In August of 2021, about 18.9% of COVID-19 deaths occurred among the vaccinated. Six months later, in February 2022, that proportional percent of deaths had increased to more than 40%.
Comparatively, in September 2021, just 1.1% of COVID-19 deaths occurred among Americans who had been fully vaccinated and boosted with their first dose. By February 2022, that percentage had increased to about 25%.
Experts said the increase in breakthrough deaths is expected with more Americans reaching full vaccination status.
"These data should not be interpreted as vaccines not working. In fact, these real-world analyses continue to reaffirm the incredible protection these vaccines afford especially when up to date with boosters," said Dr. John Brownstein, an epidemiologist at Boston Children's Hospital and an ABC News contributor.
In addition, many vulnerable Americans are more than one year out from their primary vaccinations and have yet to receive booster doses.
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To date, more than 220 million Americans have been fully vaccinated, 100 million of whom have received their first COVID-19 booster. However, about 91.5 million eligible Americans - about half of those currently eligible - have yet to receive their first booster shot.
The increase in breakthrough deaths comes as a growing proportion of older Americans enter the hospital for COVID-19 related care.
Last summer, after more vulnerable, older populations had been vaccinated, the share of Americans ages 65 years and older in the hospital had dipped to a pandemic low - with younger populations representing the largest age groups of people in need of care. However, throughout the omicron surge, the average age of those in the hospital with COVID-19 has steadily gotten older again.
More than 90% of seniors have been fully vaccinated, but a third of them have yet to receive their first booster shot. Even with overall high vaccination rates in older populations, in recent months, during the omicron surge, 73% of deaths have been among those 65 and older.
Health experts said vaccines and boosters continue to provide significant protection against severe disease. However, waning immunity re-emphasizes the urgency of boosting older Americans and high-risk Americans with additional doses.
"This trend in increased risk among the elderly further supports the need for community wide immunization. Older populations, especially those with underlying conditions, continue to be at great risk of severe complications, especially as immunity wanes. The best way to protect them is to make sure everyone around them is fully immunized," Brownstein said.
All Americans over the age of 50, immunocompromised people over the age of 12, and people who received two doses of the Johnson and Johnson vaccine, are currently eligible for a second booster.
Approximately 10.5 million people in the U.S. have received their second booster dose.
"Given the fact that immunity is waning, we've got to get people boosted," Dr. Anthony Fauci told GBH News's Boston Public Radio on Monday.
In February, unvaccinated adults were 10 times more likely to die of COVID-19 compared to vaccinated individuals and five times more likely to require hospitalization, according to data from the Centers for Disease Control and Prevention.
Compared to fully vaccinated and boosted adults, unvaccinated people were about 20 times more likely to die of COVID-19 and seven times more likely to require hospitalization.