European drug regulators have revealed that they are reviewing rare blood clots suffered by recipients of the Johnson and Johnson vaccine in the United States.

The European Medicines Agency (EMA) said it was investigating four serious cases but it was unclear at this stage whether the clot was related to vaccines or other medical problems.

Three of the cases occurred during the US launch in which nearly five million had been vaccinated by Thursday, while one case occurred during clinical trials.

In one case, the person died from complications.

Johnson and Johnson said they were aware of the review and were working with regulators to assess the matter, but insisted “there is no clear causal link between this rare event and the Janssen Covid-19 vaccine”.

The main medicinal injections are currently only available in the US and are scheduled to be published in the European Union in the coming weeks.

New Zealand has ordered up to five million doses of the Janssen vaccine but is still awaiting more data before approval.

Australia has yet to commit to Johnson and Johnson vaccines but complications have come amid concerns with AstraZeneca’s injections, which threaten to destabilize confidence in the launch.

Professor Mary-Louise McLaws, an epidemiologist at the University of New South Wales, insists that the extremely rare complications of any vaccine far outweigh the threat of Covid-19.

“It’s all about placing a proportionate risk of death in our elderly cohort representing 100 percent of all coronavirus deaths in Australia,” he told NCA NewsWire.

“They only die if they catch Covid, so to prevent them from getting infected, we have to fence them off.

“In the context of a pandemic, there is a huge risk of death for this older group so you want to protect them by vaccinating everyone, but especially children who are at greater risk of catching and transmitting it.”

Although Australia has yet to commit to a Johnson and Johnson vaccine, a hiccup from this week’s EMA threatens to complicate already-increasing supply problems around the world.

Prime Minister Scott Morrison on Thursday refused to commit to having the nation’s jab launch completed this year because Professor McLaws offers a much bleaker timeline – he expects vaccine supply delays to keep Australia’s borders closed through the end of 2022.

New Zealand has agreed to receive up to 7.6 million doses of AstraZeneca vaccine.

AstraZeneca injections will not be offered to Australians under the age of 50 following the advice of the Australian Technical Advisory Group on Immunization, which will free supplies for the older group.

“So now we have less people who need AstraZeneca, we have more space (supply),” said the professor.

“But if we want to open in the near future next year, AstraZeneca’s supply must be increased so that people get vaccinated on time because there is a three month delay between the first and second injections.

“So just because there are fewer people who need it doesn’t mean we’re out of the jungle because the government has to deploy two troops and they have to adjust the pace so we can open our borders.”