One key factor will help determine how long the Covid-19 pandemic lasts

If Covid-19 immunity doesn’t endure, the virus will.

How long will public health and the US economy be widely threatened by Covid-19? We don’t know. Without a vaccine, experts say, it’s likely to be years.

The future is murky for a lot of reasons. Namely: so much depends on human actions, both individual and collective, that are simply hard to predict and model long-term.

But scientists hope to soon answer one basic question about the virus and how it interacts with the human body, to predict when the pandemic will end: How long does immunity to SARS-CoV-2, the virus that causes Covid-19, last after an infection?

If immunity lasts a couple years or more, Covid-19 could fade in a few years’ time. If immunity wanes within a year, Covid-19 could make fierce annual comebacks until an effective vaccine is widely available. While there’s hope that a vaccine will become available, it’s not a given. The vaccine could also be less than perfectly effective. Manufacturers could struggle to produce enough of it.

Immunity is one key to understanding the duration of this pandemic. Here’s what we know about it so far, and how scientists can crack the mystery for good.

Without a vaccine, we need a lot of immune people to end an outbreak

Here’s some simple math that explains how many people need to get immunity to a virus for an outbreak to end on its own.

Outbreaks end when each new case of the virus, on average, leads to less than one new infection. Immunity can help us get there.

If every case of the virus, on average, leads to two more cases, then around half of a population needs to be immune to the virus for the outbreak to die down naturally.

If one infection causes, on average, three others, then two-thirds of the population needs to be immune for the outbreak to smother itself out.

That’s the big simplified picture.

In practice, it’s messier. R0 — pronounced r-naught, the virus’s basic reproductive number that describes how many new cases are generated, on average, from a single infection — is not a set figure. It can change in different environments, in different populations, and changes when people adopt behaviors like mask-wearing and hand-washing.

Also making things messy: Not everyone is at equal risk of catching the virus. The threshold for ending the outbreak can be reached a bit sooner, in theory, if all the most vulnerable people — and the people most likely to be infected and spread it to others (because, say, they work in a grocery store) — become immune first.

Anyway: Researchers estimate that the current R0 of SARS-CoV-2 is between 2 and 3.

So, it follows that this virus is going to keep spreading until between half and three-quarters of the population is immune. That’s the threshold epidemiologists are talking about when they talk about “herd immunity.” When herd immunity is achieved in a careful way, the number of new infections can decrease.

But there’s another potential wrinkle here. Herd immunity can only reliably be built up if immunity is lasting. If immunity wanes, then it brings down the percentage of the population that’s immune and lets the virus spread farther.

Assuming immunity lasts, how long might it take for the outbreak to burn out on its own?

So, in the best-case scenario — where immunity is lasting — how long would it take to achieve herd immunity and end the pandemic?

Ideally, immunity will build up in a population slowly, over time, to avoid hospital systems from becoming overwhelmed with cases and creating chaos and more collateral damage.

Recently, Harvard epidemiology researchers Christine Tedijanto, Marc Lipsitch, Stephen Kissler, Edward Goldstein, and Yonatan Grad published a study in Science estimating how long it would take to achieve herd immunity in a cautious manner (which would still mean a lot of infection and death).

Angela Weiss/AFP via Getty Images

They modeled an approach where nations go on and off social distancing to avoid overwhelming hospital systems in any one wave. “And we find under an approach like this, it might take until 2022 to build population immunity up in order for the virus to stop running on its own,” Tedijanto says, adding that this is just one hypothetical (and probably overly simplified) mitigation scenario. Who knows if people would abide by a stop-and-go social distancing policy?

But the result gives us a sense of how long it would take to end the pandemic if we adopted the goal of maintaining health care capacity while also easing restrictions every now and then. Their overall conclusion: “It’s going to be hard to go back to normal until we have a vaccine,” she says.

Some have advocated for an approach that builds up herd immunity faster. Perhaps we can achieve it before 2022? This would mean more infections and deaths overall (between .5 and .8 percent of all people infected with the virus die). That’s because “there’s this idea of overshoot, where if you kind of just let an epidemic go, unmitigated, it tends to have momentum,” Tedijanto says. “It doesn’t just stop when it reaches herd immunity, but actually, you tend to overshoot that number and a greater proportion of the population gets infected.”

