By Mackenzie Bean

The threshold to achieve herd immunity against COVID-19 may be lower than originally thought.

Herd immunity occurs when a large proportion of a community is immune to a virus, making person-to-person spread unlikely. Many scientists have suggested that 70 percent of a population would need to either contract COVID-19 or get vaccinated against it to achieve herd immunity.

However, we spoke with more than a dozen scientists who said the threshold may be closer to 50 percent or less of a population. Their estimates are based on statistical modeling of the pandemic, which account for real-world variations in density and demographics.

One model from Stockholm University estimates that 43 percent of a community would need to contract COVID-19 to achieve herd immunity. Other models put this figure as low as 10 percent to 20 percent, although these were in the minority.

While it's still unclear whether any city worldwide has enough immunity to prevent a second wave, scientists said there is already substantial immunity in parts of New York, London and Mumbai.

Health experts not involved with the research said these models should be taken as "food for thought," but not used to guide policies.