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COVID vaccine effectiveness against Delta variant –

During the 17 September 2021 FDA vaccine expert committee meeting, the COVID-19 vaccine effectiveness against the Delta variant was reviewed. I want to assess some of the results so that we have a better understanding of how the vaccines are working against the Delta variant.

I have read all kinds of comments and reports about the COVID-19 vaccine effectiveness against the Delta variant, but it will be good to see what data was presented to the top vaccine experts in the country. Let’s take a peek at the data.

COVID-19 vaccine effectiveness against the Delta variant

Dr. Sara Oliver, MD, MSPH, presented updates about COVID-19 epidemiology and vaccine effectiveness with respect to the Delta variant (known as B.1.617.2 by the CDC) to the Vaccines and Related Biological Products Advisory Committee (VRBPAC). Dr. Oliver’s presentation indicated that COVID-19 vaccines administered in the USA conferred significantly reduced protection against the Delta coronavirus variant (including asymptomatic and mild cases).

First, Dr. Oliver presented vaccine uptake by age group in the USA:

Overall 54.2% of the US population has been fully vaccinated, although individuals under the age of 12 are not currently indicated for any of the COVID-19 vaccines, so that makes the number appear to be lower than for the age group that is approved for the vaccine. Nevertheless, this is far below the 80-88% herd immunity level that is required to stop the disease.

vaccine effectiveness Delta variant

As you can see, the Delta variant, B.1.617.2, now represents about 99% of the cases in the USA. And this is why understanding the effectiveness of the COVID-19 vaccine against this variant to be of critical importance.

Six published papers reviewed Moderna and Pfizer mRNA vaccine effectiveness over the past few months. As you can see in the graph above, lower COVID-19 vaccine effectiveness was observed as the Delta variant became more prevalent during Summer 2021. The effectiveness ranged from 42% up to 80% depending on the study.

To be clear, these studies were in different settings or examined different vaccines, so it’s more important to see the drop in effectiveness on each line rather than compare the effectiveness data between the six studies.

vaccine effectiveness Delta variant

As the above two graphs show, there is a statistically significant drop in vaccine effectiveness between the Alpha and Delta variants of SARS-CoV-2. But, let’s be clear, the effectiveness against Delta is lower, but it still prevents infections, hospitalization, and death.

Pre-Delta vaccine effectiveness against infection ranged from 72% to 97%, which is quite good for a new vaccine. Since the introduction of Delta, it ranged from 39% to 84% against infection, which may indicate a loss of effectiveness.

More importantly, pre-Delta vaccine effectiveness against hospitalization ranged from 84% to 97%, and since the introduction of Delta, effectiveness against hospitalization has ranged from 75% to 95%. Since vaccine effectiveness against hospitalization (and mortality) is the more important measurement, the vaccine seems to still have a powerful effect in reducing hospitalization.


The purpose of this article isn’t to scare anyone that the COVID-19 is worthless against the Delta variant. In fact, it’s just the opposite – most of the research shows that there is a statistically significant drop in vaccine effectiveness, but it’s still demonstrably very effective.

Moreover, it still remains very effective against hospitalization, which is one of the most important factors in describing the efficacy of the COVID-19 vaccines.

So, if someone is telling you that the vaccine is worthless against the Delta variant, they’re inventing a narrative that is not supported by the evidence. The best vaccine experts in the US think the vaccine still works.

Michael Simpson

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