Breaking down the latest CDC data on the delta variant and vaccine efficacy


DENVER (KDVR) — The U.S. Centers for Disease Control and Prevention wants universal masking back in place, mandatory vaccinations for healthcare providers, and maybe even reconsidering some of the community strategies the country got all too familiar with in the last 18 months.  

Internal CDC documents say “the war has changed” in both pandemic policy and how that is communicated to the public.

The CDC reemphasizes that risk of infection or hospitalization are much lower in the vaccinated than among the unvaccinated. However, it also admits that breakthrough cases, especially among the delta variant, are more common than people might want to believe and may require rethinking the open-for-business atmosphere the country embraced in late spring.

Vaccinations still decrease the risk of death or severe disease by a factor of 10 and risk of infection by a factor of three. In vaccinated persons who did contract the virus, infected persons had a 40% lower viral load than unvaccinated persons, a shorter duration of viral load, a lower risk and shorter duration of symptoms.

Since more of the public is vaccinated, though, the CDC has tracked more infections among the vaccinated than previously assumed.

In January, only 0.01% of COVID hospitalizations happened to vaccinated persons. In May, 9% of hospitalizations were among the vaccinated. For hospitalized deaths, that number rose even more, from zero in January to 15.1% in May.

This reflects that there are more vaccinations now, and more vaccinations among at-risk populations such as immunocompromised or elderly persons.

For those groups, though, the vaccine is less effective. The vaccine effectiveness against hospitalization is down to 59% for immunocompromised groups, and in the 70% range for nursing home residents.

Against the backdrop of the delta variant, things change.

The delta variant is more transmissible and likely leads to more severe illness.

The original COVID strain infected between one and four people per sick person. The delta variant infects between five and 10 persons per sick person – which is about the infection risk you see with the chickenpox.

The CDC doesn’t place the delta variant too much higher on the deadliness scale. It is not as deadly as polio or smallpox or MERS or SARS or Spanish flu or ebola, but delta infections are generally stronger.

The transmissibility in delta breakthrough cases may be the same level as unvaccinated cases. They measure this through Ct value, or cycle threshold. Cycle threshold in delta cases is lower – meaning there is a higher concentration of viral material in the test sample. The relationship between Ct and transmissibility it still being researched, but the signs are that there’s a relationship.

Symptoms do appear worse with the delta variant. Health officials in other counties have reported higher odds of hospitalization – this comes from both Canada and Scotland – and in Singapore a higher need for oxygen intake.

Vaccines also appear less effective against delta. In England and Scotland, the vaccine effectiveness for symptomatic disease is 88% for the delta variant – that’s lower than the 93 or so for the alpha variant. In Israel, which has one of the highest numbers of vaccinated people, vaccine effectiveness for the delta variant is 64%.  

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