The end of the pandemic’s acute phase, or something genuinely close to it, will arrive when so-called herd immunity is reached. This will mark a meaningful turning point for public health and the economy, which are inextricably linked in the age of Covid-19. That leaves the critical question: When will herd immunity arrive?

Estimating the precise date is tricky and probably impossible, but the significance of reaching this point is clear. As WebMD advises, “herd immunity, or community immunity, is when a large part of the population of an area is immune to a specific disease. If enough people are resistant to the cause of a disease, such as a virus or bacteria, it has nowhere to go.” The bottom line: “Herd immunity protects at-risk populations.”

What percentage of the population needs to acquire resistance to Covid-19, either with vaccines or naturally with the development of antibodies? Early estimates were 60% to 70%, but revised projections are higher, some rising to nearly 90%.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, in late-December observed that “when polls said only about half of all Americans would take a vaccine, I was saying herd immunity would take 70 to 75 percent. Then, when newer surveys said 60 percent or more would take it, I thought, ‘I can nudge this up a bit,’ so I went to 80, 85.” He added that “we need to have some humility here. We really don’t know what the real number is. I think the real range is somewhere between 70 to 90 percent. But, I’m not going to say 90 percent.”

For the US, current projections generally point to some point in the second half of 2021 for herd immunity, but this is subject to change, based on several variables. Let’s dig into this a bit deeper for managing expectations. Indeed, the arrival of herd immunity in the early part of the second half vs. the latter part of this year isn’t a trivial difference in terms of health and economic consequences.

A key variable for estimating herd immunity: the pace of vaccine doses administered. The current rate, based on a seven-day moving average, is just below 1.5 million a day. Higher is better and will lead to an earlier herd-immunity date.

Estimates by Vanguard suggest that a 1.5 million-per-day vaccination rate for the US translates to roughly the middle of this year’s third quarter for herd immunity.

“The timing of when herd immunity is achieved relates directly to our outlook for the global economy,” the firm explains.

The path of economic recovery hinges critically on health outcomes; we expect to see business and social activity normalize as we approach herd immunity.

The more quickly this occurs, the more quickly we’re likely to see unemployment rates trend downward, inflation move toward central bank targets, and output reach pre-pandemic levels.

President Joe Biden on Sunday outlined a cautious outlook for the path head, telling the nation that reaching herd immunity by the end of the summer is currently a best-case scenario, albeit a challenging one. “The idea that this can be done and we can get to herd immunity much before the end of this summer is very difficult,” he said.

Not surprisingly, there are risk factors to consider to shade the forecasts. The Atlantic focuses on a critical threat, related to the transmission of the disease:

While COVID-19 vaccines are very good—even unexpectedly good—at preventing disease, they are still unlikely to be good enough against transmission of the virus, which is key to herd immunity. On the whole, we should expect immunity to be less effective against transmission than against disease, to wane over time, and to be eroded by the new variants now emerging around the world. If vaccine efficacy against transmission falls below the herd-immunity threshold, then we would need to vaccinate more than 100 percent of the population to achieve herd immunity. In other words, it becomes downright impossible.

Even if herd immunity remains theoretically within reach, 15 percent of Americans say they will never get a COVID-19 vaccine, making that threshold all the harder to hit.   

The good news is that more Americans say they’re willing to take a vaccine, according to a new survey published by the Centers for Disease Control and Prevention. The bad news: despite the increase, about half the population remains reluctant to take shots in the arm for Covid-19. 

The more immediate challenge is a limited supply of vaccines. “We have to have product available before we go out and encourage people to seek it,” says Mark Weber, who leads the federal government’s Covid-19 advertising campaign. “We’re not there yet.”

Even a substantially higher vaccine supply won’t change the logistical problems of distribution. White House coronavirus coordinator Jeff Zients cuts to the chase, noting: “This will not be easy. Vaccinating everyone in America is one of the greatest operational challenges we’ve ever faced.”

Note, too, that the emerging variants of the disease pose additional risks. Although scientists say that the current vaccines will still work against the variants, the effectiveness declines. The consensus view seems to be that the sooner we reach herd immunity, the greater the success at minimizing variant risk with the current lineup of vaccines.

Optimism on herd immunity for the US translates to expecting to reach this crucial point by Labor Day. But like so much with the disease, the outlook is constantly evolving and so current projections will likely change in the weeks ahead, for better or worse.