Vaccines are changing how we interpret COVID-19 numbers

It’s never been possible to know for sure how many people in the United States are sick with COVID-19 at a particular time. Instead, we combine all of the metrics — the percentage of tests run that come back positive, the number of new cases reported each day, people coming to the emergency room sick with COVID-19 symptoms — to get an estimate. It’s like the story of the blind men and the elephant: experts can feel out the different pieces of information to get a sense of what an outbreak looks like, even when they can’t see it directly.

We’ve seen the same elephant emerge over and over again over the past year. First, the percentage of tests that come back positive increases. After a slight lag, hospitalizations start to rise, and then...

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