What is herd immunity and why some think it could end the coronavirus pandemic

People enjoy the warm spring weather as they sit by the water at Hornstull in Stockholm on April 21 during the COVID-19 pandemic.

(CNN)The concept of herd immunity is a simple one. But achieving it? Not so much.

As the coronavirus pandemic spread throughout the world, doctors, scientists, and government leaders alike have said that once herd immunity was achieved, the spread of the virus would be less of a threat. Herd immunity is reached when the majority of a given population -- 70 to 90% -- becomes immune to an infectious disease, either because they have become infected and recovered, or through vaccination. When that happens, the disease is less likely to spread to people who aren't immune, because there just aren't enough infectious carriers to reach them.
There are just two ways to get there: widespread vaccination, which for Covid-19 is still many months away, or widespread infections that lead to immunity.

    Herd immunity by infection is not without risks

    Most doctors and experts agree that allowing Covid-19 to just plow through populations might help reach herd immunity more quickly, but that would also overwhelm hospitals. More people would die, not just from coronavirus but from other infections, too. That's why we're all stuck at home -- we're flattening the curve.
    This graphic from the National Institute of Allergy and Infectious Diseases explains the concept of herd immunity.
    "The advantage of stretching out the number of cases is that we will not exceed the capacity of hospitals to care for those who are particularly sick," Dr. H. Cody Meissner, chief of pediatric infectious disease at Tufts University Medical School, told CNN's Michael Smerconish in March.
    And then there's the issue that we don't really know how immunity works with this virus.
    Dr. Maria Van Kerkhove, an infectious disease epidemiologist at the World Health Organization (WHO), said it's not known whether people who have been exposed to the virus become completely immune to it and if so, for how long. That's why governments should wait for a vaccine, she said.
    The WHO has "seen some preliminary results, some preliminary studies, pre-published results, where some people will develop an immune response," Van Kerkhove said. "We don't know if that actually confers immunity, which means that they're totally protected."