Delta variant could drive herd immunity threshold over 80%


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 upward of 80% or more, experts say.

Dr. Ricardo Franco, assistant professor of medicine at the University of Alabama at Birmingham

Dr. Ricardo Franco

Also, it could be time to consider wearing an N95 mask in public indoor spaces regardless of vaccination status, according to a media briefing on Aug. 3 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.

“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%, Dr. 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.

Maximize mask protection

Unlike early in the COVID-19 pandemic with widespread shortages of personal protective equipment, face masks are now readily available. This includes N95 masks, which offer enhanced protection against SARS-CoV-2, Ezekiel J. Emanuel, MD, PhD, said during the briefing.

Dr. Ezekiel J. Emanuel, the Diane v.S. Levy & Robert M. Levy professor at the University of Pennsylvania in Philadelphia

Dr. Ezekiel J. Emanuel

Following the July 27 CDC recommendation that most Americans wear masks indoors when in public places, “I do think we need to upgrade our masks,” said Dr. Emanuel, who is Diane v.S. Levy & Robert M. Levy professor at the University of Pennsylvania, Philadelphia.

“It’s not just any mask,” he added. “Good masks make a big difference and are very important.”

Mask protection is about blocking 0.3-mcm particles, “and I think we need to make sure that people have masks that can filter that out,” he said. Although surgical masks are very good, he added, “they’re not quite as good as N95s.” As their name implies, N95s filter out 95% of these particles.

Dr. Emanuel acknowledged that people are tired of COVID-19 and complying with public health measures but urged perseverance. “We’ve sacrificed a lot. We should not throw it away in just a few months because we are tired. We’re all tired, but we do have to do the little bit extra getting vaccinated, wearing masks indoors, and protecting ourselves, our families, and our communities.”


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