CHICAGO / LONDON (Reuters) – With two COVID-19 vaccines now being investigated for possible links to very rare cases of blood clots in the brain, US government scientists are focusing on whether the specific technology behind the shots is at risk can contribute.
In Europe, health regulators said last week that there was a possible link between the AstraZeneca Plc vaccine and 169 cases of a rare cerebral blood clot known as cerebral venous sinus thrombosis (CVST), accompanied by a low platelet count, from 34 million shots administered in the European Economic Area.
The U.S. Food and Drug Administration on Tuesday recommended temporarily suspending the use of the Johnson & Johnson vaccine after reports of six cases of CVST in women under 50 among about 7 million people who received the shot in the United States.
Both vaccines are based on a new technology that uses adenoviruses, which cause colds, adapted to make them essentially harmful. The viruses are used as vectors to carry out instructions for human cells to make proteins on the surface of the coronavirus, which replenish the immune system to make antibodies that fight the actual virus.
Scientists are trying to find the potential mechanism that could explain the blood clots. A leading hypothesis appears to be that the vaccines elicit a rare immune response associated with these viral vectors, FDA officials said during a briefing Tuesday.
The US agency will analyze clinical trials data on various vaccines using these viral vectors, including J & J’s Ebola vaccine, to look for clues.
None of the previous vaccines using viral vectors have been administered on the scale of the AstraZeneca and J&J COVID-19 shots, which may explain why a possible link with blood clots only materialized during these massive vaccination programs.
The technology has also been used in coronavirus vaccines developed in China and Russia.
Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, was reluctant to declare the blood clot a ‘class effect’ shared by all adenovirus vector vaccines, but he sees a clear resemblance in those cases.
“It is clear to us that what we are seeing with the Janssen (J&J) vaccine is very similar to what was seen with the AstraZeneca vaccine,” Marks said. “We can not yet give a broad explanation, but it obviously comes from the same general class of viral vectors.”
‘IN THE BEGINNING’
In Europe, scientists are investigating a number of hypotheses, including a closer look at the way in which the SARS-CoV-2 virus affects blood clotting.
One team in the Netherlands plans to conduct laboratory studies that expose specific types of cells and tissues to the vaccines and monitor how they respond. They will also investigate whether the risks can be further limited by reducing vaccination.
“There are many hypotheses, and some of them may play a role,” said Eric van Gorp, a virologist at the Erasmus Medical Center in Rotterdam. “We’re at the beginning, and – as research shows – it could be that we can find the clue right away, or it can go step by step.”
Other scientists were struck by the parallel between the J&J and AstraZeneca shots.
Danny Altmann, a professor of immunology at Imperial College London, said the similar blood clotting events associated with both were “clearly notable as the definition of mechanism.” There is no sign of such problems with the vaccines that BioNTech SE makes with Pfizer Inc or Moderna Inc with another technology.
“It would be interesting to know more about Sputnik V – also a similar adenovirus vaccine,” Altmann said. The Russian vaccine, developed by the Gamaleya Institute in Moscow, uses two different human cold viruses – including the Ad26 virus in the J&J shot.
The experts say the issue could also affect the adenovirus vector vaccine from CanSino Biological in China.
To investigate whether there is a general association with adenoviruses is a reasonable speculation, and it is a line of research and investigation. But that does not mean it has been proven, ”says John Moore, a professor of microbiology and immunology at Weill Cornell Medical College in New York.
Moore, who took part in an informal White House briefing with other scientists on Tuesday, said the FDA and US Centers for Disease Control and Prevention are working closely with health officials in Europe to determine if the syndromes associated with the vaccines AstraZeneca and J&J the same.
An important clue may lie in the fact that the reported events usually occur about 13 days after the survey, this is the period in which antibodies can be expected to appear.
“It’s speculation, but the timing of something that happens on average after about 13 days indicates an immune response to a component of the vaccine,” Moore said.
Investigations of this kind can take years. But just like the vaccines themselves that were produced in record time, Moore believes that so much effort will be put into the research that it will probably be resolved within a few weeks.
“It’s so clearly important,” he said.
Reporting by Julie Steenhuysen and Kate Kelland; Additional Reporting by Michael Erman Maplewood, NJ; Edited by Michele Gershberg and Bill Berkrot