The Delta variant of coronavirus was first detected in India last October, where it helped fuel a devastating Covid-19 surge that set records for new infections and deaths. Delta has since spread to more than 100 countries. Nations that had previously kept Covid-19 cases relatively low, such as Indonesia, Australia and parts of Africa, are now seeing record growth in infections from the more transmissible variant.

Delta was first detected in the U.S. in March and by mid-July accounted for three-quarters of Covid-19 cases. It has supplanted the Alpha variant, which until recently was the most widespread version of the virus in the U.S. Its impact is acutely felt in parts of the country with low vaccination rates, where case counts and hospitalizations are surging. The Delta variant accounts for 83% of all U.S. cases, according to recent estimates from the U.S. Centers for Disease Control and Prevention. Unvaccinated individuals make up more than 95% of all hospitalizations.

What makes the Delta variant more contagious?

Researchers think Delta is about 50% more transmissible than the Alpha variant, which means the average patient would infect 50% more contacts. Alpha itself is an estimated 50% more contagious than earlier versions of the virus.

Delta’s increased infectiousness is driven by a unique combination of mutations, changes to the virus’s genetic code that affect its structure and function. Some of Delta’s most pernicious mutations affect its spike protein, which the virus uses to latch onto and infect human cells.

These mutations can make the virus better at binding to cells, as well as help it elude antibodies, which our immune systems deploy to neutralize the virus.

How do Delta’s mutations affect the spike protein?

A model generated by Robert F. Garry, a virologist at Tulane University, depicts one of the three subunits of the spike protein. It shows key parts of the protein that are affected by the Delta variant’s mutations. These locations provide clues about how the mutations may enhance Delta’s function. The variant has mutations that are unique to it, as well as ones found in other versions of the coronavirus.

How effective are vaccines against the Delta variant?

Despite Delta’s mutations, studies suggest that the Covid-19 vaccines authorized in the U.S. are effective in preventing serious illness in those exposed to the variant.

Vaccines work by stimulating the body to produce antibodies that are targeted to a particular pathogen, such as the coronavirus. After vaccination, the antibodies stay in our system. If we are infected with the actual virus, the antibodies are on hand to counter it before it can cause illness.

SHARE YOUR THOUGHTS

How should world governments respond to the Delta variant? Join the conversation below.

Currently authorized Covid-19 vaccines were created based on versions of the coronavirus that were circulating last year, before Delta emerged. The new mutations in Delta’s spike protein could make it harder for vaccine-produced antibodies to recognize and neutralize the variant, some experts have worried.

One way to measure a virus’s ability to evade vaccines is to take antibodies from patients who have been vaccinated and combine them with the actual virus in the lab, then measure how much of the virus is neutralized. In a yet-to-be-peer-reviewed study, Dr. Akiko Iwasaki, an immunologist at Yale University, and colleagues used this technique to compare how well different coronavirus variants can withstand vaccine neutralization. Blood samples were collected from patients who had received mRNA vaccines, the technology used in the Pfizer and Moderna shots.

The researchers found that Delta was better at evading neutralization than earlier versions of the virus from the start of the pandemic. It was also more resistant to neutralization than the Alpha variant, which was first detected in the U.K. last year. But it isn’t as resistant as the Gamma variant, first found in Brazil, or the Beta variant from South Africa.

Real-world studies support findings from the lab. U.S.-authorized vaccines are somewhat less effective at preventing infection from the Delta variant than they are against established versions of the virus. But they still offer considerable protection against severe illness and hospitalization.

A study of nearly 20,000 people published in the New England Journal of Medicine found that after two doses, the Pfizer vaccine was 88% effective at preventing symptomatic disease caused by Delta. It was 94% effective against the Alpha variant.

An analysis of more than 14,000 Delta cases by England’s public-health agency found that two doses of the Pfizer vaccine reduced the risk of hospitalization by 96%.

Despite the vaccines’ effectiveness, the protection they provide wanes over time. Public-health officials suspect Americans will eventually need booster shots to retain their immunity.

Vaccinated people who become infected can still transmit the virus to others. Citing the contagiousness of Delta, the CDC recommended Tuesday that vaccinated people resume masking indoors in certain parts of the country.

Covid-19’s Delta variant is proliferating world-wide, threatening unvaccinated populations and economic recovery. WSJ breaks down events in key countries to explain why Delta spreads faster than previously detected strains. Composite: Sharon Shi The Wall Street Journal Interactive Edition

Write to Josh Ulick at Josh.Ulick@wsj.com and Alberto Cervantes at Alberto.Cervantes@wsj.com