Four EU countries expressed concern Friday to European Commission chief Ursula von der Leyen over a Covid-19 vaccine candidate developed by Johnson & Johnson being shipped to the US. The leaders of Austria, the Czech Republic, Denmark and Greece said in a joint letter that the Commission should seek a way "to safeguard European supply". The warning highlighted the European Union's search to guarantee vaccine stocks for a stuttering jabs rollout after Anglo-Swedish group AstraZeneca fell far short of its early delivery schedule of its vaccine doses. The leaders hailed von der Leyen's pressure on AstraZeneca, saying: "It seems they now understand the gravity of the situation." But the Commission and EU countries must "mitigate potential problems before they arise", they said, emphasising that "time is of the essence". They also called on von der Leyen to "keep up the speed" in negotiations under way with two other makers of candidate vaccines, Novavax and Valneva. Three vaccines are currently authorised for use across the EU's 27 member states: the one by AstraZeneca, and others by BioNTech/Pfizer and Moderna. Johnson & Johnson vaccine doses made in Europe are to be sent to the US for the final stages of production. The leaders' concern is over whether US authorities will permit them to be shipped back to the EU. The European Commission has ordered 200 million doses of the single-shot Johnson & Johnson vaccine with an option for 200 more. It says 100 million doses should be delivered by June if it is approved. Johnson & Johnson has asked US regulators to authorise its vaccine for use in the United States. alm-rmb/dc