The Amsterdam-based European Medicines Agency has acknowledged that thrombosis and a lower number of blood platelets may be a very rare side effect of the Janssen coronavirus vaccine and say they should be mentioned in the official product information.

However, the EMA said at a news conference on Tuesday, the advantages offered by the vaccine, which is single dose, outweigh the disadvantages. ‘The benefits of the vaccine continue to outweigh the risks for people who receive it,’ the EMA said. ‘The vaccine is effective at preventing COVID-19 and reducing hospitalisations and deaths.’

The agency examined eight cases of blood clots in the US, one of which was fatal. In total, some seven million people have been given the Janssen vaccine.

All cases occurred in people under 60 years of age within three weeks after vaccination, the majority in women. Based on the currently available evidence, specific risk factors have not been confirmed, the EMA said. ‘The cases reviewed were very similar to the cases that occurred with the COVID-19 vaccine developed by AstraZeneca, Vaxzevria.’

As yet the health ministry has not commented on the findings. ‘It is now up to health minister Hugo de Jonge to decide what the consequences of this are for the Dutch vaccination campaign’ Ton de Boer, chairman of the Dutch medicines agency CBG, told broadcaster NOS.

Johnson & Johnson halted deliveries of the Leiden-developed vaccine to Europe pending the EMA’s verdict and De Jonge stopped its use in the Netherlands temporarily.

The Netherlands has also stopped using the AstraZeneca vaccine on the under-60s because of the possible risk. This decision went against the EMA’s recommendations and has also been widely criticized in the Netherlands

The Janssen vaccine is the fourth to be approved by the EMA and is supposed to form the backbone of the Dutch vaccination programme. De Jonge has ordered 11.3 million doses.

Currently around 100,000 people a day are being vaccinated in the Netherlands, taking the total shots placed to 4.6 million