A major worry in the world right now is Delta, a highly contagious SARS-CoV-2 virus strain, which was first identified in India in December. It then swept rapidly through that country and Great Britain before reaching the U.S., where it is now the dominant variant.

The CDC described Delta as more transmissible than the common cold and influenza, as well as the viruses that cause Ebola, smallpox, MERS, and SARS, Ebola—and called it as contagious as chickenpox in an internal document, a copy of which was obtained by and reported on in The New York Times, The highest spread of cases and severe outcomes is happening in places with low vaccination rates, and virtually all hospitalizations and deaths have been among the unvaccinated, the CDC says.

The major worry about the Delta Variant first identified in India, is not that it makes people sicker, but that it spreads far more easily from person to person, increasing infections and hospitalizations among the unvaccinated.

Evidence is also mounting that it is capable of infecting fully vaccinated people at a greater rate than previous versions, and concerns have been raised that they may even spread the virus, these experts said.

Viruses constantly evolve through mutation, with new variants arising. Sometimes these are more dangerous than the original.

The World Health Organization (WHO) has called this version of the virus “the fastest and fittest.” The CDC labeled Delta as “a variant of concern,” using a designation also given to the Alpha strain that first appeared in Great Britain, the Beta strain that first surfaced in South Africa, and the Gamma strain identified in Brazil. (The new naming conventions for the variants were established by the WHO as an alternative to numerical names.)

“It’s actually quite dramatic how the growth rate will change,” says Dr. Wilson, commenting on Delta’s spread in the U.S. in June. Delta was spreading 50% faster than Alpha, which was 50% more contagious than the original strain of SARS-CoV-2, he says. “In a completely unmitigated environment—where no one is vaccinated or wearing masks—it’s estimated that the average person infected with the original coronavirus strain will infect 2.5 other people,” Dr. Wilson says. “In the same environment, Delta would spread from one person to maybe 3.5 or 4 other people.”