The highly transmissible Delta variant is the most common strain of the COVID-19 virus circulating in the U.S., new data showed, but federal health officials said fully vaccinated people don't necessarily need to put on face masks again.
An analysis of genetic sequencing data as of June 27 showed that the Delta strain, also known as B.1.617.2, now makes up about 40% of positive COVID-19 test samples, according to Helix, a population genomics company that collects and analyzes test samples from several U.S. states.
"This is the first time we felt confident to say that Delta is the most prevalent lineage in the U.S.," said William Lee, Helix's vice president of science. He said the variant is on track to make up half or more of COVID-19 infections by early to mid-July.
Despite the Delta variant's exponential spread, the Centers for Disease Control and Prevention said it is sticking with guidance issued in May that Americans no longer need to wear masks indoors if they are fully vaccinated.
"People who are fully vaccinated are protected, including from the variants currently circulating in the country such as the Delta variant, " the CDC said Wednesday in a statement. Nearly 154.9 million people, or 46.7% of the total U.S. population, are fully vaccinated, according to the CDC.
The CDC's position on mask wearing differs from that of the World Health Organization, which said on June 25 that vaccinated people should wear masks indoors, keep a distance from others and avoid crowds when they are in a region experiencing community transmission of COVID-19. The WHO's guidance is global, and many countries have lower vaccination rates and less effective vaccines than those available in the U.S.
The CDC advises people to follow guidance from local public health authorities, which may differ from its own if there are COVID-19 outbreaks or vaccination rates are low.
Los Angeles County public health officials urged all people this week to wear a mask indoors in public places, regardless of vaccination status due to circulation of the Delta variant. According to CDC data, 50.4% of the county's population has been fully vaccinated.
CDC Director Rochelle Walensky said on June 22 that the Delta variant is expected to become dominant in the U.S. over the next month. CDC data show that as of June 19, Delta made up 26.1% of positive Covid-19 samples sequenced, while Alpha made up 47.8%.
Helix's data showed that the Delta strain is now more prevalent than Alpha, or B.117, which had been the most common strain in the U.S. since March. Helix said Alpha has dropped below 20% of positive test samples sequenced. Another variant, called Gamma, or P.1, appears to be plateauing, he said.