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UW biologist: Herd immunity ‘necessary but not sufficient’ for return to normal

A mural in Seattle's Capitol hill neighborhood. (MyNorthwest photo)

The term “herd immunity” has been thrown around frequently as vaccinations in the United States have ramped up, with many viewing it as the end goal for the COVID-19 pandemic. But as University of Washington biologist Carl Bergstrom detailed in a thread posted to Twitter, it may not be that simple.

Reaching true nationwide immunity against COVID ‘probably unlikely’

As Bergstrom described, herd immunity occurs when the amount of people no longer susceptible to a virus is enough to prevent a new outbreak from spreading. He estimates that true herd immunity for COVID-19 will likely necessitate vaccinations “somewhere in the range of 60-70%” for all Americans, or even 80% or higher depending on new variant strains.

So, can we ever reach that goal?

“In principle, yes,” Bergstrom posits.

The problem he sees is that many states across the country have already chosen to relax restrictions, despite not having reached the herd immunity threshold, and that even after it’s achieved, we won’t actually be able to return to normal right away.

“Before we can relax all our controls we also want to have a low number active cases,” he described. “Recall that herd immunity is the point at which a new outbreak cannot start from scratch. It is not the point that an ongoing outbreak is over.”

“Herd immunity will be necessary for living like 2019,” he added. “But it’s not sufficient.”

As COVID appears to wane, what will the new normal look like?

In the days and weeks ahead, Bergstrom believes that “if we continue control measures once we reach herd immunity, the number of cases will rapidly decline and we can soon relax.” But if restrictions are relaxed prior to that, “cases will only slowly decline and many unnecessary infections will occur.”

Washington has seen the byproduct of relaxing restrictions play out recently, with statewide daily case counts having risen to the same level the state saw during last summer’s second wave.

If the state — and the rest of the country — doesn’t eventually achieve herd immunity, Bergstrom predicts that we’ll likely see some modicum of seasonality akin to the flu, with “minimal cases in the summer, followed by a winter wave.”

What vaccinations will be most useful for in the near term is preventing most serious cases, and ultimately spelling the end of the pandemic as we know it once case numbers have sufficiently declined.

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