Here is a terrific analysis of the recent Israeli data that is making the rounds showing decreased efficacy of vaccines vs. Delta.
The author is Jeffrey S. Morris, PhD, Professor and Director of the Division of Biostatistics at the University of Pennsylvania’s Perelman School of Medicine.
https://www.dbei.med.upenn.edu/bio/jeffrey-s-morris-phd
The article is pretty dense, but very informative and the author does a nice job walking through things step by step. It’s worth the read. Long story short, the headlines claiming only 60-70% efficacy vs. Delta are misleading as the data are not stratified by age. When doing an apples to apples comparison of age groups the Pfizer vaccine shows the following efficacy:
In conclusion, as long as there is a major age disparity in vaccination rates, with older individuals being more highly vaccinated, then the fact that older people have an inherently higher risk of hospitalization when infected with a respiratory virus means that it is always important to stratify results by age; if not the overall efficacy will be biased downwards and a poor representation of how well the vaccine is working in preventing serious disease (the same holds for efficacy vs. death).
Even more fundamentally, it is important to use infection and disease rates (per 100k, e.g.) and not raw counts to compare unvaccinated and vaccinated groups to adjust for the proportion vaccinated. Use of raw counts exaggerates the vaccine efficacy when vaccinated proportion is low and attenuates the vaccine efficacy when, like in Israel, vaccines proportions are high.
This is not just an issue of making vaccines look worse than they are … any summary computing “proportion of hospitalized that are unvaccinated” that covers a period of time in which the proportion vaccinated was low can be similarly misleading, especially if there was a massive Covid-19 surge during that time periods. For example, computing total proportion of hospitalized covid infections in the USA from unvaccinated individuals while aggregating over the entire 2021 (January to present), a time periods that includes the early months in which virtually all USA residents were unvaccinated and there was a massive winter surge, will be similarly misleading. Thus, these artifacts can be used by some to make the vaccines look better than they in fact are, e.g. any report suggesting things like 99.9% of hospitalizations are from unvaccinated when covering a long period of time like this.
The bottom line is there is very strong evidence that the vaccines have high efficacy protecting against severe disease, even for Delta, and even in these Israeli data that on the surface appear to suggest the Pfizer vaccine might have waning efficacy. This is clearly evident if the data are analyzed carefully, and agrees with all other published results to date from other countries.