It’s called the Delta variant, and on Wednesday, the CDC joined the World Health Organization (WHO) in identifying this variant as one of “concern” that could be more transmissible and lead to more serious illness, throwing a potential wrench into worldwide reopening efforts.

This strain, also known as the B.1.617.2 coronavirus variant, was first identified in India and is one among several other local coronavirus strains that have been identified in Brazil, the U.K., and other regions. But what worries health officials is that the strain could become the dominant one throughout much of the world without containment efforts. The U.K. this week delayed its reopening plans owing to the transmissibility and increased risk of illness associated with the Delta variant, which accounted for 90% of new COVID-19 cases in the nation this past week.

In the U.S., the numbers for Delta variant cases are far lower, constituting about 10% of new infections, although the rate of spread has been increasing in the past few weeks. One reason for the relatively low case numbers that we know of to date may be the aggressive immunization campaign underway in the U.S., which has seen 146.5 million residents fully vaccinated to date, more than 44% of the overall population.

But this strain’s transmissibility and severity is giving some medical professionals pause. Deepak Srivastava, a physician and president of the Gladstone Institutes for biomedical research, told ABC News that it will be critical to keep an eye on this variant’s spread ahead of California’s reopening this week.

However, there has been early evidence that the current COVID-19 vaccines from Pfizer, Moderna, and AstraZeneca, offer some protection against the Delta variant, and the more people within a community who are vaccinated, the lower the risk of transmission.

But depending on cases and hospitalizations going forward caused by this pathogenic tweak, communities with low vaccination rates may have to continue being cautious. And experts point out that variants such as this are precisely why people shouldn’t get just one dose of a two-dose COVID vaccine and call it a day.

For instance, new research out of the U.K.’s National Health Service and British universities found that two doses of AstraZeneca’s or Pfizer’s vaccines were 92% or 96% effective, respectively, at preventing hospitalizations for patients who were infected by the Delta variant.

The Delta variant will likely have different effects in different regions. In Southern states with low immunization rates among young adults, for example, the strain could pose a threat as more and more people socialize in person and mask mandates are lifted.

“Here in the South, particularly in Louisiana, Mississippi, we’re seeing really low vaccination rates,” Peter Hotez, codirector of the Center for Vaccine Development at Texas Children’s Hospital, told CNBC. “And less than 10% of adolescents are vaccinated in many of these Southern states, so we have a real vulnerability here.”

More health care and Big Pharma coverage from Fortune:

• 12% of the world’s population has received a COVID vaccine. See how your country is doing
• England’s reopening is officially in limbo, serving up a cautionary tale for countries on the vaccination forefront
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• India needs Pfizer vaccines—but company demand for legal protection is holding things up
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