COVID-19

CDC: More Severe Breakthrough Cases Than Expected From Delta, Unvaxxed 10x More At Risk

Ultimately, the CDC study still shows vaccines are overwhelmingly effective. Even as delta's prevalence spiked, the unvaccinated were 10x as likely to die or be hospitalized with COVID-19

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What to Know

  • CDC researchers looked at severe COVID outcome rates among unvaccinated and fully vaccinated people in the time period that delta's weekly case prevalence went from less than 1% to 90%
  • In that time frame, fully vaccinated people in the 13 jurisdictions studied (including NYC) accounted for 9% of new COVID deaths and 8% of new hospitalizations
  • Still, protection afforded by vaccines is evident: even after delta became dominant, fully vaccinated people had 5x less risk of infection, 10x less risk of hospitalization and 10x less risk of death associated with COVID-19 compared with unvaccinated people

A new CDC study out Friday provides what may be the most jolting evidence yet of the threat the highly contagious delta variant may pose to the fully vaccinated, particularly as it usurped every other COVID-19 strain to dominate American cases.

What's also clear though: That danger is magnified 10 times for people who aren't fully immunized against the virus, researchers found.

The health agency analyzed percentages of total cases, hospitalizations and deaths by vaccination status across 13 jurisdictions, including New York City, between April 4 and July 17, which was when delta's U.S. prevalence soared from less than 1% of new weekly cases to 90% of new weekly cases.

(It's now nearly 99% of U.S. positives samples tested, the CDC says -- and at least 98% of all those tested in New York City and New Jersey over the last month.)

We thought we could control the pandemic with about 70% of the country vaccinated. But with the Delta variant leading to case surges even in highly vaccinated countries, that changes the math. We may need 90% to take the COVID-19 vaccine to really wrangle with this variant, says Alabama epidemiologist Dr. Suzanne Judd.
We thought we could control the pandemic with about 70% of the country vaccinated. But with the Delta variant leading to case surges even in highly vaccinated countries, that changes the math. We may need 90% to take the COVID-19 vaccine to really wrangle with this variant, says Alabama epidemiologist Dr. Suzanne Judd.

In that time frame, fully vaccinated people in the 13 jurisdictions studied accounted for 9% of new COVID deaths and 8% of new hospitalizations. They accounted for 8% of new cases during the same timeframe, though health officials at all levels of government are consistently more concerned about the first two metrics, which indicate the most severe outcomes associated with COVID breakthrough infections.

No doubt, vaccination (this CDC report didn't break out breakthrough cases by vaccine brand, specifically) has been a proven protector. Ninety-one percent, 92% and 92%, respectively, of COVID-associated deaths, hospitalizations and cases within the period April 4 through July 17 were among the unvaccinated. But any apparent risk to fully immunized people has been a critical issue these past months.

CDC

And the risk only appeared to intensify as delta's weekly prevalence grew, based on the CDC's research. This latest study broke out the research timeframe into two periods -- April 4 to June 19, when delta's prevalence was rising significantly, and June 20 to July 17, when it comprehensively seized the U.S. and the entire globe.

In the first date range assessed, fully vaccinated people accounted for 8% of new COVID deaths, 7% of hospitalizations and 5% of cases. These percentages dramatically increased during the June 20 to July 17 study period -- to 16% of new COVID deaths, 14% of new hospitalizations and 18% of new COVID cases.

CDC

Full vaccination rates across the 13 jurisdictions studied did increase between the two time periods -- from 37% in the first period, April 4 to June 19, to 53% in the second, June 20 to July 17. Based on research that presumes 90% vaccine effectiveness, the CDC says it would have expected vaccinated people to account for 6% of new cases, which is close to the 5% observed, in the first period studied and around 10% of new cases in the second, which fell well short of the 18% found.

Fully vaccinated people accounting for 18% of new COVID cases in the period June 20 to July 17 would have been expected if vaccine effectiveness were at 80%, the CDC said. As delta's prevalence topped 50%, the percentage of fully vaccinated people among new COVID cases in each age group increased at rates that reflected that lower vaccination effectiveness benchmark of 80%, researchers said.

COVID hospitalization and death increases among vaccinated people age 65 and older also appeared higher than expected, according to the CDC. Still, the overwhelming risk delta poses to unvaccinated people is clear in this latest data -- a fact officials in New York City and elsewhere strive to cement in U.S. minds.

New death, hospitalization and case rates were substantially higher in unvaccinated people versus vaccinated people in all 13 U.S. jurisdictions. This latest CDC study says its findings are consistent with previous research indicating delta is more likely to cause breakthrough infections than other strains but not more likely -- in a statistically significant sense -- to cause more severe outcomes.

Even after delta become the most dominant variant, fully vaccinated people were five times less likely to be infected with COVID, 10 times less likely to be hospitalized because of it and 10 times less likely to die of virus-associated complications compared with unvaccinated people, the CDC found.

"Findings were consistent with a potential decline in vaccine protection against confirmed SARS-CoV-2 infection and continued strong protection against COVID-19–associated hospitalization and death," the CDC study said.

"Getting vaccinated protects against severe illness from COVID-19, including the Delta variant, and monitoring COVID-19 incidence by vaccination status might provide early signals of changes in vaccine-related protection that can be confirmed through well-controlled vaccine effectiveness studies," the study said.

CDC

Researchers identify a number of limitations in their study, including adding partially vaccinated people to their non-immunized assessment, lags in death reports, assumptions around the timing of delta's weekly case prevalence hitting 50% and the fact the 13 jurisdictions represent just 25% of the U.S. population.

Locally, both New York and New Jersey, have been in the throes of their most intense new case and hospitalization loads since May -- though none of the latest numbers compare marginally to the ones of spring 2020.

New York Gov. Kathy Hochul, who addressed the delta variant in her first address as governor as well as in the days before she was officially sworn in, reported a new daily case total of 6,151 on Friday, the highest single-day toll since April 16.

She also reported 43 new daily deaths, the highest single-day number since April 30 (44), though Hochul launched her administration by reporting those numbers differently than her predecessor.

It's unclear based on New York state data how many of the new deaths and cases are driven by breakthrough cases.

The U.S. government has also forcefully acknowledged the threat of delta. President Joe Biden unveiled a six-point plan Thursday to combat that variant's threat, including sweeping new vaccination mandates and school recommendations.

See all the highlights of Biden's new delta variant strategy right here.

We thought we could control the pandemic with about 70% of the country vaccinated. But with the Delta variant leading to case surges even in highly vaccinated countries, that changes the math. We may need 90% to take the COVID-19 vaccine to really wrangle with this variant, says Alabama epidemiologist Dr. Suzanne Judd.
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