Recently, biologists Natalie Dean and biologist Carl Bergstrom calculated in the New York Times what damage an overshoot could do. “If 100,000 people are infectious at the peak and they each infect 0.9 people, that’s still 90,000 new infections, and more after that,” they write. “If the pandemic went uncontrolled in the United States, it could continue for months after herd immunity was reached, infecting many more millions in the process.” By the time the pandemic ends, they write, in this scenario, the number of people infected could outnumber the threshold for herd immunity by a large margin — perhaps by two-thirds.

“Letting it spread, even in a smart way, through the population enough to get herd immunity will result in just really a shocking number of deaths,” Dean tells Vox. “Then the other solution is being locked down forever. With those being the viable options, we certainly have to explore other ones.”

If immunity isn’t lasting, we could live with Covid-19 outbreaks for years

The Science paper’s 2022 finding assumes immunity lasts for years. If an individual’s immunity wanes in a year — as is possible with other viruses in the coronavirus family — it could take longer than two years. If immunity is particularly weak — if it lasts less than a year — Covid-19 may be with us for a long time.

“If immunity lasts a year or less, we can expect annual wintertime outbreaks of Covid-19 until there’s a vaccine,” Kissler says. “If immunity lasts longer [around 1-5 years], then there may be sporadic wintertime outbreaks; maybe not every year, but possibly every couple of years. The longer immunity lasts, though, the more likely it is that it will eventually dissipate altogether and the disease will be eliminated.”

For reasons scientists don’t quite understand, for some infections, a person’s immunity never wanes. People who are immune to smallpox, for example, are immune for life: Antibodies that protect against smallpox have been found as long as 88 years after vaccination.

Less reassuring here is that scientists have observed antibody levels for other coronaviruses (there are four strains that infect people as the common cold) can wane over a period of years. However, even if you lose the antibodies, it doesn’t mean you are again completely susceptible to the virus. Yes, none of this is simple. More on that here.

But the good news, for now, at least, is that studies suggest that nearly everyone develops antibodies to SARS-CoV-2. More work needs to be done to show which antibodies, and in which concentrations, confer the most lasting immunity.

We need further studies called “correlate of protection studies.” These are longitudinal studies that track patients over time, “following them to see if they develop reinfection; and comparing the antibody levels and other immune markers between those who are reinfected and those who are not,” Saad Omer, director of the Yale Institute for Global Health, says.

Sadly, these studies take time. We can’t know if immunity lasts for a year if a year has not yet passed. It could also be that immunity lasts different lengths of time for different people, depending on the severity of their infections.

“The worst-case scenario that I can imagine would be that mild or asymptomatic infections don’t confer much immunity, and the virus ends up transmitting a lot better come winter,” Kissler says. “This would be bad because the virus would have lots of time over the summer to percolate to most corners of the world, only to explode in a big outbreak that hits basically everyone at once.”

We may be living with Covid-19 for years. But that doesn’t mean we have to be living in lockdown for years.

Don’t take all of this to mean that we’re going to have to stay in lockdown until at least 2022. There is a middle ground between lockdown and going back completely to the way things were. A massive campaign of universal masking, testing, contact tracing, and isolating suspected cases and their contacts could help reduce transmission while getting our lives a little bit back to normal.

“I want to be optimistic and say that we can come to a place that is less extreme than what we’re in now,” Tedijanto says. There’s no one silver bullet, she says, to defeat Covid-19. But some combination of universal face mask-wearing, better data on transmission hot spots (and how to avoid them), and enhanced testing and contact tracing, is likely to pave a middle ground path.

“It’s going to be a gradual process that will require a lot of patience,” she says. And we should be prepared to live with this virus for a long time.

Support Vox’s explanatory journalism

Every day at Vox, we aim to answer your most important questions and provide you, and our audience around the world, with information that has the power to save lives. Our mission has never been more vital than it is in this moment: to empower you through understanding. Vox’s work is reaching more people than ever, but our distinctive brand of explanatory journalism takes resources — particularly during a pandemic and an economic downturn. Your financial contribution will not constitute a donation, but it will enable our staff to continue to offer free articles, videos, and podcasts at the quality and volume that this moment requires. Please consider making a contribution to Vox today.