Yes, emerging research still in preprint, and so awaiting review by other scientists does suggest that immune protection against developing symptomatic COVID appears to wane. However, protection against severe disease — generated either by immunisation or natural infection — is much longer lasting. Indeed, for the other human coronaviruses, the vast majority of infections are either asymptomatic or at worst a mild cold. How COVID will end will vary from one country to another, depending largely on the proportion of people immunised and how much infection has occurred and so how much natural immunity has built up since the start of the pandemic.
In the UK and other countries with high vaccine coverage and also high numbers of past cases, most people will have some form of immunity to the virus. The virus will remain with us, but the disease will become part of our history. But in countries without much prior illness, even with high vaccine coverage, many people will remain susceptible.
Virtually everyone who has not been vaccinated is likely to catch the virus. When infected, they will be as much at risk of severe disease and death depending on their age and medical status as at any time during the pandemic.
And in these countries, opening up will almost certainly lead to exponential growth of infections due to the large number of people with no immunity. Although COVID tends to be less severe in vaccinated people, some do still get very ill — and these countries may see a sizeable number of vaccinated people needing hospital care.
Five years from now, when childcare centres call parents to tell them that their child has a runny nose and a fever, the COVID pandemic might seem a distant memory. The virus would become a foe first encountered in early childhood, when it typically causes mild infection or none at all, says Jennie Lavine, an infectious-disease researcher at Emory University in Atlanta, Georgia.
Using data from previous studies, Lavine and her colleagues developed a model that shows how most children first come down with these viruses before the age of 6 and develop immunity to them 1. That defence wanes pretty quickly so it is not sufficient to block reinfection entirely, but it seems to protect adults from getting sick, says Lavine. Even in children, the first infection is relatively mild.
A large study of people who have had COVID suggests that their levels of neutralizing antibodies — which help to block reinfection — start to decline after around six to eight months 2. But their bodies also make memory B cells, which can manufacture antibodies if a new infection arises, and T cells that can eliminate virus-infected cells , says Daniela Weiskopf, an immunologist at the La Jolla Institute for Immunology in California, who co-authored the study.
Weiskopf and her colleagues are still tracking the immune memory of people infected with COVID to see if it persists. If most people develop life-long immunity to the virus, either through natural infection or vaccination, then the virus is unlikely to become endemic, she says. But immunity might wane after a year or two — and already there are hints that the virus can evolve to escape it.
Because the virus has spread around the world, it might seem that it could already be classed as endemic. But because infections continue to increase worldwide, and with so many people still susceptible, scientists still technically class it as in a pandemic phase.
In the endemic phase, the number of infections becomes relatively constant across years, allowing for occasional flare-ups, says Lavine. To reach this steady state could take a few years or decades, depending on how quickly populations develop immunity, says Lavine. Allowing the virus to spread unchecked would be the fastest way to get to that point — but that would result in many millions of deaths. The most palatable path is through vaccination.
Countries that have begun distributing COVID vaccines soon expect to see a reduction in severe illness. But it will take longer to see how effectively vaccines can reduce transmission. Data from clinical trials suggest that vaccines that prevent symptomatic infection might also stop a person from passing on the virus. If vaccines do block transmission — and if they remain effective against newer variants of the virus — it might be possible to eliminate the virus in regions where enough people are vaccinated so that they can protect those who are not, contributing to herd immunity.
Even if the virus remains endemic in many regions, global travel will probably resume when severe infections are reduced to levels that health services can cope with, and when a high proportion of people who are vulnerable to severe illness have been vaccinated, says Dye. The influenza pandemic, which killed more than 50 million people, is the yardstick by which all other pandemics are measured.
It was sparked by a type of virus known as influenza A, which originated in birds. Almost all cases of influenza A since then, and all subsequent flu pandemics, have been caused by descendants of the virus. These descendants circulate the globe, infecting millions of people each year. Flu pandemics occur when populations are naive to a virus; by the time a pandemic virus becomes seasonal, much of the population has some immunity to it.
Seasonal flu still has a significant toll globally, claiming roughly , lives per year. US commuters and telephone operators wore face masks in the influenza pandemic. Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle, thinks the coronavirus might follow a similar path.
