When Were the First U.S. COVID Delta Variant Cases, and How Did It Mutate?

COVID cases in the U.S. have continued to surge in recent days, with new daily infections hitting more than 125,000 on August 5.

CDC data suggests new cases are being driven by the Delta variant of COVID, which was detected in more than 80 percent of new cases surveyed in the two weeks to July 31.

On Friday, August 6, the CDC updated its information on the Delta variant, which it called the predominant strain of COVID in the U.S.

The health agency states that Delta is nearly twice as contagious as previous variants of the virus; potentially causes more severe illness than previous strains in unvaccinated people, based on two different studies; and fully vaccinated people with a Delta breakthrough infection can spread the virus to others—though vaccinated people appear to be infectious for less time than unvaccinated people.

So how did the U.S. end up with Delta, and what mutations does it have?

First cases

It's hard to say when the first cases of the Delta variant, also called B.1.617.2, emerged in the U.S., but reports from May indicate the variant was present in the country from that month at the latest.

The variant was first detected in India in October 2020, according to the World Health Organization (WHO).

On May 20, the University of Texas Southwestern Medical Center announced it had identified the first two cases of Delta in North Texas using PCR testing. It called the findings "concerning" because the variant was believed to be more infectious than the original variant of COVID.

At that time, the U.K. variant known as B.1.1.7 was still the dominant strain in the region. Two cases in Louisiana were reported the next day.

Then other counties and states started reporting cases, too. In mid-June, for example, Hawaii health officials confirmed the presence of Delta there in one individual. Later that month, officials in Orange County, Florida, announced they had detected a dozen cases of the then-new variant.

By July 1, CNN reported that Delta had been detected in all 50 states as well as Washington, D.C.

How has the variant mutated?

COVID variants become concerning when there are changes to their spike proteins—points that stick out around the outside of the virus that cause it to latch onto body cells and make us sick.

It's concerning because vaccines target these spike proteins. When the proteins change, it means the vaccine may not work as well as it was initially designed to. Other mutations may make the virus better at spreading. Current vaccines are still thought to work well in preventing people from getting severely ill from Delta.

Protein mutations happen all the time, randomly and naturally, for as long as the virus is allowed to spread, according to WebMD. Many of these mutations aren't beneficial to the virus at all and simply die off, but others, by chance, give the virus some sort of advantage.

Every COVID variant has different protein mutations. The Delta Plus variant, for instance, has the K417N protein mutation that is known to reduce the effectiveness of a cocktail of monoclonal antibody therapies. Others, such as the Beta variant first identified in South Africa, have E484K, a mutation that helps the virus slip past the body's immune defenses.

According to the CDC, Delta and its subtypes display spike protein mutations T19R, (V70F*), T95I, G142D, E156-, F157-, R158G, (A222V*), (W258L*), (K417N*), L452R, T478K, D614G, P681R, and D950N.

L452R is notable because it is thought to give the virus immunity evasion properties and also makes it more infectious, according to an international team of researchers led by Kumamoto and Tokyo Universities in Japan.

Speaking to ABC's George Stephanopoulos in a Good Morning America interview on Thursday, National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci said the virus has an opportunity to mutate for as long as it can spread, and future, more harmful variants are possible.

Urging vaccine uptake, Fauci said: "You may, sooner or later, get another variant. And it is possible that that variant might be in some respects worse than the already very difficult variant we're dealing with now, which is a major reason why you want to completely suppress the circulation of the virus in the community."

COVID test nurse
A nurse seals a COVID-19 swab test specimen bag at a mobile clinic in South Los Angeles, California, in July 2020. The Delta variant has rapidly spread throughout the U.S. this year to become the dominant strain. Mario Tama/Getty