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Madison, Wis. — Herd immunity is a phrase that has become commonplace since the early days of the pandemic. We all seem to know we want to achieve it. But what does that mean? And how will we know if we’ve gotten there?

UW Health experts are offering insights into what herd immunity is and how we’re doing on getting there in the United States and Wisconsin.

Herd immunity occurs when enough people in a community are immune to a certain infectious disease that the disease can no longer effectively spread from person to person within that community, essentially giving protection to everyone. Put another way, herd immunity can be reached when persons infected with COVID-19, on average, infect fewer than one person each, so that the disease eventually dies out. This immunity can occur through mitigation efforts, mass infection and/or mass vaccination. Mitigation efforts include mask wearing, physical distancing and frequent hand washing, and are critical while COVID-19 continues to spread.

Experts say the U.S. could reach herd immunity when 70 to 90% of the population is vaccinated. As of April 29, 30% of people in the U.S. — roughly 100 million people — were considered fully vaccinated. In Wisconsin, as of April 28, 34.6%, or 2 million people, were considered fully vaccinated.

These numbers are positive, but it means we have a long way to go, according to Dr. Matt Anderson, senior medical director, primary care, UW Health. For one thing, until a vaccine is approved for children, about 20% of the population is ineligible. This means it is especially important for as many adults as possible to get the vaccine to minimize the risk of passing it to children still vulnerable to the virus, he said.

While herd immunity is a very important step to returning to normal, health experts say getting there requires reaching vulnerable or hesitant communities.

"Herd immunity isn’t an on/off switch, but something we have to work towards," Anderson said. "Because the more people we have vaccinated, the more people we have protected from this serious, infectious and often deadly virus."

When it comes to herd immunity against COVID-19, vaccination is far more effective and safer than immunity via infection, he said. COVID-19 is highly infectious, has a 1 to 2% mortality rate and can often create long-term symptoms for those who get sick. While the vaccines have proven to be safe and highly effective against both the original COVID-19 and the emerging variants.