The novel coronavirus can still infect fully vaccinated people. But it’s not because the vaccines aren’t working. Experts said breakthrough cases happen for a number of reasons.

Why do fully vaccinated people get COVID-19?

To start, the vaccines are not 100% effective. In fact, new data from the Centers for Disease Control and Prevention found that 99.999% of fully vaccinated people did not have a severe breakthrough case that led to hospitalization or death, CNN reports.

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99.999% of fully vaccinated Americans have not had a deadly COVID-19 breakthrough case

Lawrence Young, a virologist and professor at the University of Warwick’s Medical School in the U.K., told CNBC that these breakthrough cases are often a reminder.

  • “There will always be a proportion of individuals who will still remain susceptible to infection and illness,” he told CNBC.

And then there’s another factor. The new COVID-19 strains — like the delta variant — are reportedly more transmissible, meaning they can infect fully vaccinated people and spread between them, according to CNBC.

  • There is also incomplete data about how the delta variant impacts the population.

Breakthrough COVID-19 cases expected

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Dr. Rochelle Walensky, the director of the U.S. Centers for Disease Control and Prevention, said on Aug. 6 that the world should expect breakthrough cases to happen, especially as more variants become common, as I wrote for the Deseret News.

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  • “I think we all have to recognize that with 164 million people who are vaccinated, we should expect tens of thousands, perhaps, of breakthrough infections,” Walensky told CNN 
  • “Those breakthrough infections have mild illness. They are staying out of the hospital. They are not dying, and I think that that’s the most important thing to understand,” Walensky told CNN.
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