Breakthrough infections rare among first 101 million fully vaccinated people, CDC finds
CDC investigators identified just 10,262 breakthrough SARS-CoV-2 infections in 46 U.S. states as of April 30, when approximately 101 million Americans had been fully vaccinated, according to results published today in MMWR.
“Even though FDA-authorized vaccines are highly effective, breakthrough cases are expected, especially before population immunity reaches sufficient levels to further decrease transmission,” members of the CDC’s COVID-19 Vaccine Breakthrough Case Investigations Team wrote. “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 CDC team worked with state and local health departments to identify SARS-CoV-2 infections in fully vaccinated persons. They defined a breakthrough infection as the detection of SARS-CoV-2 RNA or antigen in a respiratory sample from a patient 14 days or more after the administration of the final dose of a COVID-19 vaccine. Some specimens were also submitted for genomic sequencing.
Of the breakthrough infections, 63% occurred in women, the median patient age was 58 years (interquartile range = 40-74 years), and 27% were asymptomatic.
According to the report, 2% (n = 160) of patients with a breakthrough infection died and 10% were hospitalized. Of the 995 hospitalized patients, 29% were hospitalized for a reason unrelated to COVID-19 or did not show symptoms. SARS-CoV-2 variants accounted for 70% of the estimates of SARS-CoV-2 lineages obtained by the CDC.
The investigators noted two major limitations of their analysis: potential underreporting of breakthrough cases, and sequencing data being available for only a “small proportion” of cases.
“CDC will continue to lead studies in multiple U.S. sites to evaluate vaccine effectiveness and collect information on all COVID-19 vaccine breakthrough infections regardless of clinical status,” they wrote.