Two-Dose COVID-19 Vaccines Remain Effective Versus Delta Variant

COVID-19 vaccine
Two-dose COVID-19 vaccine effectiveness rates remain high against symptomatic disease brought on by the delta variant, according to new research in the UK.

New data from investigators at Public Health England showed 2-dose administration of Pfizer-BioNTech mRNA vaccine BNT162b2 and AstraZeneca-University of Oxford adenovirus vaccine ChAdOx1 nCoV-19 were associated with 88% and 67% effectiveness against symptomatic delta variant-derived COVID-19, respectively. The findings, published to The New England Journal of Medicine Thursday, provide support for 2-dose vaccine uptake in the wake of surging cases driven by the highly transmissible delta variant.

A team led by Jamie Lopez Bernal, PhD, sought to interpret the effectiveness of currently available 2-dose vaccines in the UK against the B.1.617.2 (delta) variant of SARS-CoV-2—at a time when cases borne from the variant were spreading from India to dozens other countries, while investigators had little indication of the COVID-19 vaccines’ effectiveness in curbing it.

The vaccines’ highly efficacious benefit in symptomatic COVID-19 prevention has been observed largely in clinical trials and real-world research involving the first prominent B.1.1.7 (alpha) variant, Lopez Bernal and colleagues noted.

“Data on the effectiveness of COVID-19 vaccines against clinical outcomes with (delta) variant have been limited,” they wrote.

Investigators conducted a test-negative case-control assessment to estimate vaccine effectiveness against symptomatic disease caused by the delta or alpha variant. They identified variants based on sequencing and spike (S) gene status observation, using data from all symptomatic cases of COVID-19 sequenced in England to determine proportion of cases with either variant according to vaccination status.

The team included 19,109 sequenced cases which included persons ≥16 years old, appropriately vaccinated with either BNT162b2 or ChAdOx1 nCoV-19 regimens, with symptomatic COVID-19 caused by either alpha or delta variant. They observed the alpha variant in 14,837 (77.6%) cases, and delta in 4272 (22.4%).

More than one-third of observed patients in each variant population were aged 16-29 at the time of sequencing; approximately 99% of both populations did not have a history of travel that would implicate international SARS-CoV-2 infection risk. A total of 307 (2.1%) sequenced COVID-19 cases from either variant group were linked to health or social care workers.

In observing a single dose of either regimen, Lopez Bernal and colleagues observed significantly lower vaccine effectiveness versus the delta variant: 30.7% (95% CI, 25.2 – 35.7). Single-dose vaccine effectiveness versus the alpha variant was 48.7% (95% CI, 45.5 – 51.7).

Two-dose BNT162b2 was associated with a 93.7% vaccine effectiveness versus the alpha variant (95% CI, 91.6 – 95.3), compared to an 88.0% effectiveness versus the delta variant (95% CI, 85.3 – 90.1).

Two-dose ChAdOx1 nCoV-19 was associated with a 74.5% vaccine effectiveness versus the alpha variant (95% CI, 68.4 – 79.4) and a 67.0% effectiveness versus the alpha variant (95% CI, 61.3 – 71.8).

“A clear effect was noted with both vaccines, with high levels of effectiveness after 2 doses,” investigators noted. “Vaccine effectiveness against either variant was smaller after the receipt of 2 doses of the ChAdOx1 nCoV-19 vaccine than after the receipt of 2 doses of the BNT162b2 vaccine, a finding that is consistent with reported clinical trial findings.”

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The team noted several limitations to the trial, including their observational nature, the lower sensitivity or specificity of relied-upon PCR testing results, and the significant differences in vaccine coverage and administration among differing races or ethnic groups adversely affected by early delta variant outbreaks—despite their control for such factors.

However, they concluded the findings show “high levels of vaccine effectiveness” with 2-dose vaccination against symptomatic COVID-19 brought on by the delta variant.

“These estimates were only modestly lower than the estimate of vaccine effectiveness against the alpha variant,” they wrote. “Our finding of reduced effectiveness after the first dose would support efforts to maximize vaccine uptake with two doses among vulnerable groups in the context of circulation of the delta variant.”

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