The delta variant, first identified in India, now makes up 83% of all sequenced cases in the United States, marking a 33% increase since July 3rd, according to the director of the Centers for Disease Control and Prevention.

The new numbers come as CDC Director Dr. Rochelle Walensky revealed that two-thirds of counties in the U.S. have vaccinated 40% or less of their residents, which has led to the rapid spread of the variant. Since initially being found, it has also spread and become the dominant strain of the virus in the United Kingdom, Germany and other countries.

“Each death is tragic and even more heartbreaking when we know that the majority of these deaths could be prevented with a simple, safe, available vaccine,” Walensky said, according to CNBC.

Deaths have risen 48% in the past week and average 239 per day. More than 34.1 million have contracted the virus, and 609,000 have died.

Studies have shown vaccination is effective against multiple variants, including Delta, meaning those who remain unvaccinated run the risk of becoming infected.

The delta variant is more contagious than the beta variant that was first discovered in the U.K. and was estimated to be 43% to 90% more transmissible than the original COVID strain, according to public health officials.

“The reason that it’s so formidable is the fact that it had the capability of transmitting efficiently from human to human in an extraordinary manner, well beyond any of the variants we have experienced up to now,” said Dr. Anthony Fauci.

Symptoms for the Delta variant are similar to the original strain of coronavirus, however, cough and loss of smell seem to be less common, and headache, sore throat, fever and runny noses seem to be more common, according to WebMD.