At the height of Europe's vaccine supply crisis in March, Johnson & Johnson's (J&J) single-dose Covid-19 vaccine was touted as a game-changer. But two months into its rollout, the shot has the lowest uptake of all four approved vaccines.

Supply problems, safety concerns, improved deliveries of rival vaccines and countries' inoculation strategies have all played a part in holding back usage.

Despite the shot's clear logistical advantage of not requiring a booster, the European Union has only administered about half of the delivered doses - an uptake rate much lower than any rival Covid-19 vaccine, including AstraZeneca's, which has also been dogged by supply and safety issues.

That is bad news for J&J, and also raises questions about the efficiency of the EU's inoculation campaign...