The clinical hold placed on the AstraZeneca Covid-19 vaccine trial highlights serious issues with the rush to approve a new Covid vaccine, and the safety of vaccines already green lit for limited use in China and Russia. AstraZeneca’s vaccine, developed by a team at Oxford University, was considered one of the leading vaccine candidates, with reports of its safety and efficacy widely welcomed when early data was released in July. The United States administration has reportedly been trying to fast-track approval of the drug before the November presidential election and European leaders have already secured access to 400 million doses before the end of this year.

The AstraZeneca vaccine, called AZD1222 or ChAdOx1 nCoV-19, is a recombinant viral vector vaccine which uses a weakened version of a common cold virus—an adenovirus—to build immunity to SARS-CoV-2 infection. The adenovirus vectors, such as the one used by the Oxford group, are the backbone of many Covid vaccine candidates, including those already approved for use in Russia and China. Pharmaceutical giant Johnson & Johnson, which is developing their vaccine candidate using the same method, calls their adenoviral vector platform a “proven” technology. But the news this week shows that this approach is far from proven.

The details of the event that caused the clinical hold have not yet been fully disclosed. STAT News, which broke the story, received a statement from the drug maker claiming the hold was a routine action triggered by a standard review process that pauses clinical trials whenever a potentially unexplained illness arises. The rumors circulating today suggest that potentially unexplained illness was in fact a serious adverse reaction among one of the trial volunteers, meaning that a previously healthy individual, when given the shot, experienced a reaction that may have required hospitalization or be life-threatening. According to the STAT report, the clinical hold on the AstraZeneca trial is affecting other AstraZeneca trials underway, as well as clinical trials being conducted by other vaccine manufacturers.

While it is true that clinical holds are not uncommon, they can be a sign that something has gone very wrong with a potential new product. In this case, we have no real way of knowing the gravity and potential impact of the hold because, as has been the case far too often during this pandemic, the drug companies aren’t releasing enough data for us to know. This lack of immediate transparency is seemingly at odds with the recent declaration by AstraZeneca and eight other Covid vaccine developers to “stand with science” and ensure public confidence in the rigorous scientific and regulatory process by which Covid-19 vaccines are evaluated and potentially approved.

The news of the hold is troubling in and of itself, but it highlights only one of the dangers of a rush to a vaccine, whether it be this AstraZeneca one or any of the others currently in development. In addition to the safety and efficacy of the vaccine itself, there are also serious matters regarding the quality and consistency of the manufacturing process that we need to be concerned about, as well as the distribution network for vaccines, the integrity of the cold chain—the system of transporting and storing vaccines at appropriate temperatures, and the safety of the needles and vials used to administer any drug.

Allowing this hold to fly past us as a routine action downplays the potential severity of the news and prevents us from seeing this for what it is: a sign that any vaccine, rushed to approval, may prove dangerous to the healthy individuals who would be receiving the drug. This isn’t the first time we’ve seen adverse reactions to potential Covid vaccines and I am fairly certain it won’t be the last. Yes, we need a Covid-19 vaccine. But rushing the approval process, as we have seen this week, may lead to more harm than good.

This post originally appeared on Forbes.