As COVID — especially the Delta variant — surges among the fully vaccinated, Brian Hooker, Ph.D., said the more the variant deviates from the original sequence used for the vaccine, the less effective the vaccine will be on that variant.

The CDC’s latest breakthrough numbers, as of July 25, show 6,587 fully vaccinated people with COVID breakthrough cases. Of those, 6,239 people were hospitalized and 1,263 people died.

In May, the CDC revised its guidance for reporting breakthrough cases, stating it would count only those cases that result in hospitalization or death. Previously, the agency had included in its breakthrough count anyone who tested positive for COVID.

According to the CDC, the surveillance system for breakthrough cases is passive and relies on voluntary reporting from state health departments, which may not be complete.

In addition, some breakthrough cases will not be identified due to lack of testing. This is particularly true in instances of asymptomatic or mild illness, the CDC said.

NBC News contacted health agencies in 50 states and the District of Columbia to collect information on breakthrough cases, citing a lack of comprehensive data available from the CDC.

Data collected from 38 states showed more than 125,000 fully vaccinated Americans tested positive for COVID, and 1,400 died.

This conflicts with the CDC’s data published July 26. Research by NBC News indicates the number who have been hospitalized or died passed 7,300 in just 30 states providing data.

The total number of breakthrough cases is likely higher than 125,683, as nine states, including Pennsylvania and Missouri, did not provide information, while 11 states did not provide death and hospitalization totals. Four states gave death and hospitalization numbers, but not total cases.

In addition, vaccinated adults who had breakthrough cases but showed no symptoms could be missing from the data altogether, officials told NBC.

According to research published last week in Scientific Reports, the highest risk for establishing a vaccine-resistant virus strain occurs when a large fraction of the population has already been vaccinated but the transmission is not controlled.

The data was consistent with a study released July 30, by the CDC which showed vaccinated people may transmit the Delta variant — now responsible for 80% of COVID cases in the U.S. — just as easily as the unvaccinated.

The team of scientists who published the data in Scientific Reports said their findings follow what’s known as selective pressure — the force that drives any organism to evolve.

“Generally, the more people infected, the more the chances for vaccine resistance to emerge,” said Fyodor Kondrashov of the Institute of Science and Technology Austria.

“So the more Delta is infectious, the more reason for concern,” Kondrashov said. “By having a situation where you vaccinate everybody, a vaccine-resistant mutant actually gains a selective advantage.”

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