By Carl Zimmer
As the Delta variant sweeps the world, researchers are tracking how well vaccines protect against it — and getting different answers.
In Britain, researchers reported in May that two doses of the Pfizer-BioNTech vaccine had an effectiveness of 88 percent protecting against symptomatic disease from Delta. A June study from Scotland concluded the vaccine was 79 percent effective against the variant. On Saturday, a team of researchers in Canada pegged its effectiveness at 87 percent.
And on Monday, Israel’s Ministry of Health announced that the effectiveness of the Pfizer-BioNTech vaccine was 64 percent against all coronavirus infections, down from about 95 percent in May, before the Delta variant began its climb to near-total dominance in Israel.
Although the range of these numbers may seem confusing, vaccine experts say it should be expected because it’s hard for a single study to accurately pinpoint the effectiveness of a vaccine.
“We just have to take everything together as little pieces of a puzzle, and not put too much weight on any one number,” said Natalie Dean, a biostatistician at Emory University.
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