Effect of Delta variant on viral burden and vaccine effectiveness against new SARS-CoV-2 infections in the UK.
Level 3 - non-randomized controlled study
Non-randomized prospective community-based cohort survey
PubMed 34650248 · doi:10.1038/s41591-021-01548-7
What was done
Investigators evaluated the effectiveness of BNT162b2 and ChAdOx1 vaccines against new SARS-CoV-2 infections in a community-based survey of randomly selected households across the United Kingdom during Alpha (B.1.1.7) and Delta (B.1.617.2) variant predominance. Outcomes assessed were PCR-positive infections, symptom presence, cycle threshold (viral burden) dynamics, and differences by dosing interval, age, and prior infection history.
What was found
Vaccine effectiveness against infections with symptoms or high viral burden was lower for Delta than for Alpha, with absolute reductions of 10% to 13% for BNT162b2 and 16% for ChAdOx1. Two vaccine doses provided at least as much protection as prior natural infection. BNT162b2 showed greater initial effectiveness than ChAdOx1 against new PCR-positive cases but experienced faster declines in protection against high viral burden and symptomatic infection. Dosing intervals did not show an effect on effectiveness, whereas protection was higher in younger adults and individuals with prior infection. Infections occurring after two doses during Delta dominance had peak viral burdens similar to those in unvaccinated individuals.
Why it matters
This study showed that the Delta variant reduced vaccine effectiveness against symptomatic disease and high viral loads compared to Alpha, and established that fully vaccinated breakthrough cases could carry peak viral loads comparable to unvaccinated individuals.
Limits
The abstract does not report the total participant sample size, confidence intervals, or precise duration of follow-up. As an observational study, residual confounding from behavior or exposure risk cannot be excluded.
Cited by
- supports A large UK study that included the AstraZeneca vaccine found that peak viral loads were similar in Delta breakthrough infections between vaccinated and unvaccinated individuals.