As the delta variant sweeps the world, researchers are tracking how well vaccines protect against it — and getting different answers.

In the U.K., researchers reported in May that two doses of the Pfizer-BioNTech vaccine had an effectiveness of 88% protecting against symptomatic disease from delta. A June study from Scotland concluded the vaccine was 79% effective against the variant, and on Saturday a team of researchers in Canada pegged its effectiveness at 87%.

On Monday, though, Israel’s Ministry of Health announced that the effectiveness of the Pfizer-BioNTech vaccine was seen to be 64% against all coronavirus infections — down from about 95% in May, before the delta variant began its climb to near-total dominance in Israel.