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Herd Immunity Threshold Could Be As Low As 43%

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June 26, 2020 -- Herd immunity might be achieved when only 43% of a population becomes immune to the coronavirus.

Herd immunity might be achieved when only 43% of a population becomes immune to the coronavirus, mathematicians from the University of Nottingham and University of Stockholm wrote in an article in Science.

That’s “substantially less than the classical herd immunity level of 60% obtained through homogeneous immunization of the population,” they wrote.

The researchers define herd immunity as the point at which immunity in the population reaches a level that spread of the disease declines, even after preventive measures have been relaxed. But if you don’t have herd immunity and  preventive measures are eased, a second wave of infection may occur, they said.

The authors calculated herd immunity by factoring in different age groups and social activity levels in a population and how that affected a person’s susceptibility to the virus. The average person would transmit the virus to 2.5 people.

"By taking this new mathematical approach to estimating the level for herd immunity to be achieved we found it could potentially be reduced to 43% and that this reduction is mainly due to activity level rather than age structure,” one of the authors, Frank Ball from the University of Nottingham, said in a news release from the university.

 “The more socially active individuals are then the more likely they are to get infected than less socially active ones, and they are also more likely to infect people if they become infected. Consequently, the herd immunity level is lower when immunity is caused by disease spreading than when immunity comes from vaccination.”

The writers caution that the estimates should be interpreted as a way of showing how population differences can affect herd immunity, rather than providing a precise prediction.

They also noted the computer model works on the idea that people who catch the virus will have immunity for “an extended period of time,” and if that’s not the case then other models will be needed.

It’s not certain that people who catch the coronavirus become immune to it, and if they do become immune, how long that lasts. Recent research on a small group of participants indicated that people who’ve tested positive, especially those who don’t show symptoms, may lose their antibodies after a couple of months.

WebMD Health News Brief Reviewed by Michael W. Smith, MD on June 26, 2020

Sources

Science. "A mathematical model reveals the influence of population heterogeneity on herd immunity to SARS-CoV-2"

Medicalxpress.com “Herd immunity threshold could be lower according to new study”

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