As the gears of the modern world grind to a near halt, one question is likely on the mind of many: When will the coronavirus pandemic — and social distancing — end?
No one knows for sure, but it’s probably not any time soon. Here’s what we do know about when it may be safe to come out of our homes and resume normal life.
It will almost certainly take herd immunity to end the pandemic.
Most experts say we’re past the point of containing the virus, like we did with SARS and MERS. That means that COVID-19 is here to stay, and the pandemic will end only with herd immunity.
Herd immunity describes what proportion of a population has to be immune to a disease for the population as a whole to be protected from outbreaks. The exact threshold depends on the infectiousness of the disease, represented by the basic reproduction number, called R0 (pronounced “R naught”).
When a new virus emerges, no one is immune. A highly transmissible virus, like the coronavirus behind the current pandemic, can spread like wildfire, quickly burning through the dry kindling of a totally naive population. But once enough people are immune, the virus runs into walls of immunity, and the pandemic peters out instead of raging ahead. Scientists call that the herd immunity threshold.
Up to two-thirds of a population would need to be infected to reach that threshold.
Current estimates put the coronavirus’s R0 between two or three, meaning anyone with COVID-19 tends, on average, to infect two or three other people. While this number can change based on our behavior, researchers estimate that the herd immunity threshold for COVID-19 is about one-third to two-thirds of any given population. Worldwide, that means anywhere from 2.5 billion to 5 billion people.
Scientists aren’t yet sure how long people infected with COVID-19 remain immune, but so far it seems that they aren’t readily reinfected
Letting the virus burn through the population would be the fastest approach.
People acquire immunity against a virus in two ways: Either they have been infected and recovered — gaining some level of antibody protection — or they get a vaccine against the virus.
Since a vaccine is at least 12 to 18 months from being available (SN: 2/21/20), the fastest way to herd immunity would be to let the virus burn through the world’s population unimpeded. According to a March 16 report released by researchers at Imperial College London, in the United States, the pandemic would peak in about three months under that scenario.
But….
The costs of such a strategy would be overwhelming. Upward of 2 million Americans would die from infection alone, according to the same report. Roughly 81 percent of the U.S. population would get infected, the team estimates.
The elderly and those with underlying health conditions would be hardest hit, but younger people, too, can experience severe illness (SN: 3/19/20). And the critical care capacity of U.S. hospitals would be exceeded as early as the second week of April, and eventually require 30 times as many critical care beds as currently exist, the team estimates. While there is much still unknown about the virus, most experts agree with this overall picture.
The costs of delaying action any further to slow the virus’ spread could be catastrophic, the researchers conclude. That’s why countries around the world are trying various strategies to quell the surge in cases, in effect flattening the exponential curve of the pandemic and lessening the strain on hospitals. Those measures primarily consist of aggressive social distancing, such as closing schools, cancelling large public events and encouraging people to work from home if possible (SN: 3/13/20).
Social distancing reduces deaths but delays herd immunity.
The necessary flipside of successful social distancing is that achieving herd immunity gets delayed as cases decrease, says Michael Mina, an epidemiologist at Harvard University’s T.H. Chan School of Public Health in Boston. Even if collectively we prevent a surge in the coming weeks, he says, the virus could reemerge as soon restrictions are lifted.
“In the absence of robust herd immunity at the population level, we have some risk of a second wave of the epidemic,” Mina says.