COVID-19 infections are exceedingly rare after full vaccination: CDC

Only 0.01% of people tested positive for COVID after getting fully vaccinated.

The United States reached a major milestone this week, with the White House reporting that over half of all adults in the U.S. are now fully vaccinated against COVID-19.

This announcement came on the same day as a new report from the Centers for Disease Control and Prevention that breakthrough COVID-19 infections are exceedingly rare in fully vaccinated people.

A breakthrough infection is when a fully vaccinated person becomes infected with COVID-19.

The new CDC report shows that such breakthrough infections may occur in just 0.01% of all fully vaccinated people.

"This report helps confirm, in a real-world setting, that breakthrough infections are rare and when they do happen, they mostly have no clinical significance," said Dr. John Brownstein, a chief innovation officer at Boston Children's Hospital and professor of pediatrics at Harvard Medical School.

Earlier this month, the New York Yankees had some staff members and players test positive despite having been fully vaccinated.

These widely publicized cases sparked confusion and concern, but experts say it's important to note that no vaccine is 100% effective, and vaccines still help protect from the worst COVID-19 symptoms.

"We always tell vaccinated people that you can still get COVID," said Dr. Simone Wildes, an infectious diseases specialist at South Shore Health and a member of the Massachusetts COVID-19 Vaccine Advisory Group. "We know in general that vaccines may not work as well in immunocompromised individuals, but, overall, we haven't seen a lot of breakthrough clusters."

The new report defines breakthrough infection as a positive test 14 days or more after full vaccination with any FDA-authorized COVID-19 vaccine, including Pfizer-BioNTech, Moderna, and Johnson & Johnson.

By the end of April, over 101 million adults in the U.S. had been fully vaccinated and there had been just 10,262 breakthrough infections, the report says. These cases were generally well-tolerated by patients -- most reported mild symptoms or were entirely asymptomatic.

Only 10% of breakthrough cases required hospitalization and of the 2% of people who died, most were elderly in their eighties and around one in five died from a cause other than COVID-19.

The CDC report notes that this is certainly an underestimate of breakthrough infections because all reporting was voluntary and many asymptomatic cases may never have been tested. Also, it's not clear from the study if breakthrough infections were more likely with new, concerning variants.

Roughly two-thirds of these breakthrough infections happened in women, but specialists are not sure why. Thus far, more women than men have been vaccinated. According to the latest data, at least 64.2 million women are now fully vaccinated against COVID-19. And of the people who have been fully vaccinated in the U.S., 53.6% of those are women.

"It is hard to know whether this particular finding is statistically or clinically significant at this point and further analysis would be helpful," Brownstein said.

Regardless, the consensus is that this new report and this week's vaccination milestone are reasons to be hopeful as the country emerges from the pandemic.

"This report gives us more evidence that the COVID-19 vaccines are extremely effective. As more people are vaccinated, we are seeing cases, hospitalizations and deaths decline across the country," said Dr. Michelle Medina, the associate chief of clinical operations for Cleveland Clinic Community Care.

"Vaccine supply is now widely available, and we need to continue reaching out to those individuals and communities who are still hesitant, addressing their concerns and encouraging them to get vaccinated," Medina said.

Deepak Ramanathan, M.D., is a chief resident in orthopaedic surgery in Cleveland and a contributor to the ABC News Medical Unit.