The Delta Covid-19 variant is spreading: What does this mean for the US?

The variant, also known as B.1.617.2, is the most worrying of a lineage of the virus that was first identified in India in late 2020 and has spread to almost 70 countries, including the U.S.

The version of the virus played a significant role in a record-setting surge of infections that overwhelmed India’s healthcare system. It has become the dominant variant in the U.K., accounting for 96% of samples sent for genetic analysis in the final week of May. After cases of the variant surged, Prime Minister Boris Johnson delayed by four weeks the country’s plans to drop all remaining Covid-19 restrictions in late June.

President Biden tweeted about the variant, noting its spread among young people in the U.K. and urging young Americans to get vaccinated. His chief medical adviser, Anthony Fauci, said in a White House briefing that the variant made up 6% of samples sequenced in the U.S.

The variant will move from country to country over a difficult-to-predict time frame, said Jeff Barrett, director of the Covid-19 Genomics Initiative at the Wellcome Sanger Institute in the U.K. “I think the U.S. will be the next place that will probably see a reasonable rise," he said.

Here’s what we know so far about the Delta variant.

What is the Delta coronavirus variant?

The B.1.617 lineage, identified in October, refers to several variants of the virus that causes Covid-19.

The most concerning of the lineage is the Delta or B.1.617.2 variant, which scientists say appears to have two advantages over earlier forms of the virus. It has been established to be more infectious and it also appears to be more effective at evading vaccines, though people who are fully vaccinated still have significant protection against illness.

Are there any cases of this new variant in the U.S.?

Yes. Almost 3,000 Delta cases have been reported in the U.S., according to the nonprofit Global Initiative on Sharing All Influenza Data, or GISAID, based on genetic samples uploaded to its database. The Delta variant has been reported in a majority of states, including California, New York and Massachusetts.

To limit the variant’s spread, the U.S. has suspended most travel from India to the U.S. The Centers for Disease Control and Prevention stipulates that all U.S.-bound air travelers have proof of a negative Covid-19 test administered no more than three days before arrival.

How contagious is this variant?

Scientists are still studying the virus and their early conclusions aren’t definitive. But British scientists, who have probably done the most work on the variant, estimate it is from 40% to as much as 80% more infectious than the so-called Alpha variant, or B.1.1.7, which was first identified in England last year, is now prevalent in the U.S. and is itself more contagious than the version of the virus that emerged in China in 2019.

Wendy Barclay, professor of Virology at Imperial College London, says swab tests suggest that Delta infects people with a heavier virus load, which means they exhale more of it for others to catch. The mutations also appear to make the variant more effective at attaching itself to cells in human airways. The combination means an infected person is, other things being equal, likely to infect more people and that people require less exposure to become infected.

Scientists don’t yet know if the Delta variant is deadlier than other variants, but a study published in “The Lancet" medical journal estimated that the risk of hospitalization after infection with Delta could be 85% higher than with Alpha.

Will existing vaccines work against this variant?

Yes, but the variant appears to be somewhat more effective at evading the vaccines and the body’s immune responses in general. No vaccine is 100% effective and while those in use in the West appear to work well in preventing symptomatic Covid-19 and serious illness, they appear to work slightly less well than against earlier versions of the virus.

An analysis of more than 14,000 Delta cases by England’s public-health agency found a double dose of the shot developed by Pfizer Inc. and BioNTech SE reduces the risk of hospitalization after infection with Delta by 96%. Two doses of the vaccine developed by the University of Oxford and AstraZeneca reduce the risk by 92%, Public Health England said.

Very few of those hospitalized in the U.K. have been fully vaccinated, with the new cases mostly among younger people who aren’t yet vaccinated. There is no evidence that young adults and children are more at risk proportionately from this variant than other age groups, and the increased transmission mostly reflects the fact that they haven’t been immunized, scientists say.

Can I get reinfected with this variant even after I’ve had Covid-19 or been vaccinated against it?

Yes, it is possible to become reinfected, though there is some evidence that second infections are on average less severe than the first.

Are the symptoms of the Delta variant any different?

There is no data yet to show that the variant is causing atypical symptoms. There were reports in India suggesting that the virus was causing hearing loss, gangrene and other unusual symptoms, but scientists said an intense epidemic such as the one in India will tend to produce a spectrum of rare conditions, simply given the large number of infections.

Write to Stephen Fidler at stephen.fidler@wsj.com and Suryatapa Bhattacharya at Suryatapa.Bhattacharya@wsj.com