NUMBERS: Israeli data: How can efficacy vs. severe disease be strong when 60% of hospitalized are vaccinated?

However, while these numbers are true, to quote them as evidence for low vaccine efficacy is wrong and misleading. Sometimes, with observational data there is confounding of multiple factors that can make it easy to misinterpret simple percentages like this, and the current vaccination situation in Israel brings a perfect storm of confounding factors that lead to confusion if not thought through carefully.

In particular, the key factors here that contribute to this confusion are:
High vaccination rates in the country (nearly 80% of all residents >12yr)

Age disparity in vaccinations, including
Nearly all older people being vaccinated (>90% of residents >50yr) and

The vast majority of unvaccinated being younger people (>85% of unvaccinated <50yr) Older people are orders of magnitude more likely to be hospitalized with a respiratory virus than young people (residents >50yr are >20x more likely to have hospitalized serious infections than residents <50yr, and residents 90+ are >1600x more likely to have hospitalized serious infections than residents 12-15yr)

After accounting for the vaccination rates and stratifying by age groups, from these same data we can see that the vaccines retain high efficacy (85-95%) vs. severe disease, showing that when it comes to preventing severe disease, the Pfizer vaccine is still performing very well vs. Delta, even in Israel from whence the most concerning data have arisen.

When the people who are much more likely to be hospitalized if they get sick are also much more likely to be vaccinated, it’s not surprising that they’re a majority of the hospitalized, even with a 95% effective vaccine.