Are Covid Vaccines Effective Against The Delta Variant?

Covid vaccines play a crucial role in slowing the spread of the virus while reducing hospitalization and mortality rates in those infected. As the Delta variant has spread and become the dominant strain, are vaccines targeting previous strains still effective? Are there Covid vaccines against the Delta variant among available vaccines? How effective are they, and are they different from current options?

Before looking at how each of the available vaccines performs against the Delta variant, the first question to answer is, how do Covid vaccines work?

How do Covid vaccines work?

Covid vaccines’ main goal is helping ‘train’ the body to recognize and fight the Covid virus when it enters the body.

They do so in three main ways:

• mRNA vaccines: Deliver pieces of harmless genetic material from the Covid virus into the body, stimulating the production of a protein that triggers the production of T-lymphocytes and B-lymphocytes. Examples: Pfizer-BioNTech (Comirnaty) and Moderna
• Vector vaccines: Use a different virus to deliver a payload (vector virus) of harmless Covid virus genetic material, which produces Covid-specific proteins that trigger the production of T-lymphocytes and B-lymphocytes. Example: Janssen/Johnson & Johnson
• Protein subunit vaccines: These more traditional vaccines deliver harmless proteins (called S proteins) derived from the virus into the body, directly triggering the production of T-lymphocytes and B-lymphocytes. Example: Novavax (not yet released)

Covid vaccines and the Delta variant

Although Covid vaccines work in different ways, are they still effective against the Delta variant? Here’s what current data says about each of the FDA-approved vaccines available in the United States:

Pfizer Delta efficacy

Pfizer-BioNTech (Comirnaty) has the highest level of effectiveness of around 95% against the Alpha Covid strain, as reported during large-scale clinical trials before approval for emergency use by the FDA. However, data from this and this study show the Delta variant Pfizer effectiveness reduces to between 78% and 60% against Delta infections.

Nevertheless, effectiveness against severe illness, hospitalization, and death from the Delta variant remains over 95%.

Moderna Delta efficacy

Moderna reported its vaccine’s effectiveness as 94.1% during clinical trials against the Alpha Covid strain. However, a preliminary study has found that Moderna Delta variant effectiveness dropped to 86.7% with Delta infections compared to 90.4 to 98.4% with previous strains.

As with the Pfizer vaccine, Moderna’s effectiveness against severe illness, hospitalization, and death from the Delta variant remains above 95%.

Johnson & Johnson Delta efficacy

Johnson & Johnson/ Janssen single-shot Covid vaccine demonstrated 66.9% effectiveness during initial clinical trials against the Alpha Covid strain. As the Delta variant spread, some data suggested that it was not only effective against the Delta variant but even more effective than against previous strains, including Alpha. Moreover, Johnson & Johnson also recently published preliminary data indicating its vaccine remains effective and maintains durable results for up to eight months.

Like Moderna and Pfizer, Johnson & Johnson’s effectiveness against severe illness, hospitalization, and death from the Delta variant remains above 95%.

AstraZeneca Delta efficacy

AstraZeneca-Oxford vaccine’s effectiveness against the Alpha Covid strain was reported as 63% during clinical trials, although this figure was later revised using real-world data to 74.5%. Against the Delta variant, two shots of the vaccine showed 67% effectiveness, with additional data from AstraZeneca confirming similar levels of effectiveness.

AstraZeneca-Oxford’s effectiveness against severe illness, hospitalization, and death from the Delta variant remains above 90%.

Interpreting the results

Although all vaccines experienced a drop in effectiveness in preventing symptomatic illness, most likely due to breakthrough infections caused by the Delta variant, all remained highly effective at preventing severe illness, hospitalization, and death.

One study found that this drop in effectiveness can be mitigated by ensuring all doses are taken, noting that: “Only modest differences in vaccine effectiveness were noted with the Delta variant as compared with the Alpha variant after the receipt of two vaccine doses.”

In addition, taking booster shots after the full dose might help in ensuring vaccine effectiveness remains high. The rise of new variants like the Delta variant named Delta Plus (AY.4.2) might also heighten the need to get regular booster shots of vaccines which have been updated to tackle these emerging variants.