by Antonella Macerollo
For October 2021, we have selected: Lopez Bernal J. et al, Effectiveness of Covid-19 Vaccines against the B.1.617.2 (Delta) Variant: test-negative case–control study. N Engl J Med. 2021 385:585-594 | DOI: 10.1056/NEJMoa2108891
The B.1.617.2 (delta) variant of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first detected in India in December 2020 and became the most commonly reported variant in the country starting in mid-April 2021. Of note, 43 countries across six continents had reported detection of the variant, including a rapid increase in cases in the United Kingdom associated with travel from India.
Our authors of the month developed a test-negative case–control study to evaluate the effectiveness of the BNT162b2 and the ChAdOx1 nCoV-19 vaccines against symptomatic disease caused by the delta variant or the predominant strain (B.1.1.7, or alpha variant) over the period that the delta variant began circulating. The extracted PCR data ranged from Oct 26, 2020 to May 16, 2021. The extracted vaccination data were from April 5, 2021 to May 16, 2021
The vaccination status of people with confirmed symptomatic Covid-19 was compared against the vaccination status of those who had symptoms but tested negative. Subsequently, the proportion of cases caused by the delta variant relative to cases caused by the alpha variant were estimated according to vaccination status.
Data on all persons in England who have been vaccinated with Covid-19 vaccines are available in a national vaccination register (the National Immunisation Management System). The data extracted by the authors were restricted to the period since at least 10 cases of B.1.617.2 were detected per week (week commencing 5th April 2021 onwards) up to May 16, 2021, including the date of receipt of each dose of vaccine and the vaccine type.
- Children younger than 16 years of age as of March 21, 2021, were excluded.
- Data were restricted to persons who had reported symptoms, and only persons who had undergone testing within 10 days after symptom onset were included.
Data on all positive PCR tests between October 26, 2020, and May 16, 2021, were extracted. Data on all recorded negative community tests among persons who reported symptoms were also extracted for the test-negative case–control analysis.
- In cases where a person had multiple positive tests within a 90-day period (which may represent a single illness episode), only the first positive test was included.
- A maximum of three randomly chosen negative test results were included for each person. Negative tests in which the sample had been obtained within 3 weeks before or after a positive result could have been false negatives; therefore, these were excluded.
- Tests that had been administered within 7 days after a previous negative result were also excluded.
- Persons who had previously tested positive before the analysis period were also excluded in order to estimate vaccine effectiveness in fully susceptible persons.
Variants were identified using sequencing and by differentiating the spike (S) gene status.
Our study of the month found that after one dose of either BNT162b2 or ChAdOx1 nCoV-19, effectiveness was notably lower among persons with the delta variant (30.7%; 95% confidence interval [CI], 25.2 to 35.7) than among those with the alpha variant (48.7%; 95% CI, 45.5 to 51.7). With the Effectiveness of two doses of BNT162b2 was 93.7% (95% CI, 91.6 to 95.3) among persons with the alpha variant and 88.0% (95% CI, 85.3 to 90.1) among those with the delta variant. The effectiveness of two doses of ChAdOx1 nCoV-19 was 74.5% (95% CI, 68.4 to 79.4) among those with the alpha variant and 67.0% (95% CI, 61.3 to 71.8) among those with the delta variant.
The authors note that they only detected slight variations in effectiveness with regard to the delta variant compared with the alpha variant after two vaccine doses had been received. The differences in effectiveness were much more notable when only the first dose had been administered, with an absolute difference of 11.9 percentage points with the BNT162b2 vaccine and 18.7 percentage points with the ChAdOx1 nCoV-19 vaccine.
These results should encourage to plan the full vaccination scheme among vulnerable populations to reduce the spread of variants and their consequences. This strategy would improve significantly the health care costs of the national health systems.