It's possible to reach herd immunity, then lose it. Repeatedly.

Murray said he believes the biggest reason we can “go in and out of herd immunity” is seasonality.

“We know now that Covid-19 is quite seasonal. There’s more transmission potential in the winter than in the summer,” he said.

“I think herd immunity can occur in the summer at pretty low levels — like 55%, 60%. And I think for the winter, it’s going to be closer to 80% immune.”

How could seasons impact how much a virus spreads?

“Part of it may be humidity. So there’s the idea that these respiratory viruses don’t like humidity, and because the air outside is cold (in the winter), it gets heated up inside the house, and therefore relative humidity is lower in the winter,” Murray said.