Delta was first identified in the United States in March. It 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. The C.D.C. now estimates that the number has hit 83.2 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,” Tim Spector, a genetic epidemiologist at King’s College London who leads the study, said in June.
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-19, regardless of the variant, can vary wildly from one person to another.
“I’ll wait for published data before I make a conclusion,” Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan, said last month. “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.