The US yesterday recommended a "pause" in using the single-dose Johnson & Johnson Covid-19 vaccine to investigate reports of rare but potentially dangerous blood clots, a development that could jeopardise the rollout of vaccines around the world.
The Centers for Disease Control and Prevention and the Food and Drug Administration announced that they were investigating unusual clots that occurred six to 13 days after vaccination. The FDA commissioner said she expected the pause to last a matter of days.
In New Zealand, Medsafe is currently meeting to consider whether to approve the Johnson & Johnson Janssen Covid-19 vaccine for use in the country. Their decision is expected to be released tomorrow.
The clots occurred in veins that drain blood from the brain and occurred together with low platelets, the fragments in blood that normally form clots.
All six cases were in women between the ages of 18 and 48. One person died, and all of the cases remain under investigation.
"We're talking about six people with this blood clot over millions of doses that have been given," said Dr Geoffrey Barnes, a vascular cardiologist at Michigan Medicine
"And so what I would say to folks is you can be reassured that this is not a common event, but it's something that doctors and other health care professionals are aware of.
"We have the ability to identify, diagnose it and initiate appropriate treatments if this rare event does occur."
More than 6.8 million doses of the J&J vaccine have been given in the US, the vast majority with no or mild side effects.
Any slowdown in the dissemination of the shots could have broad implications for the global vaccination effort.
The J&J vaccine held particular promise for less affluent countries because its single-dose regimen and relatively simple storage requirements make it easier to use in the developing world.
The reports appear similar to a rare, unusual type of clotting disorder that European authorities say is possibly linked to another Covid-19 vaccine not yet cleared in the US, from AstraZeneca.
Federally run mass vaccination sites will pause the use of the J&J shot, and states and other providers are expected to follow. The other two authorised vaccines, from Moderna and Pfizer, make up the vast share of Covid-19 shots administered in the US and are not affected by the pause.
A CDC committee will meet today to discuss the cases, and the FDA has also launched an investigation into the cause of the clots and low platelet counts.
Authorities have not seen similar clots after use of the Pfizer or Moderna vaccine.
FDA officials emphasised that Tuesday's action was not a mandate. Doctors and patients could still use J&J's vaccine if they decide its benefits outweigh its risks for individual cases.
The agencies recommend that people who were given the J&J vaccine should contact their doctor is they experience severe headache, abdominal pain, leg pain, or shortness of breath within three weeks.
J&J said in a statement that it was aware of the reports of blood clots, but that no link to its vaccine had been established. The company also said it would delay the rollout of its vaccine in Europe as a precaution.
"Worldwide this is a different story, because Johnson & Johnson is expected to produce a MBT do this by any kind of way. That's a lot of dough," said Tinglong Dai, an associate professor at John Hopkins Carey Business School.
"This especially because Johnson, it is a lone single dose of vaccine approved by FDA, all by European and UK authorities. And so basically one billion doses, you can vaccinate one billion people.
"That's a larger proportion of what people worldwide. So, this definitely is very big setback for the war, although he's a very small setback for the US."
As authorities investigate whether the clots really are related to the J&J vaccine, Barnes stressed that Americans should get vaccinated as soon as possible using the other two available vaccines, from Pfizer and Moderna.
"So, then we can really recommend to patients and to the public who should be getting the vaccine and how to best protect them from Covid-19. Because we know that risks from Covid far outweigh any risks from vaccines.
"And so it's really important that people feel confident in getting their Covid vaccine so we can get this pandemic under control," said Barnes.