Two doses of COVID-19 vaccine crucial to maximize effectiveness against delta variant

Roshini Claire Anthony
03 Sep 2021

Although both the BNT162b2 and ChAdOx1 nCoV-19 vaccines appear to be effective against symptomatic disease caused by the delta variant of the SARS-CoV-2 virus, the effectiveness is lower than that against the alpha variant. However, this reduced effectiveness was less pronounced after two doses of the vaccine, a recent study from England showed.

“Absolute differences in vaccine effectiveness were more marked after the receipt of the first dose,” the authors said. “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.”

“This finding would support efforts to maximize vaccine uptake with two doses among vulnerable populations,” they said.

Data was sourced from The National Immunisation Management System register which contains data of all individuals in England who have received COVID-19 vaccines. The authors used a test-negative case-control design to assess the effectiveness of the BNT162b2 and ChAdOx1 nCoV-19 vaccines against symptomatic disease caused by either the delta (B.1.617.2) or alpha variant (B.1.1.7) of the SARS-CoV-2 virus in individuals aged 16 years during the period when the delta variant began circulating in England (April 5–May 16, 2021).

Variants were identified through sequencing and spike (S) gene status (95.3 percent of S target-positive cases had the delta variant and 99.6 percent of S target-negative cases had the alpha variant). Of the 19,109 sequenced cases, the alpha and delta variants were detected in 14,837 and 4,272 cases, respectively, comprising 77.6 and 22.4 percent of cases.

The population was restricted to individuals who had undergone SARS-CoV-2 testing within 10 days of symptom onset and mainly comprised individuals aged 16–29 years (36.1 percent), with few patients aged 70–79 or 80 years (1.1 and 0.3 percent, respectively).

Overall, the effectiveness of one vaccine dose* was lower in individuals with the delta compared with the alpha variant (30.7 percent vs 48.7 percent). The results were comparable regardless of vaccine type (absolute difference in vaccine effectiveness against delta vs alpha variant, 11.9 percentage points [BNT162b2] and 18.7 percentage points [ChAdOx1 nCoV-19]). [N Engl J Med 2021;doi:10.1056/NEJMoa2108891]

The reduced vaccine effectiveness against the delta vs alpha variant was less pronounced following two doses of the vaccine** (79.6 percent vs 87.5 percent). This was demonstrated with a small between-variant difference with the BNT162b2 vaccine (88.0 percent vs 93.7 percent). The effectiveness after two doses was lower with the ChAdOx1 nCoV-19 than with the BNT162b2 vaccine, though the between-variant difference was still small with the ChAdOx1 nCoV-19 vaccine (67.0 percent [delta] vs 74.5 percent [alpha]).

“A clear effect was noted with both vaccines, with high levels of effectiveness after two doses,” said the authors. “[However,] vaccine effectiveness against either variant was smaller after the receipt of two doses of the ChAdOx1 nCoV-19 vaccine than after the receipt of two doses of the BNT162b2 vaccine,” they noted.

They pointed out that the case numbers and follow-up time in this study were insufficient to estimate the effectiveness of the vaccines against more severe disease. They also recommended caution when interpreting the results given the observational study design.

Differences in the populations that received the different vaccines may also have impacted the results. “For example, in younger age groups, healthcare workers are more likely to have received the BNT162b2 vaccine, whereas persons in clinical risk groups are more likely to have received the ChAdOx1 nCoV-19 vaccine,” they said.


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