A new report from the US Centers for Disease Control and Prevention indicates only a “small fraction” of cases called breakthrough infections existed among people who have been fully vaccinated for Covid-19 account. According to the agency’s Morbidity and Mortality Report published Tuesday, effective April 30, when about 101 million people had been fully vaccinated, there were 10,262 breakthrough cases reported to the CDC. The data in the report came from 46 states and territories.
The CDC does note that the numbers of reports Covid-19 vaccine breakthrough cases is more than likely substantially undercounted. The surveillance system in place relies on voluntary reporting, and data may not be complete. Meanwhile, many people with breakthrough infections may be asymptomatic or might not seek testing for mild illness.
Among the breakthrough infections, nearly 30% were asymptomatic and 10% were among hospitalized patients, but that number comes with caution: 29% of those hospitalized cases were asymptomatic or the person was hospitalized for a reason other than Covid-19.
According to the report, among the known breakthrough cases, 160, or 2%, were fatal. Most of those cases were among the elderly; the median age for a patient who died was 82.
The number of breakthrough infections caused by variants was similar to the proportion of variants circulating throughout the US. During the time period in which this report comes from, the virus was in wide circulation in many parts of the country.
“Even though FDA-authorized vaccines are highly effective, breakthrough cases are expected, especially before population immunity reaches sufficient levels to further decrease transmission,” the report said.
Since the middle of May, the CDC has stopped publishing data on all Covid-19 breakthrough cases. The CDC is now reporting only severe cases that result in hospitalization and death on its website. As of mid-May, there had been more than 1,900 severe breakthrough cases; among those severe cases 93% were hospitalized and 18% were fatal.
While the reporting of these cases has changed, the definition of a breakthrough case which is a confirmed Covid-19 case at least two weeks after a second dose of Pfizer or Moderna’s vaccines or a single dose of the Johnson & Johnson vaccine, has remained the same. The CDC still highly recommends people get the Covid-19 vaccine. “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 report concluded.