transition to normalcy sooner, possibly in the first or second quarter of 2021. Every day matters. Beyond the impatience that most feel to resume normal life, the longer it takes to remove the constraints on our economies, the greater will be the economic damage.

The epidemiological end point

Most countries have deferred the hope of achieving herd immunity until the arrival of a vaccine. When herd immunity is reached, ongoing public-health interventions for COVID-19 can stop without fear of resurgence. The timing of the end point will vary by country and will be affected by a number of factors:

  • the arrival, efficacy, and adoption of COVID-19 vaccines—the biggest drivers in the timeline to herd immunity 20
  • the level of natural immunity in a population from exposure to COVID-19; in our estimate, between 90 million and 300 million people globally may have natural immunity 21
  • potential cross-immunity from exposure to other coronaviruses 22
  • potential partial immunity conferred by other immunizations, such as the bacille Calmette–Guérin (BCG) vaccine for tuberculosis 23
  • regional differences in the ways that people mix, which will produce different thresholds for herd immunity

Consider the first and most crucial variables: the arrival of vaccines, their efficacy, and their adoption. We see four plausible scenarios for vaccine efficacy and adoption, illustrated in Exhibit 1. 24 Different combinations of those two factors will drive varying levels of conferred immunity, implying the extent of natural immunity that will be required to reach herd immunity under each scenario. Combinations of efficacy and adoption beyond those shown are possible.

Exhibit 1

The other variables will also have much to say about the timeline to reach herd immunity (see sidebar, “Key factors affecting the timeline to herd immunity”).

Sidebar

Based on our reading of the current state of the variables and their likely progress in the coming months, we estimate that the most likely time for the United States to achieve herd immunity is the third or fourth quarter of 2021. As we wrote in July 2020, one or more vaccines may receive US Food and Drug Administration Emergency Use Authorization before the end of 2020 (or early in 2021) and the granting of a Biologics License Application (also known as approval) during the first quarter of 2021.

Vaccine distribution to a sufficient portion of a population to induce herd immunity could take place in as few as six months. That will call for rapid availability of hundreds of millions of doses, functioning vaccine supply chains, and peoples’ willingness to be vaccinated during the first half of 2021. We believe that those are all reasonable expectations, based on public statements from vaccine manufacturers and the results of surveys on consumer sentiment about vaccines. 25

Herd immunity could be reached as soon as the second quarter of 2021 if vaccines are highly effective and launched smoothly or if significant cross-immunity is discovered in a population (Exhibit 2). (For more on the potential for a faster resolution of the COVID-19 crisis in the United States, see “An optimistic scenario for the US response to COVID-19”.) On the other hand, the epidemiological end of the pandemic might not be reached until 2022 or later if the early vaccine candidates have efficacy or safety issues—or if their distribution and adoption are slow. At worst, we see a long-tail possibility that the United States could be still battling COVID-19 into 2023 and beyond if a constellation of factors (such as low efficacy of vaccines and a short duration of natural immunity) align against us.

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The paths to herd immunity in other high-income countries are likely to be broadly similar to the one in the United States. The timelines will vary based on differences in vaccine access and rollout and in levels of natural immunity—and potentially, in levels of cross-immunity and previous coverage of other vaccines, such as the BCG vaccine. Even as some locations reach herd immunity, pockets of endemic COVID-19 disease are likely to remain around the world, for example in areas affected by war or in communities with persistently low adoption of vaccines. In such places, until herd immunity is reached, COVID-19 might be analogous to measles—not a day-to-day threat to most people, but a persistent risk. If immunity wanes—for example, if booster vaccines are not fully adopted—then COVID-19 could become more widely endemic.

The arrival of herd immunity won’t mean a complete end to all public-health interventions. It’s possible that regular revaccinations would be required to maintain immunity, and ongoing surveillance for COVID-19 will be required. But herd immunity would mean that the emergency measures currently in place in many countries could be lifted.

The pace at which governments relax public-health measures will be critical. Some of those measures (such as full lockdowns and restrictions on certain industries) have significant social and economic consequences, and others (such as testing and tracing), while expensive, don’t. Many governments are employing packages of measures that aim to minimize the number of COVID-19 cases and excess mortality while maximizing social and economic degrees of freedom.

The transition to normal

The second end point of the pandemic may be reached earlier than the first. We estimate that the mostly likely time for this to occur is the first or second quarter of 2021 in the United States and other advanced economies. The key factor is diminished mortality.

Society has grown used to tracking the number of COVID-19 infections (the case count). But case counts matter primarily because people are dying from the disease and because those who survive it may suffer long-term health consequences after infection. The latter is an area of scientific uncertainty, but there is concern that some recovered patients will face long-term effects. 26

Most countries have made significant progress in reducing the numbers of deaths and hospitalizations associated with COVID-19. Some are close to eliminating excess mortality. Those results have generally been achieved through a combination of moderately effective interventions rather than a single “big bang” (Exhibit 3).

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A transition to the next normal, in whatever form that takes, will come gradually when people have confidence that they can do what they used to do without endangering themselves or others. Gaining that confidence will require a continuation of the progress made to reduce mortality and complications, as well as further scientific study regarding long-term health consequences for recovered patients. When confidence is restored, people will again fill bars, restaurants, theaters, and sports venues to full capacity; fly overseas (except for the highest-risk populations); and receive routine medical care at levels similar to those seen prior to the pandemic.

The timing of such a transition will depend on the progress toward herd immunity, as previously detailed (since more people with immunity means fewer deaths and long-term health consequences), and on the effectiveness of a country’s public-health response. Transitions will be gradual. They have already begun in some locations and could be well advanced in most countries by the first or second quarter of 2021. Given the interconnectedness of the global economy, country timelines to normalcy are not fully independent of one another.

To achieve that, we will need to see significant progress on the epidemiological end point, including an effective vaccine receiving Emergency Use Authorization approval during the fourth quarter of 2020 or the first quarter of 2021, followed by a smooth rollout and adoption by high-risk populations. Favorable findings on natural and cross-immunity would help accelerate timelines. Five additional criteria will also contribute to the transition to a form of normalcy—the more of these that are achieved, the faster the milestone is likely to be reached:

  • continued improvement by governments in the application of public-health interventions (such as test and trace) that don’t significantly limit economic and social activities
  • compliance with public-health measures until we achieve herd immunity
  • accurate, widely available, rapid testing that effectively enables specific activities
  • continued advancements in therapeutics (including pre- and postexposure prophylactics) for and clinical management of COVID-19, leading to lower infection-fatality ratios—substantial progress has already been made through a combination of effective drugs, such as dexamethasone and remdesivir, and changes in clinical management
  • public confidence that there aren’t significant long-term health consequences for those who recover from COVID-19

Both the epidemiological and normalcy ends to the COVID-19 pandemic are important. The transition to the next normal will mark an important social and economic milestone, and herd immunity will be a more definitive end to the pandemic. In the United States, while the transition to normal might be accomplished sooner, the epidemiological end point looks most likely to be reached in the second half of 2021. Other advanced economies are probably on similar timetables.

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