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UW-Madison Population Health Expert On Herd Immunity & What It Means For Wisconsinites

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While herd immunity from COVID-19 for the entire United States may require over 90% of people to be vaccinated, smaller groups like dorms or families can achieve protection by getting to 100% vaccination.

The term “herd immunity” was used widely as the coronavirus pandemic began last year. The main way the United States can reach herd immunity is through high vaccination rates, but is this goal something that can realistically be achieved and how would public health experts know when the country is there?

Currently, more than half of American adults have already received at least one dose of the COVID-19 vaccine and now children ages 12 to 15 can get the Pfizer vaccine, increasing the population of people protected from the virus.

Ajay Sethi is an associate professor of population health sciences at UW-Madison. He says while the vaccine is the best route to herd immunity, vaccination rates aren’t the best indicator of when a population has reached the goal.

“The best measure of when herd immunity has been achieved is if the case rates for COVID is essentially zero or near zero, it’s being eliminated,” says Sethi.

When public health officials give numbers of what percent of the population needs to be vaccinated to reach herd immunity, he says those are just projections based on what is currently known about COVID-19.

“We often think of maybe 60 to 90% as what the level of vaccination needs to be in order to get herd immunity and right away, that 60 to 90% is an awfully wide range, which tells you that it’s very hard to calculate what that level should be,” he says.

Sethi believes the number is closer to 90% or above but says the true marker will continue to be when case rates reach that near zero level.

“As long as we have outbreaks in settings where unvaccinated people are gathering, we can’t claim to have herd immunity,” he says.

But the term "herd immunity" doesn’t just apply to large scale populations like the United States or the state of Wisconsin. Smaller communities like a household or a group of friends can achieve herd immunity by becoming 100% vaccinated, Sethi says.

When the vaccines were first introduced, there were concerns that vaccinated individuals could still carry and spread the disease while experiencing no symptoms. Sethi explains that new data shows vaccinated individuals are not only protected from severe cases of COVID-19 but also from contracting or spreading the virus.

That means that indoor gatherings can be done without masks within these small groups with herd immunity but that larger events without required vaccination could still lead to transmission of COVID-19.

“We’re at that phase that anyone who is vaccinated should feel confidant that they’re protected against getting COVID and having a severe case of COVID, and hopefully we can get to the point where an entire neighborhood, zip code, county, an entire state, maybe our entire country can feel very protected,” he says.

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