Delta, other variants make ‘herd immunity’ harder to reach

Experts estimate >90% immunity needed to achieve mark

BOSTON — It was, early on, one of the more empowering phrases of the pandemic. For if we could reach ‘herd immunity,’ public health experts promised, life could return to normal.

“So the basic idea behind herd immunity is a balance between how many people in the population are susceptible versus not susceptible to an infection,” said Dr. Robin Colgrove, a virology and infectious disease specialist at Mt. Auburn Hospital in Cambridge, MA. “So, for example, in a case where everybody in a population is vaccinated with a highly effective vaccine, the herd – the population – would be very resistant to infection spreading.”

Back before the time of COVID variants, herd immunity did seem plausible given the public’s apparent appetite for normalcy and the rapid deployment of stunningly effective vaccines. Early in 2021, some were even willing to pin a number on it.

“Calculations were that if 60-to-70% of people were immune, that would be enough to stop the spread,” Colgrove said.

But once COVID variants hit, all bets were off, most especially with the Delta variant.

“It’s really impressive how much more transmissible it is than what was happening in January,” said Dr. Justin Lessler, a professor in the Department of Epidemiology at the University of North Carolina, Chapel Hill. “And that ups that number. It means we have to get in the 90% range or 85% range before we have cases start going down.”

Colgrove likens the Delta/herd immunity situation to measles, another highly contagious virus.

“The fraction of the population you need to immunize [against measles] to get herd immunity so the infection can’t spread is about 95%,” he said. “And that was really only achievable with measles because the vaccine is required to go into school.”

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It seemed like a reasonable shot that 70% of people would sign up to get vaccinated initially against COVID, Colgrove said. But the public health community surely underestimated vaccine resistance and didn’t anticipate the rise of the variants.

“So now we’re in a different world, with the Alpha, Beta, Gamma and now Delta variant, all of them successively more contagious,” Colgrove said. “It’s almost like we’re dealing with a different pathogen in how contagious it is.”

Colgrove agrees a number above 90% to achieve herd immunity would not be unreasonable now; but on another level, it might be.

“Whether that’s an achievable number then becomes no longer a medical question but a social question,” Colgrove said. “Could you get there? Could you practically get there? And could you get there without some kind of universal mandate?”

Colgrove calls herd immunity a totally valid concept that is achievable even when the odds are long, such as with measles.

“Whether we can get there with COVID-19 is a complicated question with practical as well as virology aspects,” he said.

Among the practical aspects to consider is the current state of vaccinations in the United States. The Centers for Disease Control and Prevention puts the national ‘fully vaccinated’ rate at 51.1% but finds 26 states have less than half their populations fully vaccinated, with the bottom five ranking states – West Virginia, Idaho, Wyoming, Mississippi and Alabama – posting rates under 40%.

But even the five top-ranked states for full COVID vaccination – Vermont, Massachusetts, Connecticut, Maine and Rhode Island – have not quite achieved that 70% ‘fully vaccinated’ rate thought to be optimal back before the time of Delta.

Still, Lessler thinks the focus shouldn’t so much be on the missed herd immunity goal but more on continuing to boost the overall vaccination numbers.

“Even if, as a nation, we’re beyond that threshold, [infections] go down a lot faster if we have more immunity,” Lessler said. “So, exceeding that 85% really has a lot of benefit.”

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