Norway on Friday extended a suspension of the use of the AstraZeneca Covid-19 vaccine until April 15th, with health officials saying they needed more time to investigate a potential link to severe blood clotting.

“It is a difficult but correct decision to extend the pause for the AstraZeneca vaccine. We believe it is necessary to carry out more investigations into these cases,” Geir Bukholm, Director of the Division of Infection Control at the Norwegian Institute of Public Health (NIPH), said in a statement.

EU drugs regulator EMA last week said the vaccine was “safe and effective” and not linked to a higher risk of blood clots, but could not “rule out definitively” its role in the rare clotting disorder.

The World Health Organization has also urged countries to continue using the vaccine, arguing the benefits outweigh any potential risks.

The suspension is expected to delay the Norwegian vaccination programme by one to two weeks, the institute said, and people who have already received a first dose of the vaccine would not be offered a second injection at this stage.

In the Nordic country, several healthcare workers under the age of 55 have suffered symptoms including blood clots, bleeding and a drop in blood platelets after receiving the AstraZeneca jab.

Four deaths have been reported, with three from combinations of these complications and one from a brain haemorrhage.

No link has been established with the Anglo-Swedish drugmaker’s vaccine, although a Norwegian medical team said it saw these rare but serious cases as the result of a “powerful immune response” triggered by the serum.

“The most important thing is to determine whether there is a causal link between the vaccination and these very serious side effects, as we suspect,” NIPH director Camilla Stoltenberg told public broadcaster NRK radio.

Among the first to suspend AstraZeneca’s vaccine, the Nordic countries are now adopting different strategies.

While Denmark on Thursday announced a three-week extension of the pause to allow time for a closer look at possible side effects of the vaccine, Finland, Iceland and Sweden are restarting their rollouts, although only for older people.