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| The spread of the super-contagious Delta variant is prompting new lockdowns around the world and spurring new warnings from public health officials. |
| The World Health Organization, citing the rise of Delta, the dearth of vaccines and high rates of community transmission in many parts of the world, has encouraged fully vaccinated people to continue wearing masks. |
| Australian cities facing outbreaks of the variant have instituted lockdowns, Malaysia has extended stay-at-home orders, Ireland has delayed plans to reopen indoor dining and Hong Kong has restricted incoming flights from Britain, where Delta is widespread. |
| The Delta variant’s potential risk to the United States, where about 57 percent of adults have been fully vaccinated, is more uncertain. The Centers for Disease Control and Prevention has given no indication that it will change the recommendations it released in May saying that fully vaccinated people do not need masks. |
| But some local governments are reacting more conservatively. On Monday, the Los Angeles County Department of Public Health recommended that everyone, even people who have been fully vaccinated, wear a mask indoors, noting that the Delta variant accounted for nearly half of all variants genetically sequenced in the county. |
| Last week, health officials announced that the Delta variant was responsible for about one in every five Covid-19 cases in the United States, and that its prevalence had doubled in the last two weeks. |
| Two doses of the Pfizer-BioNTech vaccines work well against the variant, and the Moderna vaccine is expected to be similarly effective. But pockets of unvaccinated people may be vulnerable to outbreaks in the coming months, scientists said. |
| “The Delta variant is currently the greatest threat in the U.S. to our attempt to eliminate Covid-19,” Dr. Anthony S. Fauci, the nation’s leading infectious disease expert, said at a White House press briefing last week. |
| First identified in India, Delta is one of several “variants of concern,” as designated by the C.D.C. and the W.H.O. It has spread rapidly through India and Britain and poses a particular threat in places where vaccination rates remain low. |
| Here are answers to some common questions about the Delta variant. |
| Why are people worried about the Delta variant? |
| Delta, formerly known as B.1.617.2, is believed to be the most transmissible variant yet, spreading more easily than both the original strain of the virus and the Alpha variant first identified in Britain. Public health officials there have said that Delta could be 50 percent more contagious than Alpha, though precise estimates of its infectiousness vary. |
| Other evidence suggests that the variant may be able to partially evade the antibodies made by the body after a coronavirus infection or vaccination. And the variant may also render certain monoclonal antibody treatments less effective, the C.D.C. notes. |
| Delta may also cause more severe illness. A recent Scottish study, for instance, found that people infected by the Delta variant were roughly twice as likely to be hospitalized than were those infected with Alpha. But uncertainties remain, scientists said. |
| “The severe disease piece I think is the one question that really hasn’t been answered yet,” said Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. |
| Where is it spreading? |
| Delta has been reported in 85 countries. It is now the most common variant in India and Britain, where it accounts for more than 90 percent of cases. |
| Delta was first identified in the United States in March. Although Alpha remains the most prevalent variant here, Delta has spread quickly. In early April, Delta represented just 0.1 percent of cases in the United States, according to the C.D.C. By early May, the variant accounted for 1.3 percent of cases, and by early June, that figure had jumped to 9.5 percent. Last week, Dr. Fauci said that the estimate had hit 20.6 percent. |
| Does the Delta variant cause different symptoms? |
| It’s not clear yet. “We’re hurting for good data,” Dr. Osterholm said. |
| In Britain, where the variant is widespread, reports have emerged that Delta may cause different symptoms than other variants do. Researchers conducting the Covid Symptom Study, which asks people with the disease to report their symptoms in an app, have said that the most common symptoms of Covid have changed as the variant has spread through Britain. |
| “What we’ve noticed is the last month, we’re seeing different sets of symptoms than we were seeing in January,” said Tim Spector, a genetic epidemiologist at King’s College London, who leads the study. |
| Headaches, a sore throat, and a runny nose are now among the most frequently reported symptoms, Dr. Spector said, with fever, cough and loss of smell less common. |
| These data, however, have not yet been published in a scientific journal, and some scientists remain unconvinced that the symptom profile has truly changed. The severity of Covid, regardless of the variant, can vary wildly from one person to another. |
| “I’ll wait for published data before I make a conclusion,” said Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan. “The fact is Covid is generally associated with a wide variety of symptoms, so it’s hard to say if this is truly unusual or if this is anecdotal.” |
| Even if the data hold up, it does not necessarily mean that Delta itself causes different symptoms than other variants do. A milder symptom profile could be a result of the fact that the variant is primarily infecting younger people, who are the least likely to be vaccinated, or those who may already have some immunity to the virus from a previous infection, for example. |
| If I’m vaccinated, do I need to worry? |
| Although there is not yet good data on how all of the vaccines hold up against Delta, several widely used shots, including those made by Pfizer-BioNTech and AstraZeneca, appear to retain most of their effectiveness against the Delta variant, research suggests. |
| “If you’re fully vaccinated, I would largely not worry about it,” said Dr. Ashish K. Jha, dean of the Brown University School of Public Health. |
| According to one recent study, the Pfizer-BioNTech vaccine was 88 percent effective at protecting against symptomatic disease caused by Delta, nearly matching its 93 percent effectiveness against the Alpha variant and 95 percent against the original version of the virus. But a single dose of the vaccine was just 33 percent effective against Delta, the study found. |
| “Fully immunized individuals should do well with this new phase of the epidemic,” said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine. “However, the protection offered by a single dose appears low, and of course if you are not at all vaccinated, consider yourself at high risk.” |
| Delta is likely to infect “large numbers” of unvaccinated people, he said. |
| Will it cause a new surge in the United States? |
| It is unclear how much of a problem Delta will cause in the United States, where more than half of adults have been fully vaccinated. “I think we are not going to see another big, national surge in the United States because we have enough vaccination to prevent that,” Dr. Osterholm said. |
| Still, vaccination rates have been highly uneven, and are lower in certain states and demographic groups. Delta could fuel outbreaks in the South, where vaccinations lag, or among young people, who are less likely to be vaccinated than their elders. |
| “In places where there’s still a lot of susceptibility to the virus, it opens a window for cases to start going up again,” said Justin Lessler, an infectious disease epidemiologist at Johns Hopkins University. “But even in those states, and certainly nationally, we’re probably not getting back to the numbers we were seeing last winter.” |
| In countries where vaccination rates remain low, including many low- and middle-income countries that have struggled to get access to vaccines, Delta could be disastrous. |
| The variant is “spreading rapidly among unvaccinated populations,” Dr. Tedros said at the news briefing. “As some countries ease public health and social measures, we’re starting to see increases in transmission around the world. More cases means more hospitalizations, further stretching health workers and health systems, which increases the risk of death.” |
| What can I do? |
| Get vaccinated. If you’re already vaccinated, encourage your family, friends and neighbors to get vaccinated. Vaccination is likely to slow the spread of all the variants and reduce the odds that new, even more dangerous variants emerge. |
| “I encourage people who are vaccinated to trust in the vaccines but be cognizant that new variants will continue to occur where transmission exists,” said Saskia Popescu, an infectious disease epidemiologist at George Mason University. “So it’s really about ensuring local, national and global vaccination.” |
| Face masks, which remain a particularly important tool for those who are ineligible for or do not have access to vaccines, can provide additional protection. |
| “Mask wearing remains an effective tool for reducing transmission, especially indoors where the virus may be easily spread through inhalation of aerosols emitted by an infected person,” Barbara Ferrer, the director of public health for Los Angeles County, said in a statement this week. |
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