Why Some Nations Have Deadlier Outbreaks With the Same Vaccines

It’s one of the great puzzles of the pandemic. Most developed economies are now highly vaccinated with some of the most effective shots on offer, so why are the latest Covid-19 outbreaks more deadly in some places than in others?

While it’s clear vaccines led to a drop in fatalities during the most recent delta variant-driven waves compared with earlier bouts with the virus, some countries saw deaths fall to a greater degree than others, an outcome scientists still don’t have answers for.

Countries like Germany, Denmark and the U.K. have seen Covid deaths fall to roughly a tenth of ...