VE calculations provided primarily focus on death and disease, show reduced effectiveness compared to earlier projections, and predominantly cover a blended time frame in which contributions from Beta predominates and Delta infection is minimal or non-existent.
As summarized in slides #16 and 17, the viral load observed during Delta infection is significantly higher than observed with the ancestral strain.
Slide #19 summarizes data then available concerning reduced vaccine effectiveness for the Delta strain relative to the ancestral Alpha strain, although more recent data from Israel indicate that Pfizer vaccine effectiveness against Delta infection is under 40%. Among the most surprising and alarming aspects of the entire presentation are the epidemiologic risk management modeling findings presented Slides #20 and #21. This pair of slides are complicated and difficult to understand for those who have not previously encountered these types of analyses, which may explain why the Washington Post did not discuss the information and conclusions.
These represent statistical modeling projections for the potential impact of different intervention options based on assumptions concerning the reproductive coefficient of the virus, the effectiveness of mask or vaccine use, the extent of vaccine acceptance in the population, and the fraction of the population which has been previously exposed/recovered/naturally immune due to prior SARS-CoV-2 infection.
Based on the flawed assumptions used, these projections indicate that at approximately 60% vaccine uptake, in the absence of mask use the Delta variant has an approximately 100% probability of spreading at greater than 50 cases/100,000/week.
If only the unvaccinated were to wear masks, the probability of seeing further spread ranges from 40 to 100%. If both vaccinated and unvaccinated practice optimal mask use, the probability of further Delta viral spread ranges from 0 to 90%. From this analysis, the CDC concludes that "Given higher transmissibility and current vaccine coverage, universal masking is essential to reduce transmission of the Delta variant".
Based on these projections, it appears that if the more realistic variables of Ro =8, 40% vaccine effectiveness, and suboptimal mask use are employed, it is unlikely that the eventual spread of the Delta variant through the general population can be controlled even with considerably higher uptake of these leaky vaccines and perfect mask compliance.
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