Breakthrough cases: What we know right now

While epidemiologists may consider any new outbreak a possible source of new variants, one key way to monitor the virus/variant race is by analyzing breakthrough cases—those infections that occur after someone has been fully vaccinated. Here’s how states and the CDC are tracking these cases now, and what we know about vaccine protection against the variants.

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Delta and Gamma are starting to dominate

Delta, or B.1.617.2, is particularly dangerous. As I’ve written before, this variant spreads much more quickly than other strains of the coronavirus and may cause more severe illness, though scientists are still investigating that second point. Thanks to this variant, it’s now much more dangerous to be unvaccinated than it was a year ago. It’s quickly becoming dominant in the U.S.

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National numbers, June 20

The CDC updated its variant prevalence estimates this week, reporting that the Delta variant makes up 10% of U.S. cases as of June 5. This aligns with other estimates I cited last week, and suggests that the variant is spreading here at a truly rapid pace—its prevalence multiplied by four times in two weeks, according to CDC data.

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New variant names from the WHO

We finally have a straightforward variant naming system: on May 31, the WHO announced a system using letters of the Greek alphabet. B.1.1.7 (first identified in the U.K. is now Alpha, B.1.351 (first identified in South Africa) is now Beta, and so on.

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National numbers, May 30

Cases, deaths, and hospitalizations all continue to drop nationwide. The U.S. reported about 3,000 COVID-19 deaths last week, in total—at the peak of the winter surge, we saw more than 3,000 deaths a day.

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