World Health Organization officials, concerned about the Delta variant, have urged even fully vaccinated people to continue wearing masks and taking other precautions.
The Centers for Disease Control and Prevention, on the other hand, told fully vaccinated Americans in May that they no longer needed to wear masks indoors or to stay six feet from other people. The agency also eased advice about testing and quarantine after suspected exposure.
Asked on Monday about the W.H.O.’s cautions, a C.D.C. spokesman pointed to the existing guidance and gave no indication it would change.
The Delta variant, a highly infectious form of the virus that has spread to at least 85 countries since it was first identified in India, is now responsible for one in every five Covid-19 cases across the United States. Its prevalence here has doubled in the past two weeks, and Dr. Anthony Fauci, the nation’s top infectious disease doctor, has called it “the greatest threat” to eliminating the virus in the United States.
Los Angeles County said on Monday that it strongly recommended that everyone wear masks indoors as a precaution against the Delta variant, adding that it accounted for nearly half of all cases sequenced in the county.
“Until we better understand how and to who the Delta variant is spreading, everyone should focus on maximum protection with minimum interruption to routine as all businesses operate without other restrictions,” county officials said in a statement.
The rise of new variants “makes it even more urgent that we use all the tools at our disposal,” Dr. Tedros Adhanom Ghebreyesus, the director-general of the W.H.O., said at a news briefing on Friday.
Though fully vaccinated people are largely protected, studies suggest the Pfizer vaccine’s efficacy against the Delta variant is slightly lower than against other variants, and significantly lower for individuals who have received only one dose.
Britain — where some two-thirds of the population have received at least one dose of the Pfizer or AstraZeneca vaccine and just under half have received two — has seen a sharp rise in cases driven by the variant. And Israel, with one of the highest vaccination rates in the world, has partially reimposed mask mandates in response to an uptick in cases.
Given how fast-moving the variant is, “the vaccine approach is not enough,” said Eric Feigl-Ding, senior fellow at the Federation of American Scientists in Washington. “We’re not at the level of vaccinations where we can release the brakes on everything else.”
Other scientists disagreed, saying guidance has to be tailored to local conditions.
“The W.H.O. is looking at a world that is largely unvaccinated, so this makes sense,” said Dr. Ashish Jha, the dean of the Brown University School of Public Health, adding that parts of the United States might also need different advice.
“If I were living in Missouri or Wyoming or Mississippi, places with low vaccination rates,” he said, “I would not be excited about going indoors without wearing a mask — even though I’m vaccinated.”
In other news from around the world:
The African Union and the Africa Centers for Disease Control and Prevention have voiced concerns that a planned European Union digital travel pass does not cover people vaccinated with Covishield, the shot made at the Serum Institute of India on which many low-income countries have relied. The vaccine is the same as the one manufactured by AstraZeneca, which has E.U. approval and is included in the E.U. travel pass, but the Serum Institute has not received its own separate E.U. authorization to market the shot under the name Covishield. In a joint statement, the A.U. and the Africa C.D.C. said the exclusion was “concerning” and urged Brussels to expand its list of acceptable vaccines.
Abu Dhabi, the capital of the United Arab Emirates, will bar unvaccinated residents over the age of 15 from accessing most public places from Aug. 20, including schools and universities as well as malls, gyms, restaurants, cafes and cultural venues. The new rules were announced on Twitter by the government’s media office on Monday, and come as an Emirati federal authority reported a rise in cases and deaths linked to the more infectious Delta variant, according to Reuters.