Dr. Mike Ryan, the World Health Organization's emergency director, offered some stern words Wednesday about hopes that herd immunity could stymie the coronavirus.

"Humans are not herds," he said.

Herd immunity is a concept in epidemiology that describes how people can collectively stave off infections if some percentage of the population has immunity to a disease.

But herd immunity in relation to the coronavirus is far from a reality, particularly without a vaccine. Ryan said the term "herd immunity" emerged from veterinary epidemiology, typically involving business decisions of whether to let animals die for the overall health of a herd.

"An individual animal in that sense doesn't matter, from the perspective of the brutal economics of that decision-making," Ryan said.

"So I think we need to be really careful when we use terms in this way around natural infection in humans because it can lead to very brutal arithmetic which does not put people and life and suffering at the center of that equation," he said.

Health professionals and officials continue to warn that there is no easy way out of the coronavirus pandemic, particularly with parts of the U.S. beginning to ease lockdowns. Herd immunity remains out of reach, even in the places hardest hit by the pandemic.

There are two main ways to achieve high levels of immunity in a population: Either enough people have already been infected and their immune systems have developed antibodies to protect against future infections — at least for the short term — or there is a vaccine. Without herd immunity or effective treatments — and in the absence of social distancing measures — it's expected that countries will need to prepare for periodic and unpredictable spikes of new infections until a vaccine is widely available.

For COVID-19, the disease caused by the coronavirus, it's estimated that 50 percent to 70 percent of the population will need to be immune to achieve herd immunity.

Despite more than 4.3 million confirmed coronavirus cases worldwide, no country is even approaching the levels of immunity needed to collectively slow transmissions.

In Wuhan, China, thousands of people returning to work in April were tested for antibodies, and preliminary results found that only 2 percent to 3 percent had developed them. Early results from a nationwide study in Spain found that about 5 percent of the roughly 90,000 people tested were positive for antibodies. And even in hard-hit regions, such as New York City, preliminary testing of 1,300 people found that 21.2 percent were positive for antibodies.

"That means 80 percent of the population would appear to still be susceptible," said Dr. Robert Atmar, an infectious disease specialist at Baylor College of Medicine in Houston. "So even in an area that has been highly affected, we're not seeing the levels that you would expect for herd immunity."

Herd immunity, which varies depending on the pathogen, is calculated based on the transmissibility of a disease — the number of people one infected person will then go on to infect. In general, a disease that is more transmissible needs a higher proportion of the population to be immune for herd immunity to be effective.

"For a disease like measles, it's necessary for more than 95 percent of the population to be immune, but for other pathogens, the number of people required to be immune can be lower," Atmar said.

That raises concerns for any country trying to lift lockdown restrictions, said Dr. David Dowdy, an associate professor of infectious disease epidemiology at the Johns Hopkins Bloomberg School of Public Health.

Epidemiologists and public health officials generally agree that allowing herd immunity for the coronavirus to build through natural infections would be disastrous.

"Right now, unless you're New York City, chances are that 5 percent or less of the population is immune, so to get to herd immunity, you'd have to take the number of cases and deaths we've already experienced and multiply that by about 15," Dowdy said. "If we were to let that happen over a short period of time, it would obviously be catastrophic for the health system."

But that raises a public health quandary. With no vaccine and even the rosiest projections estimating that an effective candidate is at least nine months away, can social distancing be eased without inviting new waves of infections?

In the U.S., where unemployment is soaring and anti-lockdown protests are brewing, some demonstrators and lawmakers have been pointing to Sweden as a model for how to operate without strict social distancing measures — and as a possible way to build herd immunity.

Sweden notably didn't enact official lockdowns and kept schools open for students under 16 even as other countries in Europe took unprecedented steps to curb the spread of the virus. In a meeting, Tuesday of the Health, Education, Labor, and Pensions Committee, Sen. Rand Paul, R-Ky., who is also a medical doctor, suggested that the U.S. should follow Sweden's lead.

But while the "Swedish model" has been hailed as a success story, details are emerging that show that the country has had its own struggles. Sweden has reported more than 28,000 confirmed cases and more than 3,500 deaths from the coronavirus. And although those figures pale in comparison to the 1.4 million cases and more than 85,000 deaths in the U.S., the coronavirus death rate in Sweden is higher than it is in the U.S.

And there's little evidence that Sweden is close to herd immunity. The country's Public Health Agency said in late April that about one-third of Stockholm's 1 million residents may have been exposed to the virus by early May, marking some progress toward herd immunity but far from the 70 percent estimated as necessary.

And Sweden's response has come with a heavy human toll.

"Sweden has, if not the highest per capita mortality rate in Europe, then just about the highest, so they have paid a price for their approach," Dowdy said.

A further wrinkle is that scientists are still unsure whether coronavirus antibodies translate into immunity.

"It's likely, but we don't know for sure," Atmar said. "I think it's reasonable to expect that there will be some level of immunity and protection from reinfection, but we just don't have that information yet."

Even if antibodies can protect someone from being reinfected, it's not known how long the immunity could last.

"If natural immunity to this virus is going to be gone within three to six months, then we don't even need to be talking about herd immunity," Dowdy said.

Still, Atmar said, there are ways the virus' spread could be contained even as some social distancing restrictions are gradually lifted.

"Wearing masks in public, modifying how we use public transportation and increasing testing and contact tracing are ways that we can mitigate some of the risks of transmission," he said. "If we can identify people early who are potentially infected and quarantine them, we can still decrease transmission."