It’s time to worry about the Delta variant

The Delta variant (also known as B.1.617.2) was first identified in India earlier this spring. It’s now known to spread more easily than any other variant found so far and evade immunity from a prior COVID-19 infection. Scientists are also investigating the variant’s potential to more easily cause severe disease—as well as links to a “black fungus” that has become a secondary epidemic in India.

Harvard epidemiologist William Hanage called the variant “really, really anxiety-inducing,” STAT’s Andrew Joseph reports. (If you’d like to read more on the biology of this variant, Joseph’s article provides a useful overview.)

The Delta variant was first identified in the U.S. in April. It’s making up a small fraction of new cases at the moment, but is spreading rapidly: from an estimated 1.3% of cases on May 8, to 2.5% of cases on May 22, to 6.1% of cases on June 5. The June 5 estimate comes from CDC’s Nowcast predictions, which extrapolate from the most recent available sequencing data (typically reported with a lag of two weeks or more.)

The share of cases caused by this variant appears to be doubling every two weeks, which means that Delta could become the dominant variant here this summer. Some data suggest that domination could happen within a month—a dashboard run by the testing company Helix puts Delta at 10% of new cases as of May 31, suggesting an even faster transmission rate for the variant.

Helix scientist Alexandre Bolze wrote on Twitter that Delta could become dominant “next week or next 2 weeks” based on these trends.

Other variant trends also support Delta’s dominance. This variant, along with Gamma (or P.1, the variant first identified in Brazil), appears to be outcompeting other variants of concern in the U.S. Alpha (or B.1.1.7) has now plateaued at around 70% of U.S. cases, according to CDC data. The variants found in California and New York, both of which made up more than 10% of new cases earlier in the spring, are now declining.

While the CDC is not yet publishing data on Delta’s prevalence in individual states, we can assume that state-by-state variant trends—especially in those states where Delta cases were first identified—are reflecting the variant’s rise on a national level.  

Many experts are now looking at Delta’s spread in the U.K. as a portent for its spread here. The variant has become dominant in the U.K., thoroughly outcompeting Alpha, and is driving a new surge—even though over half of the British population has received at least one vaccine dose. In fact, the U.K. has delayed its full reopening plans by a month due to this case resurgence.

The COVID-19 vaccines currently in use in both the U.K. and the U.S. do work well against Delta, especially the Pfizer and Moderna vaccines—and especially after a full two-dose regimen is complete. But anyone not yet vaccinated is highly vulnerable to this variant. In the U.K., the current case surge is driven by young adults and teenagers who aren’t yet eligible for vaccination.

As physician and public health expert Vin Gupta put it: “Being unvaccinated on June 9, 2021 is much more risky to your own wellbeing than being unvaccinated on June 9, 2020.” And the longer one waits, the riskier this condition becomes.

The Delta variant should serve both as an additional reason for those in wealthy nations who aren’t yet vaccinated to get their shots—and a reason for wealthy nations to share doses with the rest of the world.

More variant data