There are a lot of caveats to these numbers. They’re not from a peer reviewed study but rather the latest data from the Israeli health ministry. While the Pfizer mRNA vaccines remains highly effective at preventing hospitalization and serious illness (over 90%), the efficacy against infection with the Delta variant has dropped markedly.
The Delta variant became much more prevalent in Israel between May and June. It now accounts for 90% of new infections. In May, the vaccine’s efficacy against infection was 94.3%. But in June it fell to 64%, according to these Israeli Ministry of Health numbers. This shift isn’t a perfect match for efficacy against the Delta variant. But given how quickly Delta became the dominant strain over this period it’s pretty close. Meanwhile a separate study from researchers at Hebrew University estimated the vaccine’s efficacy at between 60% and 80% against infection with the Delta variant.
The same figures also show diminished but still very robust protection against serious symptoms and hospitalization: 98.2% in May, 93% in June.
These numbers are significantly different from other recent studies. Another recent study of the same vaccine and same variant in the UK found a still very robust 88% efficacy. Here’s a cautionary note on these figures from the head of Israel’s biggest HMO. Basically the absolute numbers are small and not evenly distributed. They are mostly from a set of small outbreaks. So the numbers can be hard to interpret.
As I’ve noted in other posts, one of the key questions about this data is just what these infections represent. How many of these infections are asymptomatic and possibly of no real consequence either to the infected person’s health or community spread? What number of these cause significant illness and possibly longterm health consequences for the people who get infected? At least in the pre-vaccine period there is abundant evidence of cases where an initially ‘mild’ or non-debilitating case produced ‘long covid’ and serious longterm health consequences.
When reading up on these questions it is often hard to pry apart what is actually unknown and what is simply unspecified either in a mainstream media write up of new data or reports in more specialist sources.