Delta Variant Could Drive Herd Immunity Threshold Over 80%

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In addition to driving up case numbers, the Delta variant could also push the threshold to achieve herd immunity higher.
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Because the Delta variant of SARS-CoV-2 spreads more easily than the original virus, the proportion of the population that needs to be vaccinated to reach herd immunity could be upwards of 80% or more, experts say.

Also, it could be time to consider wearing an N95 mask in public indoor spaces regardless of vaccination status, according to a media briefing today sponsored by the Infectious Diseases Society of America.

Furthermore, giving booster shots to the fully vaccinated is not the top public health priority now. Instead, third vaccinations should be reserved for more vulnerable populations — and efforts should focus on getting first vaccinations to unvaccinated people in the United States and around the world.

"The problem here is that the Delta variant is...more transmissible than the original virus. That pushes the overall population herd immunity threshold much higher," Ricardo Franco, MD, assistant professor of medicine at the University of Alabama at Birmingham, said during the briefing.

Dr Ricardo Franco

"For Delta, those threshold estimates go well over 80% and may be approaching 90%," he said.

To put that figure in context, the original SARS-CoV-2 virus required an estimated 67% of the population to be vaccinated to achieve herd immunity. Also, measles has one of the highest herd immunity thresholds at 95%, Franco added.

Herd immunity is the point at which enough people are immunized that the entire population gains protection. And it's already happening. "Unvaccinated people are actually benefiting from greater herd immunity protection in high vaccination counties compared to low vaccination ones," he said.

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