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COVID vaccines effectiveness against new Delta variant

With the spread of the Delta variant (known as B.1.617.2 by the CDC) from India, scientists have been concerned about the effectiveness of COVID vaccines against it. The good news is that the Pfizer and AstraZeneca vaccines seem to fairly effective against this new variant of SARS-CoV-2 virus.

Let’s take a look at the paper that describes the effectiveness of the vaccines against the COVID-19 Delta variant.

What is the Delta variant?

The SARS-CoV-2 Delta variant, also known in the CDC nomenclature as the B.1.617.2 lineage, was first detected in India in late 2020. The World Health Organization (WHO) named it the Delta variant on 31 May 2021.

It has mutations in the genes encoding the spike protein which are known to affect the transmissibility of the virus as well as whether it can be neutralized by antibodies for previously circulating variants.

Public Health England (PHE) in May 2021 observed infectiousness to be 51–67% higher than the alpha variant. In June, the Delta variant was linked to an increased risk of hospitalization.

On 7 June 2021, researchers at the National Centre for Infectious Diseases in Singapore posted a pre-print paper suggesting that patients testing positive for Delta are more likely to develop pneumonia and/or require oxygen.

Photo by Spencer Davis on Unsplash

COVID vaccines effectiveness against new Delta variant

In an article on the medRxiv preprint server, Jamie Lopez Bernal, Ph.D., of Public Health England in London, and colleagues showed that two doses of Pfizer COVID vaccine showed 87.9% effectiveness (95% CI 78.2%-93.2%) against the Delta variant, while two doses of the AstraZeneca showed 59.8% effectiveness (95% CI 28.9%-77.3%).

Unfortunately, effectiveness after one dose against the Delta variant was similarly low for both vaccines, at 33%.

There are some contradictory data out of Israel, which has a high rate of vaccination with the Pfizer vaccine. The Israeli Health Ministry is reporting that the Pfizer vaccine’s effectiveness in preventing symptomatic COVID-19 is around 64% given that the Delta variant appears to be responsible for 90% of the new cases in Israel over the past two weeks.

I don’t know how many times I have to repeat this, but even at 64% effectiveness that is not equivalent to 0% effectiveness. It’s still very effective especially in light of the seriousness of the disease.

However, and this is very important, the data also shows that the Pfizer vaccine is still highly effective against preventing serious symptoms and hospitalization, around 93%. In other words, even if you contract a breakthrough infection with the Delta variant, you will probably avoid the more serious consequences of the disease.

Photo by Hakan Nural on Unsplash


  • Shang J, Wan Y, Luo C, Ye G, Geng Q, Auerbach A, Li F. Cell entry mechanisms of SARS-CoV-2. Proc Natl Acad Sci U S A. 2020 May 26;117(21):11727-11734. doi: 10.1073/pnas.2003138117. Epub 2020 May 6. PMID: 32376634; PMCID: PMC7260975.
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