The virus has evolved over time, becoming faster and more adept at infecting humans, Crotty says. Nearly everyone has also now had at least one brush with COVID-19, whether through vaccination or illness. Each encounter leaves behind instructions for the immune system, helping it recognize the virus faster the next time it appears.
The answer has changed from the earliest days of COVID-19, experts say. In the beginning of the pandemic, we were really looking at seven to 10 days as the window of time where people had to quarantine or isolate after an exposure, says Andrew Pekosz, a virologist at Johns Hopkins University. That has shortened significantly now.
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