The Oxford-AstraZeneca vaccine has become a bastion of British medical innovation and pride - and one the British Prime Minister has been a fierce cheerleader of.
"These vaccines are safe, they've saved many thousands of lives. And people should come forward to get their jabs," Prime Minister Boris Johnson said.
But not if you're under 30, with the UK's regulator saying the benefits don't outweigh the risk of potential blood-clots.
Dr June Raine, Chief Executive of the Medicines and Healthcare products Regulatory Agency (MHRA) says it only affects a small number of people.
"The UK has careful monitoring systems in place and these systems are now detecting a potential side effect of the vaccine AstraZeneca in an extremely small number of people."
So far, 20.2 million people have received a dose of the AstraZeneca vaccine since its rollout in December.
Seventy nine people have reported blood-clots as a side-effect - that's around four in every million - and of those 79, 19 people have died.
Three were under the age of 30, according to the UK's Joint Committee on Vaccination and Immunisation (JCIV).
The vaccine isn't more dangerous to under-30s, it's just not as beneficial because COVID-19 itself is less likely to harm them - so it's not worth the risk.
"It would be preferable to offer the very youngest people who have no other health conditions the alternative vaccine instead of the AstraZeneca vaccine really just on the side of safety rather than any particular concern," Professor Wei Shen, chair of the JCVI says.
New Zealand has 7.5 million doses of AstraZeneca on order, but it's one of numerous vaccines that've been purchased. And experts say side-effects were always expected - others could pop up with the other brands like Pfizer or Moderna.
"This is a massive beast we are driving along at pace and if you sail a massive liner then it's not reasonable that you don't have to make a course correction," Professor Jonathan Van-Tam, Deputy Chief Medical Officer for England says.
Wales has just begun the rollout of the Moderna vaccine - an American-made option that is being recommended for under 30s, alongside the Pfizer vaccine.