Vaccinated patients who acquired so-called breakthrough infections carried nearly the same amount of the coronavirus as those who did not get the injections, according to scientists who analyzed a large COVID-19 outbreak in the US state of Massachusetts.

The findings of that study were pivotal in the US Centers for Disease Control and Prevention's decision this week to recommend that vaccinated persons return to wearing masks indoors in areas where the Delta variety is fueling illness outbreaks.

The data suggest that the CDC's mask guidance should be broadened to include the entire country, even areas outside of hot zones, according to the authors.

The discoveries have the potential to change how we think about how disease spreads in the future. Previously, it was assumed that vaccinated people who were infected had low amounts of the virus and were unlikely to spread it to others. However, new evidence suggests that this is not the case with the Delta variety.

More than 900 cases have been reported in Provincetown, a coastal tourist destination on Cape Cod in the county with Massachusetts' highest immunization rate. About three-quarters of them had received all of their vaccines.

Travis Dagenais, one of the many vaccinated people afflicted, said it was a mistake to "throwing caution to the wind" and party in groups for long hours over the July Fourth holiday.

"The dominant public messaging has been that the vaccine means a return to normal," said the 35-year-old Boston resident.

"Unfortunately, I've now learned it's a few steps toward normal, not the zero-to-sixty that we seem to have undertaken."

Dagenais believes that getting vaccinated helped him get over the worst of his flu-like symptoms in a matter of days. He has made a full recovery.

Massachusetts, like many other states, abolished all COVID-19 limitations in late May, ahead of the summer season's typical opening on Memorial Day. The requirement for everyone to wear an inside mask was reinstated in Provincetown this week.

Internal records on breakthrough infections and the Delta variety imply the CDC is exploring significant adjustments to its coronavirus recommendations, such as advising masks for everyone and requiring immunizations for doctors and other health professionals, according to leaked internal documents.

According to the documents, the Delta strain, which was first discovered in India, causes infections that are more contagious than the common cold, flu, smallpox, and the Ebola virus, and it is as infectious as chickenpox.

COVID-19 vaccinations are still extremely effective in avoiding major diseases and death from the Delta version.

The White House defended its approach to rising viral infections and shifting public health standards on Friday, deferring to the Centers for Disease Control and Prevention (CDC) while emphasizing the need for immunizations.

While most experts agreed with the CDC's revised indoor masking policy, several said the Provincetown epidemic report did not establish that vaccinated people are a substantial source of new illnesses.

"The (CDC) recommendation is scientifically plausible. However, it is not based on this research "Jennifer Nuzzo, a public health expert at Johns Hopkins University, agreed.

The CDC's findings are based on roughly 470 COVID-19 cases connected to Provincetown's holiday celebrations, which included densely packed indoor and outdoor holiday parties at bars, restaurants, guest houses, and rental residences.

Researchers tested a sample of them and found that individuals who were fully vaccinated and those who have not had nearly the same level of virus.

Three-quarters of the illnesses occurred in those who had been fully immunized. Around 80% of individuals who were fully vaccinated suffered symptoms, the most common of which were cough, headache, sore throat, muscle aches, and fever.

According to Dr. Angela Rasmussen, a virologist at the University of Saskatchewan, the method researchers used to quantify how much virus an infected individual carries does not indicate whether they are actually transferring the virus to other people.