The Delta variant of the virus that causes Covid-19 is often described as highly transmissible. So, what does that actually mean?
Scientists studying Covid-19 say that Delta’s increased contagiousness means we need to update our thinking about exposure risks. Because people infected with Delta carry higher levels of virus than with earlier strains, the old rules of thumb no longer apply, they say—including the conventional wisdom that it takes 15 minutes of close contact with someone to get infected.
Delta has already changed public-health advice about masking. The Centers for Disease Control and Prevention on Tuesday recommended that vaccinated people resume masking indoors in certain parts of the country. And local officials have begun to reinstate mask mandates, including in Los Angeles County and St. Louis County.
CDC Director Rochelle Walensky said the guidelines were revised because new data on the Delta variant show that breakthrough infections for fully vaccinated people, though rare, carry a similar level of viral load as infections among the unvaccinated.
Here’s what scientists say we need to know about how Delta changes our risk calculus.