In this post, I will focus on vaccine efficacy vs. severe disease/hospitalization, which is the key factor for public health. I will not deal with vaccine efficacy vs. symptomatic or asymptomatic disease here — that has its own set of nuances that I will save for a future post.
… 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.
While this is just a snapshot of currently active infections on August 15, 2021, the principles apply to other analyses done on Israeli data, as well as others.
One caveat with any efficacy analyses with the Israeli dashboard data is that the previously infected are not separated out. Note that:
- Israel did not allow previously infected to be vaccinated until 3 months into the vaccination campaign (in March)
- Then made only optional (given they awarded immunity passports to previously infected even if unvaccinated) and only limited them to one shot.
Given the high vaccination rate, it is plausible that a substantial proportion of unvaccinated were previously infected. Given the overwhelming evidence that previous infection confers strong and lasting immune protection from dozens of published papers, this means those unvaccinated have strong immune protection (possible comparable to vaccinated). This would serve to attenuate the efficacy estimates, and may be one reason why the efficacy vs. severe disease is not higher than 85-92%. Also, this might make their single-dose efficacy appear much higher than other places since it also includes those previously infected who were eventually vaccinated. More caveats to keep in mind …
[editor’s note: this post deserves a full read as I omitted the backup explanations for the conclusion]