When a wave of coronavirus infections crashed over India in the spring, scientists wondered whether a new variant, Delta, was partly to blame. Time appears to have answered the question: Having spread to at least 85 countries, the Delta variant is now fuelling outbreaks around the world and prompting new lockdowns — even in countries that seemed to have the virus under control. “The Delta variant is currently the greatest threat in the U.S. to our attempt to eliminate COVID-19,” Dr. Anthony Fauci said last week. What makes the Delta variant so concerning, and how will it affect the course of the pandemic? Here’s what people are saying. The Delta variant has multiple mutations that appear to make it 40 to 60 percent more transmissible than Alpha, the variant first identified in Britain, which is itself estimated to be about 30 to 50 percent more transmissible than the original coronavirus. In Australia, security cameras even documented a transmission that occurred between two people passing each other in a shopping mall.

“It is the most hyper-transmissible, contagious version of the virus we’ve seen to date, for sure — it’s a superspreader strain if there ever was one,” Eric Topol, a professor of molecular medicine and an executive vice president at the Scripps Research Institution, told Scientific American. The good news: Several of the vaccines in use still appear to work well against Delta. A Public Health England study published in May found that the variant only slightly reduced the effectiveness of two doses of the Pfizer and AstraZeneca vaccines. The Moderna vaccine is expected to perform similarly well. A preprint from Public Health England found that two doses of either vaccine also provided exceptional protection against hospitalisation from the Delta variant. But: The Delta variant does seem to pose a greater risk to people who have received only one dose. According to Public Health England, a single dose of either AstraZeneca’s or Pfizer’s vaccine reduced a person’s risk of developing COVID-19 symptoms by 33 percent, compared with 50 percent for the Alpha variant.

A single dose of AstraZeneca showed reduced protection against hospitalisation. (There was no major difference with Pfizer.) Preliminary evidence also suggests that the two Chinese vaccines, Sinovac and Sinopharm, may be less effective against Delta. Those vaccines are being used in more than 90 countries, including the Seychelles, Chile, Bahrain and Mongolia, which have all outpaced the U.S. vaccination rate. All four ranked among the top 10 countries with the worst COVID outbreaks as recently as last week. “If the vaccines are sufficiently good, we should not see this pattern,” said Jin Dongyan, a virologist at the University of Hong Kong.

Whether the Delta variant causes more severe illness than other variants remains an open question. One Scottish study published in The Lancet found that the hospitalisation rate of patients was about 85 percent higher than that of patients with the Alpha variant. But virulence is a difficult trait to measure, as Katherine Wu explains in The Atlantic, and any one study should be taken with a grain of salt: “If a surge in virulence happens, it’s often incidental — collateral damage from an increase in contagiousness,” she writes.

It is also not yet known how effective the Johnson & Johnson vaccine is against the Delta variant, though studies are in progress. “Generally people are agreeing that they anticipate that J&J will perform well against the Delta variant as it has so far against other variants circulating in the United States,” said Dr. Rochelle P. Walensky, the director of the Centers for Disease Control and Prevention.

Yet others say it may be time for people who received the Johnson & Johnson vaccine to consider supplementing their protection with a dose of Pfizer or Moderna — an idea the C.D.C. and the F.D.A. have not yet weighed in on. “I just decided that it was a good idea for me to get a boost,” Angela Rasmussen, a virologist at the University of Saskatchewan, told Slate. “We know that at least two doses of an adenovirus vector vaccine and an mRNA vaccine provide protection equivalent to having two mRNA vaccines.”

Many experts are concerned by Delta’s contagiousness, regardless of whether it causes more severe disease. “In unvaccinated populations, experts generally have greater fears about more transmissible variants than ones that are, say, just deadlier, because by causing more cases than would have occurred otherwise, the faster spreading strains can result in greater hospitalisations and deaths overall,” Andrew Joseph writes in Stat. “Delta appears to be able to do that and more.” Health care systems may strain even in places where vaccination rates are relatively high. In Britain, for example, where about half the population is fully vaccinated and the variant accounts for 90 percent of new infections, cases have increased sixfold in recent weeks. “What we’re seeing in U.K. is very likely to show up in other Western countries soon,” The Financial Times’s John Burn-Murdoch predicted at the end of May.

He was right. At the beginning of June, the Delta variant was responsible for 6 percent of U.S. infections. Now, it has overtaken the Alpha variant and is expected to account for a majority of infections by mid-July. Already, The Washington Post reports, it appears to be taxing hospitals in a lightly vaccinated part of Missouri, and hospitalisations are rising in Arkansas, Nevada and Utah, where less than 50 percent of the eligible population has received at least one vaccine dose.

By far the biggest risk, though, is to countries that have limited access to vaccines, Ewen Callaway writes in Nature. Experts are especially concerned about the potential for spread in Africa, where most nations have vaccinated less than 5 percent of their populations. Africa has weathered the pandemic much better than North America and Europe, but cases and deaths on the continent have increased by 40 percent in the past week.

“Many nations that excelled at protecting their citizens are now facing a triple threat,” Ed Yong writes in The Atlantic. “They controlled COVID-19 so well that they have little natural immunity; they don’t have access to vaccines; and they’re besieged by Delta.”

The C.D.C. has given no indication that it plans to revise its guidance from May that fully vaccinated people can forgo masking in most situations, which paved the way for restrictions to be lifted across the country. But the W.H.O. disagrees, warning that while immunisation is highly effective at preventing severe illness and death, the degree to which vaccines prevent transmission is unknown.

And globally, democratising vaccine access still remains the most urgent priority for containing the virus. “Vaccines that can save lives and turn this disease into a pre-pandemic baseline risk are available but in dire short supply,” says Zeynep Tufekci, a sociologist at the University of North Carolina, noting that even breakthrough infections from the Delta variant are overwhelmingly asymptomatic or mild. “Billions around the world deserve this. And there’s no time.”

Spencer Bokat-Lindell is a staff editor in the Opinion section of NYT©2021

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