The delta variant is prompting a spike in coronavirus cases across the US. But the pandemic may soon take yet another turn.On CNBCs Squawk Box last week, former Food and Drug Administration (FDA) commissioner and current Pfizer board member Scott Gottlieb said were transitioning from this being a pandemic to being more of an endemic virus, at least here in the United States and other Western markets. Gottlieb was responding to a question about the point at which the US might claim something close to victory over Covid, in light of upcoming approvals for booster shots and vaccines for children under age 12.This is going to become more of an endemic illness where you see sort of a persistent infection through the winter, but not at the levels that were experiencing certainly right now, Gottlieb said (noting that this would be less true in countries with low vaccination rates). Booster shots will play a role in getting the US to that phase, he says, as will the delta variant.So what does this transition actually mean? As Ed Yong explained in The Atlantic, the pandemic ends when almost everyone has immunity, preferably because they were vaccinated or alternatively because they were infected and survived. Because the delta variant is so transmissible, particularly among the unvaccinated, Gottlieb is suggesting that deltas spread plus vaccinations will push wealthy Western countries into a new Covid era.Endemic versus pandemicTo understand the import of Covids eventual shift from pandemic to endemic, its necessary to define both terms. This is a bit trickier than one might think.In March 2020, the World Health Organization (WHO) declared Covid-19 to be a global pandemic. Although there is no universally agreed-upon definition of what constitutes a pandemic, the WHO had previously called a pandemic the worldwide spread of a new disease. The International Epidemiology Associations Dictionary of Epidemiology says that a pandemic is an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of people. (The term epidemic, meanwhile, is typically used to describe a sudden, sharp increase in a disease throughout a particular community, population, or region.)So its safe to say that a pandemic involves a disease thats contracted by a lot of people in geographically diverse locations, though as a recent Nature article notes, such descriptions are qualitative in nature. A 2009 article in the Journal of Infectious Diseases adds that most diseases that get described as pandemics tend to be newer, and characterized by high attack rates (the proportion of people getting sick with the disease), explosive spread in a short period of time, lower levels of immunity, and higher levels of contagiousness. Covid-19 checks all those boxes.Endemic diseases, like chicken pox or malaria, are not novel, and the rates of infection within a given population are fairly predictable. The Centers for Disease Control and Prevention (CDC) says that endemic refers to the constant presence and/or usual prevalence of a disease or infectious agent in a population within a geographic area. Speaking with the New York Times this spring, professor of infectious disease epidemiology David Heymann said that becoming endemic was the natural progression of many infections we have in humans, whether it is tuberculosis or HIV.We have learned to live with them and we learn how to do risk assessments and how to protect those we want to protect, he explained.The end of herd immunity hopesIt wasnt always a foregone conclusion that Covid-19 would become endemicfor a while, the hope was that vaccines might allow populations to reach a level of herd immunity that would stomp out the virus almost entirely. But as Yong notes, because the delta variant spreads so quickly, most experts think herd immunity is no longer realisticeven if vaccination rates rose to the levels previously thought necessary. (At the low end of estimates, the CDC says that 90% of Americans would need to be vaccinated to reach herd immunity with delta; at the high end, Yang says, herd immunity is mathematically impossible with the vaccines we have now.)And so endemic Covid is where were heading, sooner or later. The optimistic view is that enough people will gain immune protection from vaccination and from natural infection such that there will be less transmission and much less Covid-19-related hospitalization and death, even as the virus continues to circulate, immunologist Yonatan Grad tells the Harvard Gazette.Will Covid last forever?Not all countries have given up on the hope of Covid zero. China, for example, is pursuing a zero tolerance approach to the virus, with strict lockdowns, contact tracing, testings, and quarantines. Australia, too, has sought to shut out the virus with closed borders, lockdowns, and widespread testing. The highly contagious delta variant, however, is still managing to penetrate both countries, calling the long-term viability of a zero-Covid strategy into question.We dont know yet when Covid-19 will cross over into endemic territory, in the US or anywhere else. But when it does, it should present a much less significant threat to our health and our daily lives. Vaccines and immunity from previous infections will give most people protection against severe cases of the virus, and measures like regular testing and outbreak tracing can hopefully keep a lid on any resurgences.It wont be the ending to the pandemic we may have hoped for. But it will be better than where were at now.