On Tuesday, the Centers for Disease Control and Prevention released the latest data on breakthrough COVID-19 infections, which are infections among people who have been fully vaccinated against the disease. Yet again, the data suggests that the vaccines are highly effective against infection, as well as severe disease and death. The data breakdown also hints that vaccines are winning the race against variants, which don’t seem to be breaking through at higher rates than expected.

Among approximately 101 million vaccinated people in the US as of April 30, the CDC collected reports of 10,262 breakthrough cases from 46 states and territories. That works out to about 0.01% breakthrough cases among the fully vaccinated. This number is almost certainly a significant undercounting, the CDC acknowledges.

Breakthrough monitoring is passive and voluntary; vaccinated people who had asymptomatic or mild COVID-19 infections may not have gotten tested or reported their cases. Only about 27 percent of the cases tallied by the CDC were considered asymptomatic. Transmission of COVID-19 was also very high during the monitoring period reported, with about 355,000 COVID-19 cases reported nationally in the week ending on April 30.

Still, the numbers are more reliable for severe and deadly breakthrough cases—and those figures are equally comforting. Among the 10,262 cases, only 995 cases (10 percent) were known to require hospitalization. And among those 995 people in the hospital, 289 people (29 percent) had asymptomatic COVID-19 infections or were hospitalized for a reason other than COVID-19.

Less than two percent of all breakthrough infections resulted in death; the CDC tallied just 160 deaths among people with breakthrough COVID-19 infections. Of those who died, 28 people (18 percent) were asymptomatic at the time of death or died from a cause unrelated to COVID-19. The median age of those who died was 82.

Unconcerning variant data

The CDC also collected genetic sequencing data from the breakthrough infections to monitor which version/variant of SARS-CoV-2 was infecting the vaccinated people. The agency only had sequencing data from 555 of the reported cases (5 percent). But the data looked as expected, mirroring the proportions of variants circulating in the whole population. Of the 555 cases with sequencing data, 356 (64 percent) were caused by a variant of concern. During the monitoring period, the variants of concern were estimated to account for about 70 percent of cases. The B.1.1.7 variant was the most common variant identified, turning up in 199 of the 555 cases.

Likewise, there were no red flags among the people who reported breakthrough infections. Age and sex breakdowns simply reflected the characteristics of the fully vaccinated proportion of the population.

Overall, the CDC concluded that the vaccines are working as expected and are highly effective. Despite the success of the vaccines, “breakthrough cases are expected, especially before population immunity reaches sufficient levels to further decrease transmission,” the agency noted. “However, vaccine breakthrough infections occur in only a small fraction of all vaccinated persons and account for a small percentage of all COVID-19 cases. The number of COVID-19 cases, hospitalizations, and deaths that will be prevented among vaccinated persons will far exceed the number of vaccine breakthrough cases.”

The agency also noted that as of May 1, it switched from nationally monitoring all breakthrough cases—which is likely to continue missing a large number of asymptomatic cases—to only monitoring breakthrough cases that result in hospitalization or death.