The European drug regulator has revealed it is 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 if the clots were related to the vaccine or another medical issue.
Three of the cases occurred during the US rollout where nearly five million had been given the vaccine by Thursday, while one was during a clinical trial.
In one of the cases, the person died from the complication.
Johnson and Johnson said it was aware of the review and was working with regulators to assess the issue, but insisted "no clear causal relationship has been established between these rare events and the Janssen Covid-19 vaccine".
The major pharmaceutical's jab is currently only available in the US and was scheduled to be issued in the European Union in coming weeks.
New Zealand has ordered up to five million doses of the Janssen vaccine but is still waiting for more data before approval.
Australia has not yet committed to the Johnson and Johnson vaccine but the complication comes amid concerns with the AstraZeneca shot, which is threatening to derail confidence in the rollout.
Professor Mary-Louise McLaws, an epidemiologist at the University of New South Wales, insisted the extremely rare complications in each of the vaccines far outweighed the threat of Covid-19.
"It's all about putting into proportion the risk of death to our elderly group who represent 100 per cent of all coronavirus deaths in Australia," she told NCA NewsWire.
"They only die if they get Covid, so to keep them from getting infected we have to ring-fence them.
"In the context of a pandemic, there's this enormous risk of this older group dying so you want to protect them by vaccinating everybody, but particularly the young ones who have a greater risk of acquiring it and transmitting it."
Despite Australia not yet committing to the Johnson and Johnson vaccine, the hiccup from the EMA this week threatens to complicate an already increasing supply issue across the globe.
Prime Minister Scott Morrison on Thursday refused to commit to the nation's jab rollout being completed this year as Professor McLaws offered a far more gloomy timeline — she expects delays to vaccine supply leading to Australia's borders remaining closed up until the end 2022.
New Zealand has agreed to receiving up to 7.6 million doses of the AstraZeneca vaccine.
The AstraZeneca shot won't be offered to Australians under the age of 50 following advice from the Australian Technical Advisory Group on Immunisation, which will free-up supply for the older cohort.
"So now that we have less people requiring AstraZeneca, we've got a little bit more (supply) space," the professor said.
"But if we want to open up anytime soon next year, that supply of AstraZeneca is going to have to be increased to get people vaccinated in time because there's a three-month delay between the first and the second shot.
"So just because fewer people need it, doesn't mean we're out of the woods yet because the government is going to have to run two cohorts and they're going to have to match the speed so we can open up our borders."