In­ves­ti­ga­tors at As­traZeneca have or­dered a halt to dos­ing their close­ly-watched Covid-19 vac­cine in or­der to probe a po­ten­tial­ly se­vere ad­verse event — re­port­ed­ly in­volv­ing in­flam­ma­tion of the spinal cord — suf­fered by one of the tri­al par­tic­i­pants.

In a state­ment out Tues­day evening, a spokesman for the phar­ma gi­ant told End­points News that the study had been put on hold to in­ves­ti­gate an ill­ness. Lat­er Tues­day, they sent a re­vised state­ment, which not­ed that the halt is vol­un­tary, not or­dered by reg­u­la­tors. It reads:

As part of the on­go­ing ran­domised, con­trolled glob­al tri­als of the Ox­ford coro­n­avirus vac­cine, our stan­dard re­view process was trig­gered and we vol­un­tar­i­ly paused vac­ci­na­tion to al­low re­view of safe­ty da­ta by an in­de­pen­dent com­mit­tee. This is a rou­tine ac­tion which has to hap­pen when­ev­er there is a po­ten­tial­ly un­ex­plained ill­ness in one of the tri­als, while it is in­ves­ti­gat­ed, en­sur­ing we main­tain the in­tegri­ty of the tri­als. In large tri­als ill­ness­es will hap­pen by chance but must be in­de­pen­dent­ly re­viewed to check this care­ful­ly. We are work­ing to ex­pe­dite the re­view of the sin­gle event to min­imise any po­ten­tial im­pact on the tri­al time­line. We are com­mit­ted to the safe­ty of our par­tic­i­pants and the high­est stan­dards of con­duct in our tri­als.

The spokesman did not say what ill­ness was in­volved, but the New York Times re­port­ed that the halt was trig­gered by a case in the UK of trans­verse myelitis, an in­flam­ma­tion of the spinal cord that can in­ter­rupt mes­sages that spinal cord nerves trans­mit. Ac­cord­ing to Johns Hop­kins, “it is char­ac­ter­ized by symp­toms and signs of neu­ro­log­ic dys­func­tion in mo­tor and sen­so­ry tracts on both sides of the spinal cord.”

The con­di­tion can be trig­gered by a va­ri­ety of things, in­clud­ing vac­ci­na­tions.

As­traZeneca’s shares slid 6% af­ter the bell on Tues­day, a sharp drop for a Big Phar­ma of that size. Shares of Mod­er­na and BioN­Tech, the two mR­NA lead­ers in Phase III, each jumped 5%.

This is the first such pause in the fren­zied race to get a Covid-19 vac­cine to the goal line. And the in­ter­rup­tion on a Covid-19 leader like this trig­gered head­lines world­wide. As­traZeneca, like the rest of the lead­ing de­vel­op­ers work­ing on a pan­dem­ic vac­cine, has been bar­rel­ing ahead in­to late-stage piv­otal work on a jab for the coro­n­avirus that has dis­rupt­ed the world. But at every quick step, ex­ecs at the multi­na­tion­al have al­so re­peat­ed­ly as­sured all in­volved that they will not hes­i­tate to slow things down if war­rant­ed.

The next big ques­tion is how long As­traZeneca’s vac­cine could be on pause and whether there could be a spillover in­volv­ing oth­er vac­cine mak­ers.

An­drew Berens at SVB Leerink put it this way, siz­ing up the im­pact if it is trans­verse myelitis:

AZN’s vac­cine study may be post­poned by weeks to months, as the safe­ty data­base is scru­ti­nized and tri­al pro­to­cols re­vised. Fur­ther, this AE could have the po­ten­tial to slow down vac­cine de­vel­op­ment more broad­ly. Trans­verse myelitis has not been de­fin­i­tive­ly linked to any com­mer­cial vac­cines, but it has been shown to be re­lat­ed to nat­ur­al virus in­fec­tion, and a few cas­es of trans­verse myelitis were de­scribed af­ter ad­min­is­tra­tion of the MMR, vari­cel­la, and he­pati­tis B vac­cines (the mech­a­nis­tic ev­i­dence was de­ter­mined to be weak per the In­sti­tute of Med­i­cine, cur­rent­ly the Na­tion­al Acad­e­my of Med­i­cine).

Safe­ty is­sues — par­tic­u­lar­ly when they could be tied to a se­ri­ous ad­verse event — loom par­tic­u­lar­ly large here. Any sug­ges­tion of a safe­ty prob­lem could raise con­cerns for all in­volved, pos­ing added prob­lems in the event they pro­voke pub­lic re­sis­tance to a shot. And there are po­lit­i­cal im­pli­ca­tions as well. As Pres­i­dent Trump has promised a vac­cine by year’s end, or even soon­er, the top vac­cines have a shot at a snap OK. Safe­ty is­sues would put a vac­cine fur­ther be­hind in the line.

Jef­feries’ Michael Yee was quick to pick up on that as­pect, writ­ing:

While we see this event as more of a short- to medi­um-term top­ic for de­bate, we do be­lieve these vac­cines will like­ly end up be­ing dif­fer­en­ti­at­ed on ef­fi­ca­cy and safe­ty in their Phase II­Is, which could dri­ve opin­ions on which is “best” and even­tu­al mar­ket up­take.

At our lat­est count, we in­clude the As­traZeneca vac­cine in 3rd place among 29 com­pa­nies in or near the clin­ic, right be­hind the mR­NA lead­ers at Pfiz­er/BioN­Tech and Mod­er­na. They’ve been work­ing with $1.2 bil­lion in pub­lic fi­nanc­ing for this work, with the Phase III get­ting un­der­way in Ju­ly. In­ves­ti­ga­tors plan to re­cruit 30,000 pa­tients in the US and 50,000 world­wide as they look for a broad pa­tient group to test safe­ty and ef­fi­ca­cy.

For a look at all End­points News coro­n­avirus sto­ries, check out our spe­cial news chan­nel.

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