This evidence synthesis has been compiled by members of the CITF Secretariat and does not necessarily represent the views of all CITF members.

By Mercedes Yanes Lane

In a recent study published in the New England Journal of Medicine, researchers from the UK have estimated the effectiveness of vaccination in preventing symptomatic disease caused by the delta or alpha variants. One dose of the Pfizer or the AstraZeneca vaccine was less than 50% effective against symptomatic COVID-19 caused by delta or alpha. However, when two doses of Pfizer were administered, an 88% effectiveness was observed against the delta variant specifically; two doses of the AstraZeneca vaccine also showed a 67% effectiveness against delta.

Researchers used a test-negative case-control design, in which vaccination status in individuals with symptomatic COVID-19 was compared to the vaccination status of individuals who reported symptoms but had a negative test. Information on vaccination status and demographics was collected from the national vaccination registry in the UK (the National Immunization Management System). Data on PCR testing and variant sequencing were collected from hospital and public health laboratories. Study participants included individuals older than 16 years of age who had symptomatic COVID-19 caused by the alpha or delta variant and who had been vaccinated with either the AstraZeneca or the Pfizer vaccine.

In total, 19,109 sequenced cases were included, with the alpha variant detected in 14,837 samples, and the delta variant in 4272 samples.

For both vaccines, the effectiveness of the first dose in preventing symptomatic disease was notably lower among individuals infected with the delta variant (30.7%; 95% confidence interval [CI], 25.2 to 35.7) than among those infected with the alpha variant (48.7%; 95% CI, 45.5 to 51.7). When examining vaccine effectiveness following two doses, the effectiveness of the AstraZeneca vaccine against symptomatic disease was better: alpha variant 74.5% [95% CI, 68.4 to 79.4] and delta variant 67.0% [95% CI, 61.3 to 71.8]), but still lower compared to the Pfizer vaccine: alpha variant 93.7% [95% CI, 91.6 to 95.3] and delta variant 88.0% [95% CI, 85.3 to 90.1]).

Overall, two doses of either vaccine appear to be highly effective against symptomatic cases of the delta variant.


Lopez Bernal J, Andrews N, Gower C, Gallagher E, Simmons R, Thelwall S, Stowe J, Tessier E, Groves N, Dabrera G, Myers R. Effectiveness of Covid-19 vaccines against the B.1.617.2 (delta) variant. NEJM. 2021 Jul 21. DOI: 10.1056/NEJMoa2108891