'Breakthrough' COVID Found in More Than 10K Vaccinated Americans: CDC

Damian McNamara

May 25, 2021

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Centers for Disease Control and Prevention (CDC) researchers report that more than 10,000 fully vaccinated Americans have experienced "breakthrough" COVID-19 through April 30, 2021. They note that the figures are likely an underestimate.

The 10,262 SARS-CoV-2 vaccine breakthrough infections were reported from January 1 through April 30 in 46 US states and territories. About one quarter of affected people, 27%, were asymptomatic. Two percent, or 160 people, with breakthrough infections died, preliminary data reveal.

A total of 995 people were hospitalized, including 289 hospitalized for asymptomatic infection or for reasons unrelated to COVID-19, lead study author Meseret Birhane, MPH, and colleagues at the CDC note.

The study was published online May 25 in Morbidity and Mortality Weekly Report (MMWR).

"Ten thousand is a very small number compared to the hundreds of millions we have vaccinated. It's way better than any flu vaccine we have had in our lives," Ali H. Mokdad, PhD, professor of health metrics sciences at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington in Seattle, told Medscape Medical News.

Breakthrough infections are expected, he said, because no vaccine is 100% effective. Furthermore, breakthrough cases have been asymptomatic or mild in general. "We defang COVID-19, that's how I see it. And that's what we want."

The researchers recommend that all Americans aged 12 years and older get vaccinated. "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."

A majority of breakthrough cases, 63%, occurred in women. The median age of people in the study was 58 years (interquartile range, 40–74 years).

Genetic sequencing was performed on 5% of these reported cases. Nearly two thirds, 64%, of these cases involved a SARS-CoV-2 variant of concern. These variants included B.1.1.7 (UK) in 56% of breakthrough cases, B.1.427 (California) in 29%, P.1 (Brazil) in 29%, B.1.429 in 25% (California), and B.1.351 (South Africa) in 13%.

Because the reporting of breakthrough cases was passive and voluntary, it is likely that the numbers are an underestimate, the CDC researchers note.

"I totally agree this is an underestimate," Mokdad said.

The CDC plans to continue collecting information on all COVID-19 breakthrough cases going forward. However, since May 1, the agency has limited its investigation of breakthrough cases to those in which people are hospitalized or die.

"My concern is moving into the future," Mokdad said.  We need to keep our eye on the ball." The CDC needs to continue rigorous surveillance, he added, to answer:  Are breakthrough infections increasing with time? Are breakthrough infections increasing with the new variants? Which vaccines are most effective against future variants?

"None of this right now is a concern but is it something we need to keep an eye on? Definitely yes."

The study authors and Dr Mokdad have disclosed no relevant financial relationships.

MMWR Morb Mortal Wkly Rep. Published online May 25, 2021. Full text

Damian McNamara is a staff journalist based in Miami. He covers a wide range of medical specialties, including infectious diseases, gastroenterology and neurology. Follow Damian on Twitter: @MedReporter.

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