Effectiveness of Pfizer, AstraZeneca COVID-19 Vaccines Against Delta Variant

While a significantly reduced effectiveness was seen with 1 dose, only a small reduction was observed after 2.

A recent study conducted by investigators from Public Health England has found that 1 dose of either the Pfizer or AstraZeneca COVID-19 vaccines have a significantly reduced effectiveness against the Delta variant. However, after 2 doses of either vaccine only small reductions were observed.

Results from the study were published in The New England Journal of Medicine.

“These findings suggest a modest reduction in vaccine effectiveness,” the authors wrote. “Nevertheless, a clear effect of both vaccines was noted with high levels of effectiveness after two doses. This is consistent with reported clinical trial findings.”

For the study, the team of investigators employed a test negative case control design to estimate the effectiveness of the 2 vaccines against symptomatic disease from both the Delta and Alpha variants of COVID-19.

Data from all symptomatic, sequenced cases from England was used to estimate the proportion of Delta cases compared to Alpha cases by vaccination status. This data was gathered from the national vaccination register, the National Immunization Management System (NIMS).

In total, 12,675 sequenced cases were included in the study, of which 11,621 were the Alpha variant and 1,054 were the Delta variant.

Findings from the study demonstrated that vaccine efficacy was notably lower after a single dose of either vaccine against the Delta variant when compared to the Alpha variant, at approximately 33%.

After 2 doses of the Pfizer vaccine, a reduced dose effectiveness was observed, dropping from 93.4% against Alpha to 87.9% against Delta. After 2 doses of the AstraZeneca vaccine, dose effectiveness was reduced from 66.1% against Alpha to 59.8% against Delta.

After 1 or 2 doses of either vaccine, detected sequenced cases had higher odds of infection with the Delta variant compared to unvaccinated cases.

“Overall, we found high levels of vaccine effectiveness against symptomatic disease after two doses. These estimates were only modestly lower than vaccine effectiveness against the B.1.1.7 variant,” the authors wrote. “It is likely that vaccine effectiveness against more severe disease outcomes will be greater. Our finding of reduced effectiveness after dose 1, would support maximizing vaccine uptake with two doses among vulnerable groups in the context of circulation of B.1.617.2.”