As the coronavirus sweeps across the globe in relentless waves, a succession of more-infectious strains has washed up. So far delta, the variant first reported in India in October, is the most worrisome. About twice as transmissible as the original virus that emerged in late 2019, delta combines a multitude of genetic features that have enabled it to pierce public-health defenses to stoke severe epidemics and trigger fresh rounds of movement restrictions or delay planned easings of them.

1. Why is delta worse than other strains?

It’s the most infectious variant of SARS-CoV-2, the coronavirus that causes Covid, to emerge yet -- about 55% more transmissible than alpha, the previously most-infectious form, and almost twice as contagious as other circulating strains. The World Health Organization expects delta to rapidly outcompete other coronavirus variants and become the dominant strain of SARS-CoV-2 globally in coming months.

2. What makes delta more transmissible?

It’s not entirely clear, but it’s thought that the variant replicates faster within those harboring it and may be expelled in larger volumes by them. Delta has also been associated with an increased incidence of Covid in younger patients, who may be more socially mobile and less likely to have been immunized.

3. What’s different about it?

The variant carries a genetic mutation that helps it to bind more easily to ACE2, a protein on some human cells, potentially enabling it to cause infections more readily. Concentrations of virus particles, or viral loads, in the airways of individuals recently infected with delta were about 1,200 times greater than in those infected with the original coronavirus strain, a study in China found. The research, released without pre-publication review by experts in the same field, also noted that delta was detectable in patients four days after picking up the virus -- two days sooner than was previously observed, indicating the strain replicates faster and may make individuals more infectious during the early stages of infection. A short incubation period may make it more difficult to prevent outbreaks by tracing the contacts of infected people and requiring them to quarantine, a strategy relied on heavily by countries such as China and Australia; a shorter incubation makes it more difficult to identify contacts before they are themselves infectious.

4. Is delta more virulent?

Possibly. In India, which had an unprecedented wave of infections peaking at close to 400,000 daily cases in mid-May, doctors linked delta to a broader array of Covid symptoms, including hearing impairment. Early data from Scotland indicated that delta-infected Covid patients were 1.8 times more likely to be hospitalized than those with an alpha infection. Other U.K. data support the increased risk of hospitalization but do not provide clear evidence that delta patients experience more severe illness once in the hospital. A study from Ontario, Canada, analyzing data from more than 200,000 Covid patients found delta carried a higher risk of hospitalization, admission to an intensive care unit, and death compared with other strains.

5. Is delta more likely to infect those already immune?

Delta appears better at evading the immunity gained from either immunization or a previous coronavirus infection. It also appears less responsive to some of the antibody therapies used to treat Covid. The reduction in the effectiveness of vaccines against Covid caused by delta may vary by inoculation. For example, the shot made by Pfizer Inc. and its partner BioNTech SE appeared to be more effective than AstraZeneca Plc’s in a study from England published in the New England Journal of Medicine in July. A full course of most vaccines, though, provides strong protection against severe illness and death. Recent modeling from Germany found the continued use of face masks and testing would considerably reduce the chance of a further surge in infections.

6. How prevalent is delta?

Delta has spread rapidly, turning up in about a dozen new countries each week in July and reaching 124 nations as of July 18. It’s now the dominant strain in the U.S., U.K., Russia, China and at least 10 other nations.

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