Get ready to hear this again and again until global vaccination rates rise: There's another concerning COVID variant to keep an eye on. No, not quite the Delta coronavirus mutation that’s surging in under-vaccinated regions across the U.S. This is the COVID Delta Plus variant, and it's a seemingly more contagious coronavirus strain that sprang from the original Delta variant first detected in India—a mutation of a mutation, as tends to happen with many viruses.

This week, India's health ministry said it had identified at least 22 cases of people infected with the Delta Plus variant in the country across four states and is now dubbing the strain a "variant of concern," a mutation that may be more transmissible and dangerous or less susceptible to available treatments and vaccines—one to keep an eye on. This particular variant was first seen in Europe (even though the Delta variant was originally identified in India) but has since been hitting Indian states such as Maharashtra, Kerala, and Madhya Pradesh.

The Indian government also says that nine countries have now reported cases of the strain, which is being called either AY.1 or B.1.617.2.1 as official scientific names. Those nations include the U.S., the U.K., Portugal, Switzerland, Japan, Poland, Nepal, China, and Russia.

Just how worried should you be about COVID's latest mutation? It’s a bit early to tell, as some public health officials have pointed out. The Indian government's decision to upgrade this new strain's risk status from "variant of interest" to variant of concern with such little data available to date has been the main source of confusion, since a variant of interest seems to better fit the bill for the Delta Plus strain under World Health Organization (WHO) definitions.

"There is no data yet to support the variant of concern claim," Dr. Gagandeep Kang, a virologist and fellow at the Royal Society of London, told the BBC regarding the decision. She added that in order to actually assess a pathogen as a variant of concern, you have to monitor hundreds of patients over time and see whether or not the mutation makes hospitalization and death more likely, or a response to COVID drugs and vaccines less likely.

That would require both real-world monitoring of patients to see how sick they get and tracing factors such as immune responses to the Delta Plus strain. It may simply be a matter of extreme caution in India, which has already seen two distinct COVID waves and desperately wants to avoid a third.

There's already plenty to worry about in the U.S. with the original Delta variant, which was dubbed a variant of concern by the Centers for Disease Control (CDC) last week. At the beginning of the month, about 10% of new reported COVID cases in America were caused by the Delta strain. Just two weeks later, it's now making up 20% of new reported cases and seems to be spreading particularly quickly among younger people.

That figure could continue to climb without even more vaccine vigilance. But preliminary studies suggest vaccines from Pfizer and AstraZeneca were 92% to 96% effective in preventing Delta variant–related hospitalizations following two doses of those shots.

There have been at least 10 major COVID variant classes, either of interest or of concern, identified by the WHO.