Lots of uncertainty in the vaccine world today: as many will have heard, the Oxford/AstraZeneca trial in the UK, the US, and Brazil has been paused due to a serious adverse event. The New York Times, citing a source with knowledge of the event, reported last night that this was transverse myelitis. And Nature has noted that the volunteer information sheet mentions such a case as having happened in July – it’s unclear if we’re seeing a second case, or more details on that earlier one, or what. Update: as mentioned in the comments, a later version of the information sheet says that this person was later diagnosed with an unrelated neurological problem, so that might be good news – we’ll see!)
Transverse myelitis is a rare disorder, an inflammation in the spinal cord that runs right across it (thus the “transverse”) The worrying part is that it (like Guillain-Barré, discussed here) is known to be associated with an autoimmune response that can be a sequel of viral infection. Indeed, it has been associated with SARS-CoV-2 infection itself. It has also been thought to be a rare event after vaccination, but there’s disagreement about that. Here’s a 2009 review of the literature that suggests that it is such, but it’s also worth noting that the Institute of Medicine concluded in 2012 that the evidence for causality was weak. Nonetheless, the association with the immune response to viral infection itself seems solid – there are surely other causes of transverse myelitis, but that’s one of them. And mimicking an immune response to a viral infection is just what a vaccine is supposed to accomplish, and keep in mind that the Oxford vaccine (like the other viral-vector candidates) is in fact a virus, a re-engineered chimpanzee adenovirus in this case, whose entire mode of action is to infect cells in the body. So I think this is indeed an event to be taken seriously, and I think pausing the trial to take stock of what’s going on is entirely appropriate.
Unfortunately, it might end up being very difficult to come to a conclusion, especially if this is a single case. (Edit: if this is a second case, though, that will be very bad news indeed) Welcome to vaccine safety trials and the difficulty of working with such data! As mentioned before, since vaccines are designed to be given exclusively to people who are not sick (a very unusual situation in drug development work!), the safety standards have to be very high. But the adverse events themselves (especially the serious ones) can be extremely rare, and the only way to get a statistical foothold on them is to have a very large controlled patient population under study. The Oxford/AstraZeneca trial is enrolling nearly 30,000 people, and the problem is that that may still not be enough for a definite answer on something like this. We’ll all await more information and I’ll revisit this as it becomes available.