Scientists in the United States are keeping a close eye on the Delta variant of Covid-19 as it spreads through an unevenly vaccinated American public and a rapidly reopening economy.

The Delta variant, first identified as B.1.617.2 in India, is thought to be more transmissible than both the original Covid-19 strain and the Alpha strain, which was discovered in the United Kingdom.

In the United States, the Delta variant is spreading at an inopportune time. Covid-19 cases have dropped dramatically since the winter peak, from more than 250,000 new diagnoses per day in January to around 14,000 per day in June. There have been fewer hospitalizations and deaths as a result of fewer cases.

As a result, state after state has lifted all social distancing guidelines, including California, which has approved large indoor gatherings such as sporting events. Social distancing and mask requirements are now mostly enforced on the honor system.

Despite this, Delta has roughly doubled every two weeks in the US, a pattern once followed by Alpha, the variant first discovered in the UK that eventually came to represent the vast majority of new US infections. The Delta variant has also pushed back the planned reopening of the UK. The doubling of cases has prompted some, including former Food and Drug Administration (FDA) commissioner Dr Scott Gottlieb, to predict that Delta could account for up to 10% of US cases by mid-June.

This week, the CDC officially designated Delta as a “variant of concern.” Delta is classified as a “variant of concern,” putting it in the same category as Alpha and Gamma in terms of increased surveillance (the variant first identified in Brazil).

The most serious classification for new strains is variants of “high consequence.” This designation implies significantly reduced vaccine efficacy, failures in testing, and more severe disease. This designation does not apply to any variants currently in circulation in the United States. Scientists are concerned about the Delta variant because a more transmissible virus can make social distancing less effective. There are also concerns about severe localized outbreaks in a country that is unevenly vaccinated. According to the CDC, the United States has fully vaccinated 43.9 percent of the population, and 52.6 percent have received at least one dose. These rates, however, vary by region.

Large swaths of the south, such as Alabama, Louisiana, and Tennessee, have only vaccinated less than 35% of their populations. In contrast, much of the northeast, including Vermont, Maine, Massachusetts, Rhode Island, and New Hampshire, has fully vaccinated more than half of its population.

Vaccination rates also differ greatly by age. In the south, despite being eligible for weeks, less than 10% of adolescents are vaccinated, according to Dr. Peter Hotez, a vaccine researcher and dean of Baylor College’s National School of Tropical Medicine. However, lower vaccination rates in some states are also due to a lack of public health infrastructure. People who have delayed getting vaccines but are not necessarily opposed to getting one said they would be more likely to get one if they had time off work, transportation, or if their state provided financial incentives, according to the Kaiser Family Foundation. Furthermore, it is unclear whether Delta will result in more serious disease.

A recent Lancet study in Scotland discovered that the Delta variant was twice as likely to hospitalize people as the Alpha variant.

The Lancet study confirmed that vaccines against Delta are still effective. Pfizer’s mRNA vaccine provided “very good” protection. AstraZeneca’s vaccine had “substantial but reduced” efficacy against Delta. To elicit an immune response, AstraZeneca’s vaccine employs a different technology known as viral vector.

In the five to six weeks it takes for more Americans to be fully vaccinated, the Delta variant “will be the majority of US cases,” said Dr Cyrus Shahpar, an epidemiologist and Covid-19 data director at the White House, on Twitter. “It is critical to begin building protection now.”

In turn, America’s uneven vaccination status highlights another long-term issue: unvaccinated people may put pressure on the virus to evolve once more.