What Makes the Delta Variant of Covid-19 So Dangerous for Unvaccinated People

The Delta variant of coronavirus was first detected in India last October, where it helped fuel a devastating Covid-19 surge that set records for new infections and deaths. Delta has since spread to more than 100 countries. Nations that had previously kept Covid-19 cases relatively low, such as Indonesia, Australia and parts of Africa, are now seeing record growth in infections from the more transmissible variant.

Delta was first detected in the U.S. in March and by mid-July accounted for three-quarters of Covid-19 cases. It has supplanted the Alpha variant, which until recently was the most widespread version of the virus in the U.S. Its impact is acutely felt in parts of the country with low vaccination rates, where case counts and hospitalizations are surging. The Delta variant accounts for 83% of all U.S. cases, according to recent estimates from the U.S. Centers for Disease Control and Prevention. Unvaccinated individuals make up more than 95% of all hospitalizations.

What makes the Delta variant more contagious?

Researchers think Delta is about 50% more transmissible than the Alpha variant, which means the average patient would infect 50% more contacts. Alpha itself is an estimated 50% more contagious than earlier versions of the virus.

Delta’s increased infectiousness is driven by a unique combination of mutations, changes to the virus’s genetic code that affect its structure and function. Some of Delta’s most pernicious mutations affect its spike protein, which the virus uses to latch onto and infect human cells.

These mutations can make the virus better at binding to cells, as well as help it elude antibodies, which our immune systems deploy to neutralize the virus.