We've achieved herd immunity for the measles. Will we ever get there on COVID-19?

Dr. Marc Siegel

These days, everyone is talking about herd immunity. But what actually is it? We are all a part of a larger herd, and when a critical mass of immunity is reached, the contagion slows to a crawl or even a stop. Herd or population immunity is based on how transmissible a pathogen is. If it spreads easily, then a higher percentage of immune hosts are needed to slow it down, until the pathogen runs out of available options. With measles, probably the most easily transmissible respiratory virus, that number is around 95%, and since almost all of us take the vaccine as young children, we achieve it.

So when can we expect to get to herd immunity on COVID-19? First, it depends on how many people have been infected and have developed a natural immune response. Antibody tests project this number to be close to three times higher than reported cases, or about 100 million people. This group provides a substantial blockade to the virus, as long as they’re not confronted with a variant to which they’re not immune.

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