The United States passed a significant milestone this week, with the White House reporting that more than half of all adults in the country are now fully vaccinated against COVID-19. This announcement was made on the same day that the Centers for Disease Control and Prevention released a new report stating that breakthrough COVID-19 infections are extremely rare in fully vaccinated people. A breakthrough infection occurs when a person who has been fully vaccinated becomes infected with COVID-19.

According to the new CDC report, such breakthrough infections may occur in only 0.01 percent of all fully vaccinated people. Vaccines are extremely effective at preventing infection, but none are completely effective. Even when these rare breakthroughs occur, the vaccines are still overwhelmingly effective at preventing hospitalization or death.

“This report confirms, in a real-world setting, that breakthrough infections are rare, and when they do occur, they mostly have no clinical significance,” said Dr. John Brownstein, chief innovation officer at Boston Children’s Hospital and professor of pediatrics at Harvard Medical School. Earlier this month, some New York Yankees staff and players tested positive despite being fully vaccinated.

These widely publicized cases sparked confusion and concern, but experts say it’s important to remember that no vaccine is 100% effective, and vaccines still help protect against the most severe COVID-19 symptoms.

“We always tell people who are vaccinated that they 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 that vaccines may not work as well in immunocompromised people in general, but we haven’t seen a lot of breakthrough clusters.”

According to the new report, a breakthrough infection is defined as a positive test 14 days or more after receiving full vaccination with any FDA-approved COVID-19 vaccine, including Pfizer-BioNTech, Moderna, and Johnson & Johnson.

According to the report, by the end of April, over 101 million adults in the United States had been fully vaccinated, with only 10,262 breakthrough infections. Patients tolerated these cases well in general; most reported mild symptoms or were completely asymptomatic. Only 10% of breakthrough cases required hospitalization, and of the 2% who died, the majority were elderly people in their eighties, with one in every five dying from a cause other than COVID-19.

According to the CDC report, this is almost certainly an underestimation of breakthrough infections because all reporting was voluntary, and many asymptomatic cases may never have been tested. Furthermore, the study does not indicate whether breakthrough infections were more likely with new, concerning variants.

“Sequencing capabilities have improved overall since the study began in January 2021,” Brownstein said. “Routine surveillance and sequencing could be useful for public health planning in the future, particularly in anticipating variant-related surges. However, on an individual level, they may not mean much because most fully vaccinated people do well clinically after breakthrough infections.”

Approximately two-thirds of these breakthrough infections occurred in women, but experts are unsure why. So far, women have been vaccinated at a higher rate than men. According to the most recent statistics, at least 64.2 million women have been fully immunized against COVID-19. Women make up 53.6 percent of those who have been fully vaccinated in the United States.

“At this point, it’s difficult to say whether this particular finding is statistically or clinically significant, and more research would be beneficial,” Brownstein said. Regardless, the general consensus is that this new report, as well as this week’s vaccination milestone, are reasons to be optimistic as the country emerges from the pandemic.

“This report adds to the body of evidence that COVID-19 vaccines are extremely effective. As more people are vaccinated, we are seeing a decrease in cases, hospitalizations, and deaths across the country “Cleveland Clinic Community Care’s associate chief of clinical operations, Dr. Michelle Medina, explained.

“Vaccine supplies are now widely available, and we must continue to reach out to those individuals and communities who are still hesitant, addressing their concerns and encouraging them to get vaccinated,” Medina said.