Herd Immunity: Do you even understand it?

In 2020, the phrase "herd immunity" has become as ubiquitous as the phrase "social distancing" or the word "antibodies."

Gypsyamber D'Souza is a professor at Johns Hopkins Bloomberg School of public health. She tells Inverse that, when talking about herd immunity, there's one thing people really need to understand.

When no one has immunity to a virus, it can tear through the population – as seen in the top panel.

When a few people develop immunity, the virus begins to run out of potential footholds (seen in the middle panel)

The measles vaccine essentially eliminated the highly contagious disease from North America in 2000.

(The virus resurged in 2019, beginning in places that did not have high enough rates of vaccination to achieve herd immunity.)

The more infectious a disease is, the more people need to be vaccinated to achieve herd immunity, D'Souza explains.

For Covid-19, the World Health Organization estimates that between 60 and 80 percent of the population would need to be immune for herd immunity to be achieved.

Some scientists place their estimates as low as 50 percent, The New York Times reported.

D'Souza says that the threshold is "certainly above 50% for coronavirus and we don't know exactly where it is."

Herd immunity is not a strategy for controlling the pandemic for a clear reason:

It comes at an extremely high cost.

Sweden tried to achieve herd immunity naturally by letting the virus run uncontrolled. Sweden didn't impose social distancing or lockdowns this winter.

Some reports have shown that pockets of communities around the world have higher percentages of people with Covid-19 antibodies.

68 percent of people had coronavirus antibodies at one clinic in Corona, Queens. In pockets of Mumbai's poorest neighborhoods, between 51 and 58 percent of people had antibodies.

That's not enough to truly guarantee immunity in the US for a few reasons.

"Our lifestyles, in the US, are not lived very locally [or] without travel," D'Souza says.

"If having had that infection does not prevent you from getting reinfected at all, well then the idea of herd immunity doesn't apply at all," says D'Souza.