One of the prominent COVID-19 vaccine trials has hit the pause button. Yesterday, the health news publisher STAT broke the story. AstraZeneca, a British-Swedish pharmaceutical company that partnered with the University of Oxford in the United Kingdom to release a vaccine, said in a statement that an illness in one of the trial participants had triggered a shutdown. “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,” the company wrote.

This action does not necessarily mean there is an issue with AZD1222, AstraZeneca’s COVID-19 vaccine. “In large clinical trials, illnesses will happen by chance and must be independently reviewed,” the company explained.

The vaccine was 1 of 9 possible candidates to reach the last stage of testing, called phase 3. AstraZeneca’s vaccine was considered a promising candidate. In May of this year, the government agency BARDA, Biomedical Advanced Research and Development Authority, agreed to pay $1.2 billion dollars for as many as 300 million doses of the vaccine – only one of many preorders.

Just this week AstraZeneca had joined other leading pharmaceuticals, pledging not to rush a vaccine to market, “[E]very day, we follow the science in seeking safe & effective solutions for patients,” the company said in a tweet. Pascal Soriot, the Chief Executive Officer of AstraZeneca, explained that the trials would restart after review by a committee of independent experts. “We will be guided by this committee as to when the trials could restart, so that we can continue our work at the earliest opportunity to provide this vaccine broadly, equitably and at no profit during this pandemic.”

Earlier this year the company shared its findings from earlier trials in The Lancet. The study participants reported tenderness and pain at the injection site, and varying degrees of fatigue, headache and muscle ache in the week after the vaccine. Despite these side effects, the article reported that volunteers did reach the study’s goal by developing an antibody response and that there were no serious side effects.

This new illness will need to be fully investigated before trials continue, but as of now AstraZeneca’s vaccine is not out of the running. At the same time, there are 8 other vaccines in phase 3 trials and many more advancing through phase 1 and 2. Whether it will be enough to have a vaccine by 2021, is still unclear.