Pfizer-BioNTech COVID-19 Vaccine Shows Dip in Effectiveness Against Delta Variant

With the delta variant now dominant in the United States, study results highlight the need for officials to do everything they can to increase vaccine uptake.

As the United States grapples with a surge of COVID-19 cases driven by the Delta variant, a new study says that 2 doses of the Pfizer-BioNTech vaccine is 88% effective against the more-contagious strain, somewhat lower than the 95% to 100% efficacy against severe disease seen in pivotal, randomized, late-stage trials.

A single dose of either the Pfizer-BioNTech vaccine or the AstraZeneca vaccine is even less effective against Delta, which is now the dominant strain in the country.

The research, published Wednesday in The New England Journal of Medicine, underscores what lies beneath the urgency that public health officials are feeling in efforts to combat misinformation about COVID-19 vaccination. Last week, US Surgeon General Vivek Murthy, MD, issued the first Surgeon General's Advisory regarding the threat that inaccurate information and lies about the pandemic poses to public health.

An earlier version of the study, conducted in England, was published online in May.

It compared the effectiveness of the Pfizer-BioNTech vaccine (known as BNT162b2) and the Astra Zeneca vaccine (or ChAdOx1 nCoV-19) against 2 variants—Alpha, or B.1.1.7, Dnd delta, or B.1.617.2—after 1 or 2 doses. The Pfizer-BioNTech vaccine has been available in England and the United States since December 2020; the AstraZeneca vaccine is available in the United Kingdom and 137 other countries, but has not yet received emergency use authorization by the FDA.

Unlike the pivotal randomized, double-blind, placebo-controlled vaccine trials, this work used observational data on all individuals in England who received COVID-19 vaccinations up to May 16, 2021. The study, using what is called a test-negative design, was made possible because England has a national vaccination register, which includes the date of each administered dose and vaccine type. Via the unique National Health Service patient identifier number, vaccine receipt information was linked with positive results for SARS-CoV-2 as determined by polymerase chain reaction testing, whole-genome sequencing, and spike gene status to identify Delta and Alpha variants.

Test-negative designs are used to estimate vaccine effectiveness, in which patients who test positive for a disease make up the cases and controls are selected from those who test negative in the same situation. Each participant has their vaccination status noted, and the odds of vaccination are then compared between cases and controls.

Effectiveness after 1 dose of either vaccine was lower against the Delta variant (30.7%; 95% CI, 25.2%-35.7%) compared with the Alpha variant (48.7%; 95% CI, 45.5%-51.7%).

When examining the effect of 2 doses, the Pfizer-BioNTech vaccine was 93.7% effective against the Alpha variant (95% CI, 91.6%-95.3%) and 88.0% effective (95% CI, 85.3%-90.1%) against the Delta variant.

Effectiveness of 2 doses of the AstraZeneca vaccine was 74.5% (95% CI, 68.4%-79.4%) against the Alpha variant and 67.0% (95% CI, 61.3%-71.8% against the Delta variant.

The AstraZeneca vaccine, like the Johnson & Johnson (J&J) vaccine, uses an adenovirus platform. Earlier this week, a preprint study also said that the J&J vaccine is less effective against the Delta variant compared with mRNA vaccines, such as the Pfizer-BioNtech one.


Lopez Bernal J, Andrews N, Gower C, et al. Effectiveness of Covid-19 vaccines against the B.1.617.2 (delta) variant. N Engl J Med. Published online July 21, 2021. doi:10.1056/NEJMoa2108891

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