Colorado county hit early by COVID-19 delta strain saw more breakthrough cases, CDC says

What to know about the delta variant

The delta variant has become the dominant strain of COVID-19 in the U.S. in the summer of 2021, driving up cases and hospitalizations.

The COVID-19 vaccine appeared to be less effective against some of the earliest reported cases of the delta variant in Mesa County, Colorado, according to the U.S. Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.

In the report, released on Aug. 6, the CDC compiled data collected by the Colorado Department of Public Health and Environment to identify some of the earliest cases of the delta variant of the COVID-19 virus in the country.

In May of this year, Mesa County, which has a population of 154,933, reported its first five cases of the delta variant, all of which "were associated with school settings," according to the CDC.

Mesa County’s delta variant case count more than doubled from 43% to 88% between May 1 to June 5. And as of June 6, more than half of Colorado’s delta variant cases were being reported out of Mesa County, according to the CDC.

FILE - People wait to get vaccinated for COVID-19 at a baseball game on Aug. 05, 2021 in Springfield, Missouri.

In early May, Mesa County’s vaccinations were at 36%, which was lower than the rest of Colorado which had reached 44% vaccinated during the same time.

"Compared with that in all other Colorado counties, incidence, intensive care unit (ICU) admissions, and COVID-19 case fatality ratios were significantly higher in Mesa County during the analysis period," the CDC said.

The CDC highlighted that breakthrough cases, which happen when a fully vaccinated person is still infected with COVID-19, were markedly higher in Mesa County.

"The slightly lower crude VE estimate against symptomatic infection in Mesa County may lend support to previous findings that COVID-19 vaccines provide modestly lower protection against symptomatic infection with the Delta variant," according to the report.

The COVID-19 vaccine was only 78% effective in Mesa County while the vaccine was 89% effective for the rest of Colorado, according to the CDC.

The CDC also posits that due to the higher levels of the delta variant circulating through Mesa County, there was higher exposure to vaccinated individuals, thus making way for higher rates of breakthrough infections.

Despite the report, the CDC continued to push vaccinations, citing how some protection is better than none and that the vaccines still prevent severe COVID-19 or hospitalization or death.

"Vaccination is a critical strategy for preventing infection, serious illness, and death associated with SARS-CoV-2 (including the Delta variant). Additional targeted prevention strategies (e.g., masking in indoor settings irrespective of vaccination status) and adherence to prevention strategies (e.g., surveillance testing and infection prevention and control procedures) are prudent in areas with high circulation of the Delta variant and in higher risk settings, such as residential care facilities," the report concluded.

Meanwhile, health officials on July 30 released details of research conducted by the CDC which was key in the decision by the administration to recommend that vaccinated people return to wearing masks indoors in parts of the U.S. where the delta variant is fueling infection surges. The authors said the findings suggest that the CDC’s mask guidance should be expanded to include the entire country, even outside of hot spots.

An outbreak in Provincetown — a seaside tourist spot on Cape Cod in the county with Massachusetts’ highest vaccination rate — has so far included more than 900 cases. About three-quarters of them were people who were fully vaccinated.

Health authorities have warned that even though the COVID-19 vaccines are incredibly effective -- the Pfizer and Moderna ones being about 95% effective against symptomatic infection in studies -- they’re not perfect. No vaccine is.

But it wasn’t until the delta variant began spreading that the risk of breakthroughs started getting much public attention. The barrage of headlines is disconcerting for vaccinated people wondering how to balance getting back to normal with more exposure to unvaccinated strangers — especially if they have vulnerable family members, such as children too young to qualify for the shots.

While there’s not a specific count, it’s clear severe breakthrough infections are rare. As of July 12, the CDC had tallied 5,492 vaccinated people who were hospitalized or died and also tested positive for coronavirus — out of more than 159 million fully vaccinated Americans. CDC Director Dr. Rochelle Walensky has said 99.5% of all deaths from COVID-19 are in the unvaccinated.

There isn’t a separate count of mild or asymptomatic breakthroughs, although CDC is tracking those through studies such as one that gives weekly virus tests to more than 5,000 essential workers, she told senators.

Breakthroughs tend to be mild because a vaccinated person’s immune system doesn’t have to start from scratch to fight the coronavirus. Even if the virus sneaks past vaccine-spurred antibodies and starts replicating in your nose or throat, secondary defenses jump into action and usually, "the virus is stopped in its tracks within a few days," said University of Pennsylvania immunologist Scott Hensley.

"The vaccines were developed to keep us out of those terrible institutions we call hospitals," said Dr. William Schaffner, an infectious disease expert at Vanderbilt University. "We have to keep coming back to that."

The Associated Press contributed to this report.