Johnson & Johnson, one of four U.S. companies whose Covid-19 vaccine has advanced to the last phase of clinical trials, has hit pause on a study of 60,000 patients due to a case of “unexplained illness,” according to an official announcement released last week. Whether the illness was caused by the vaccine or a placebo remains unknown.

The temporary pause, their statement goes on to clarify, differs from a clinical hold, which fellow vaccine developer AstraZeneca initiated after a participant in a U.K. trial developed serious neurological symptoms. Under normal (which is to say pre-Covid) circumstances, the interruption of the Johnson & Johnson trial would go unreported—so expected is it in a study that large. But it could be millions, not tens of thousands, who potentially receive this vaccine, and as such even the slightest hiccups warrant scrutiny.

The same could be said for yesterday’s report that one of the participants in the AstraZeneca trial in Brazil has died, though the independent safety and monitoring committee that evaluated the case ultimately made the decision not to pause the trial. While their judgment gives us reason to believe that the death was unrelated to the vaccine or placebo that participant received, it still chalks up as yet another—and not likely the last—complication that AstraZeneca has noted but barreled past. 

Any vaccine, if efficacious, works furiously and potently to rev up the body’s immune system. Ideally, the effects are protective, stopping infection and preventing disease. The conditions that form a perfect storm in some, however, can unleash a cyclonic nightmare in others, triggering adverse reactions powerful enough to become illnesses in and of themselves, like Guillain-Barre syndrome. When pharmaceutical products of any kind, not just vaccines, are rushed to market, these rare yet serious side effects are more likely to end up brushed under the rug.

While desperate times call for desperate measures, ignoring the adverse outcomes of vaccine trials, however select the few, cannot be one of them. At least one patient in the AstraZeneca trial—later discovered to have multiple sclerosis, a condition undiagnosed prior to their participation—developed a rare disease known as transverse myelitis that involves inflammation of the spinal cord. The second patient to cause the AstraZeneca trial to pause due to illness, a source told the New York Times, was also ultimately confirmed to have transverse myelitis. I’ve spoken with a number of former FDA regulators who tell me this would typically be reason enough not just to put trials on hold, but also to terminate development of a particular vaccine altogether.

The Johnson & Johnson vaccine, like the AstraZeneca vaccine, was created using an adenovirus—a common cold-causing virus—vector, specifically adenovirus serotype 26 (Ad26). Though the technology has been implemented in the creation of gene therapies and experimental Ebola vaccines, it has yet to yield a fully licensed vaccine and, tragically, has been associated with fatalities, like the death of Jesse Gelsinger. To be sure, the Johnson & Johnson adenovirus vector differs from that of AstraZeneca, but they do share the property of being neurotropic—that is, having the propensity to infect neural tissue of a variety of types.

Somewhat worrying is that Ad26 is one of two adenovirus vectors used in the Russian Covid-19 vaccine, which the country’s Ministry of Health approved before a large-scale trial had even begun. The second of the two vectors, Ad5, was also used in one of four vaccines that hundreds of thousands are reportedly receiving in China, developed by CanSino Biologics. It would be beneficial to researchers and developers everywhere if data on the adverse side effects of these vaccines was made available to the general public, especially since other countries are buying them up by the millions. But such accounts, if they do exist, are currently shrouded in secrecy.

If vaccines for Covid-19 aren’t held to the highest safety and efficacy standards we can muster, they could very well exacerbate, not mitigate, the pandemic. Across Europe, we’re seeing what happens when people let their guard down; in countries like Spain, Italy, and France, rates of infection and hospitalization now exceed those accumulated during the first wave. The public health risks that contribute to such resurgences, like congregating in large groups, and the risks of rushing out a vaccine tend to be treated as separate concerns. But if an only partially effective vaccine is authorized for mass use, whatever effectiveness there may be in reducing infection could be more than swamped by new infections that are the result of people letting down their guard.

We all want this crisis to be over with—and for good. In our haste to recapture a sense of normalcy, however, we risk prolonging the pandemic by not just months, but potentially years. Safety, not a need for speed, must absolutely come first and foremost, and in the context of vaccine trials that means taking each pause or hold as seriously as needed. Proceeding with care and caution, contrary to what our basest instincts tell us, is what will dig us out of this mess.