While the number of new coronavirus cases reported each day remains relatively low, they have started to trend upwards again. The study’s experts warned (as did Anthony Fauci recently), if there is one thing likely to spoil such celebration and reverse the progress the U.S. has made in the pandemic, it’s the fast-spreading, highly-transmissible Delta variant that now accounts for the majority of COVID infections in the country.

According to data released by the Centers for Disease Control and Prevention last night, which relies upon genomic surveillance data reported through July 3, 51.7% of new COVID cases are estimated to be caused by the Delta strain of the coronavirus, up from 30.4% for previous the two-week period.

The Delta variant is particularly dominant in parts of the Midwest and the Upper Plains regions, where the CDC estimates it now accounts for upwards of 74% of new COVID infections. Many of those states lag in vaccinations, with rates trailing the national average. For southern states west of the Mississippi, and New York and New Jersey, the share of new cases due to the Delta variant is between 55% and 60%. In all other regions in the country, the Delta variant accounts for roughly 30% to 40% of new cases, according to the CDC data.

While America’s vaccinated population is at very low risk for getting severely ill from the Delta variant—182.9 million people, or 55.1% of the population have at least one shot—the sizeable population that has not yet gotten a vaccine is vulnerable to the virus. Anecdotally, the symptoms of those infected with the Delta variant are slightly different than those who experienced COVID early in the pandemic; people with Delta variant cases, according to some experts, are less likely to lose their senses of smell and taste, and more likely to manifest like a common cold.

The Delta variant is not just dominant in the U.S., of course, but surging in many parts of world, including Europe, Africa, and Asia and in a number of countries that have not yet been able to access a supply of vaccines for their populations. Many countries are struggling to cope with the fast-spreading variant, and growing number of people are under lockdown or state of emergencies due to the virus. South American countries, meanwhile, are experiencing the Lambda variant.

Share of the U.S. population 18+ that has received at least once shot