Delta Variant Could Drive Herd Immunity Threshold Over 80%

Damian McNamara

August 03, 2021

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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.

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.

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 Emanuel, who is Diane v.S. Levy & Robert M. Levy professor at the University of Pennsylvania in 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-micron 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.

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."

Dealing With a Disconnect

Dr Ezekiel J. Emanuel

In response to a reporter's question about the possibility that the large crowd at the Lollapalooza music festival this past week in Chicago could become a superspreader event, Emanuel said, "it is worrisome."

"I would say that, if you're going to go to a gathering like that, wearing an N95 mask is wise, and not spending too long at any one place is also wise," he said.

On the plus side, the event was held outdoors with lots of air circulation, Emanuel said.

However, "this is the kind of thing where we've got a sort of disconnect between people's desire to get back to normal...and the fact that we're in the middle of this upsurge."

Another potential problem is the event brought people together from many different locations, so when they travel home, they could be "potentially seeding lots of other communities."

Boosters For Some, For Now

Even though not officially recommended, some fully vaccinated Americans are seeking a third or booster vaccination on their own.

Asked for his opinion, Emanuel said: "We're probably going to have to be giving boosters to immunocompromised people and people who are susceptible. That's where we are going to start."

More research is needed regarding booster shots, he said. "There are very small studies — and the 'very small' should be emphasized — given that we've given shots to over 160 million people."

"But it does appear that the boosters increase the antibodies and protection," he said.

Instead of boosters, it is more important for people who haven't been vaccinated to get fully vaccinated.

"We need to put our priorities in the right places," he said.   

Emanuel noted that, except for people in rural areas that might have to travel long distances, access to vaccines is no longer an issue. "It's very hard not to find a vaccine if you want it."

A remaining hurdle is "battling a major disinformation initiative. I don't think this is misinformation. I think there's very clear evidence that it is disinformation — false facts about the vaccines being spread," Emanuel said.

The Breakthrough Infection Dilemma

Breakthrough cases "remain the vast minority of infections at this time...that is reassuring," Franco said.

Also, tracking symptomatic breakthrough infections remains easier than studying fully vaccinated people who become infected with SARS-CoV-2 but remain symptom free.

"We really don't have a good handle on the frequency of asymptomatic cases," Emanuel said. "If you're missing breakthrough infections, a lot of them, you may be missing some [virus] evolution that would be very important for us to follow." This missing information could include the emergence of new variants.

The asymptomatic breakthrough cases are the most worrisome group," Emanuel said. "You get infected, you're feeling fine. Maybe you've got a little sneeze or cough, but nothing unusual. And then you're still able to transmit the Delta variant."

The Big Picture

The upsurge in cases, hospitalizations, and deaths is a major challenge, Emanuel said. "We need to address that by getting many more people vaccinated right now with what are very good vaccines."

"But it also means that we have to stop being US-focused alone." He pointed out that Delta and other variants originated overseas, "so getting the world vaccinated...has to be a top priority."

"We are obviously all facing a challenge as we move into the fall," Emanuel said. "With schools opening and employers bringing their employees back together, even if these groups are vaccinated, there are going to be major challenges for all of us."

Based on an August 3 media briefing by the Infectious Diseases Society of America.

Damian McNamara is a staff journalist based in Miami. He covers a wide range of medical specialties, including infectious diseases, gastroenterology, and critical care. Follow Damian on Twitter: @MedReporter.

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