A new study by the U.S. health protection agency showed Friday that around three-quarters of the individuals infected with the novel coronavirus in outbreaks detected after large public events in a Massachusetts town had been fully vaccinated.

The Centers for Disease Control and Prevention said the data underscored that the Delta variant, the dominant strain of the coronavirus in the United States, is highly contagious. The White House, however, maintained that the view of the CDC is that breakthrough cases among vaccinated people are "rare."

A separate CDC internal document, a copy of which The Washington Post says it has obtained and made available on its website, characterized the Delta variant as being more transmissible than the common cold, seasonal flu or Ebola, and as infectious as chickenpox.

"Acknowledge the war has changed," the document warned, while recommending universal masking as an essential measure to reduce the transmission of the Delta variant.

In the study analyzing the outbreaks in the eastern state of Massachusetts, 469 COVID-19 cases were identified among residents who had traveled to a town in Barnstable County during the two weeks beginning July 3, of which 74 percent occurred in fully vaccinated persons.

Multiple summer events and large public gatherings were held in the town at that time, attracting thousands of tourists from across the United States, according to the report. People with COVID-19 reported having attending densely packed indoor and outdoor events at venues that included bars and restaurants.

According to the study, testing identified the Delta variant in 90% of specimens from 133 patients.

Overall, 79 percent of vaccinated patients with breakthrough infection were symptomatic, with the most common being cough, headache, sore throat, myalgia and fever.

Among five COVID-19 patients who were hospitalized, four were fully vaccinated. No deaths were reported, according to the study.

The CDC said infection with the Delta variant resulted in similarly high viral loads in vaccinated and unvaccinated people, suggesting an increased risk of transmission and raising concern that, unlike with other variants, vaccinated people infected with the variant can transmit the virus.

"This finding is concerning and was a pivotal discovery leading to CDC's updated mask recommendation," Rochelle Walensky, the head of the agency, said in a statement.

The CDC on Tuesday recommended that all persons, including those who are fully vaccinated, wear masks in indoor public settings in areas where COVID-19 transmission is high or substantial.

The administration of President Joe Biden has been stepping up its warning against the Delta variant as the country is seeing a sharp rise in coronavirus cases, while roughly half of the population remains unvaccinated.

On Thursday, Biden said millions of federal employees will be asked to attest to their vaccination status, and those not inoculated will be required to wear a mask on the job and comply with a weekly or twice-weekly virus testing requirement.

Supplied electron micrograph shows the new pneumonia-causing coronavirus. (Photo courtesy of the U.S. National Institute of Allergy and Infectious Diseases)(Kyodo)