J&J pause adds to adenovirus vaccine doubts

The Covid-19 vaccine candidates being developed by Astrazeneca and Johnson & Johnson have a couple of things in common. Both have now been linked with a safety scare, and both use an adenoviral vector.

So far, little is known about the adverse event that caused J&J to pause all of its Covid-19 vaccine trials, including the phase III Ensemble study. But if it turns out to be similar to the neurological issue that caused Astra’s US trial to be put on hold, it would be a big problem for the adenovirus approach.

Fortunately, there are only a handful of Covid-19 vaccine candidates in development that use an adenoviral vector. Apart from J&J’s Ad26.COV2-S and Astra’s AZD1222, Cansino Biologics’ Ad5-nCoV and Reithera’s GRAd-COV2 have entered clinical trials, an analysis by Evaluate Vantage reveals. In addition, an oral adenovirus-based vaccine from Vaxart went into phase I today.

And Sputnik V, the vaccine approved in Russia although only phase I/II data are available, is based on two adenoviral vectors, type 5 and type 26.

Many unknowns

It will take time to ascertain whether there is an issue with Ad26.COV2-S and, indeed, adenoviral vaccines more broadly.

J&J only found out about the event on Sunday evening, Mathai Mammen, head of R&D for J&J’s Janssen unit, said during the company’s third-quarter earnings call today. The company informed the study’s data safety monitoring board immediately, then “quickly” posted a statement on its website on Monday, he explained.

He also stressed that the study remains blinded and J&J does not know whether the event occurred in the vaccine or placebo arm. When asked if the event had been neurological – a nod to the suspected case of transverse myelitis that caused Astra’s trial halt – Mr Mammen answered that the company “knows very little at this point”.

He added that J&J was committed to transparency and intends to provide an update once more details emerge, but that it would take “a few days, minimum” for the information to be evaluated.

Mr Mammen declined to answer a question on how many patients have been dosed in Ensemble so far, but it cannot be many – the study only began in late September. He maintained that J&J’s plan to complete enrolment in two to three months “remains on track”.

Warning signs?

However, the signs so far with adenoviral vaccines do not augur well. Neurological events also led to the halt of studies of J&J’s Ebola vaccine, which uses adenovirus 26, the same vector as its Covid vaccine candidate. Still, the programme was resumed and the Ebola vaccine was approved in the EU in July.

J&J was keen to stress the difference between a clinical trial halt and a pause in its statement: the former is a requirement of the FDA but the latter is not. Still, investors and the public at large will no doubt be watching to see if this pause becomes something more serious.

Notably, while Astra has restarted studies of AZD1222 in the UK, Brazil and Africa, the US study is still on hold.

Other approaches

On the face of it, the latest news looks like a positive for the most advanced Covid-19 vaccine developers, Pfizer/Biontech and Moderna, as it puts them further ahead of some of their closest rivals. Both projects are mRNA vaccines.

But it might not play out that way: public confidence in vaccines is already shaky, and another adverse event, even with a different platform, could add to overall vaccine hesitancy. People might be particularly reluctant to receive vaccines using novel technologies such as mRNA, should these receive emergency use authorisation.

Until there is long-term follow-up in large numbers of patients, concerns about potential toxicity with new platforms are unlikely to be assuaged.

So the winners might end up being companies testing more traditional approaches like recombinant protein vaccines. The most advanced of these is Novavax’s NVX-CoV2373, which began a 10,000-patient phase III UK trial of in September.

Meanwhile, Sanofi and Glaxosmithkline have a candidate in phase I/II. The companies expect to have data by December; if this is positive, they will push on into phase III later that month. They hope to be able to submit their vaccine for approval in the first half of 2021.

This story has been updated to include details of Vaxart's oral adenovirus vaccine.