Shaman and others say the virus could also settle into a seasonal pattern of annual winter outbreaks similar to flu. Still, the coronavirus might be able to dodge immunity acquired by infection, and possibly outsmart vaccines. Already, laboratory studies show that neutralizing antibodies in the blood of people who have had COVID are less capable of recognizing a viral variant first identified in South Africa called Y.
V2 , than variants that circulated earlier in the pandemic 4. Trial results suggest that some vaccines might be less effective against Y. Among high-income countries, cases caused by the Delta variant reversed the transition toward normalcy first in the United Kingdom, where a summertime surge of cases led authorities to delay lifting public-health restrictions, and more recently in the United States and elsewhere.
The Delta variant increases the short-term burden of disease, causing more cases, hospitalizations, and deaths. Our own analysis supports the view of others that the Delta variant has effectively moved herd immunity out of reach in most countries for now, although some regions may come close to it. While the vaccines used in Western countries remain highly effective at preventing severe disease due to COVID, recent data from Israel, the United Kingdom, and the United States have raised new questions about the ability of these vaccines to prevent infection from the Delta variant.
Serial blood tests suggest that immunity may wane relatively quickly. This has prompted some high-income countries to start offering booster doses to high-risk populations or planning for their rollout. Data from the US Centers for Disease Control and Prevention also suggest that vaccinated people who become infected with the Delta variant may transmit it efficiently.
These events and findings have raised new questions about when the pandemic will end. Beyond that, a more realistic epidemiological endpoint might arrive not when herd immunity is achieved but when countries are able to control the burden of COVID enough that it can be managed as an endemic disease. Raising vaccination rates will be essential to achieving a transition toward normalcy.
Vaccine hesitancy, however, has proven to be a persistent challenge, both to preventing the spread of the Delta variant and to reaching herd immunity. Vaccines are also likely to be made available to children in the coming months, making it possible to protect a group that comprises a significant share of the population in some countries.
In this article, we review developments since our March update, offer a perspective on the situation and evidence as of this writing, and present our scenario-based analysis of when a transition toward normalcy could occur. We have written previously about two endpoints for the COVID pandemic: a transition toward normalcy, and herd immunity. The transition would gradually normalize aspects of social and economic life, with some public-health measures remaining in effect as people gradually resume prepandemic activities.
Many high-income countries did begin such a transition toward normalcy during the second quarter of this year, only to be hit with a new wave of cases caused by the Delta variant and exacerbated by vaccine hesitancy. Indeed, our scenario analysis suggests that the United States, Canada, and many European countries would likely have reached herd immunity by now if they had faced only the ancestral SARS-CoV-2 virus and if a high percentage of those eligible to receive the vaccine had chosen to take it.
But as the more infectious Delta variant becomes more prevalent within a population, more people within that population must be vaccinated before herd immunity can be achieved Exhibit 1. Vaccine hesitancy makes it all the more difficult to reach the population-wide vaccination level rates that confer herd immunity.
While it now appears unlikely that large countries will reach overall herd immunity though some areas might , developments in the United Kingdom during the past few months may help illustrate the prospects for Western countries to transition back toward normalcy. Having suffered a wave of cases caused by the Delta variant during June and the first few weeks of July, the country delayed plans to ease many public-health restrictions and eventually did so on July 19, though expansive testing and genomic surveillance remain in place.
As cases decline, our analysis suggests that the United States, Canada, and the European Union could restart the transition toward normalcy as early as the fourth quarter of , provided that the vaccines used in these countries continue to be effective at preventing severe cases of COVID Allowing for the risk of another new variant and the compound societal risk of a high burden of influenza, respiratory syncytial virus, and other winter respiratory diseases, the question for these countries will be whether they manage to arrive at a different epidemiological endpoint, as we discuss next.
We have previously written about herd immunity as a likely epidemiological endpoint for some countries, but the Delta variant has put this out of reach in the short term. Instead, it is most likely as of now that countries will reach an alternative epidemiological endpoint, where COVID becomes endemic and societies decide—much as they have with respect to influenza and other diseases—that the ongoing burden of disease is low enough that COVID can be managed as a constant threat rather than an exceptional one requiring society-defining interventions.
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