What is herd immunity? What you need to know

Chatter about herd immunity amid the coronavirus pandemic has recently cropped up thanks in part to remarks from Sweden's ambassador to the U.S.

Karin Ulrika Olofsdotter told NPR that the European county's capital, Stockholm, could reach herd immunity in the next few weeks.

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“About 30% of people in Stockholm have reached a level of immunity," Karin Ulrika Olofsdotter said. “We could reach herd immunity in the capital as early as next month.”

Rather than locking down like neighboring countries Denmark and Norway, which CNN reported have begun easing restrictions, Sweden has kept businesses open while the government has issued social distancing guidelines.

Here’s a breakdown of herd immunity.

What is herd immunity?

NPR reported herd immunity is when the majority of a given population has become immune to an infectious disease through vaccination or by recovering from it.

Some researchers say the threshold for reaching herd immunity for the coronavirus is 60% in some areas.

According to Johns Hopkins Bloomberg School of Public Health, however, that may depend on the contagiousness of the infection.

“Depending (on) how contagious an infection is, usually 70% to 90% of a population needs immunity to achieve herd immunity,” it said.

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Are people who have recovered from COVID-19 immune?

The World Health Organization reported there is no proof that people who have recovered from COVID-19 cannot get it again.

"Some governments have suggested that the detection of antibodies to the SARS-CoV-2, the virus that causes COVID-19, could serve as the basis for an 'immunity passport' or 'risk-free certificate' that would enable individuals to travel or to return to work assuming that they are protected against re-infection," the WHO wrote in a scientific brief. "There is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection."

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When will herd immunity be achieved?

Herd immunity won’t be achieved soon, according to Gypsyamber D’Souza, a professor of epidemiology at the Johns Hopkins University Bloomberg School of Public Health.

"We will not get to herd immunity in the close future, in the next year or two, but the preventive measures that we're taking now (through social distancing) are buying us time to develop a vaccine that we can hopefully use to immunize ourselves and get herd immunity that way," D'Souza told Time magazine.

How will we achieve herd immunity with COVID-19?

While there is a best case and worst case scenario to achieve herd immunity, Johns Hopkins University Bloomberg School of Public Health stated that the most likely case will see a rise and fall in infection rates over time. Social distancing measures may relax "when numbers of infections fall and then may need to re-implement these measures as numbers increase again. Prolonged effort will be required to prevent major outbreaks until a vaccine is developed."

Still, the virus could infect children before they can be vaccinated or adults after their immunity wanes.

“But it is unlikely in the long term to have the explosive spread that we are seeing right now because much of the population will be immune in the future,” the school said.