AstraZeneca Plc’s antibody cocktail was only 33% effective in preventing symptomatic Covid-19 in people who had been exposed to the virus, failing a key study in the drug-maker’s pandemic push.

The trial of 1,121 adult volunteers sought to determine whether the long-acting antibody combination could protect people who had recently come into contact with the SARS-CoV-2 virus in settings such as nursing homes. The company stated that it is conducting additional studies on the medicine that may help to clarify the findings.

The result is a setback for Astra for a drug that was hoped to be a bright spot in the company’s pandemic efforts after the mixed success of its vaccine with the University of Oxford. Other pharmaceutical companies, such as GlaxoSmithKline Plc, have had some success in getting similar therapies through clinical trials and approved for people who are at high risk of severe disease or are unable to be vaccinated.

The study, which was conducted in the United States and the United Kingdom, found that 23 volunteers who received the AZD7442 cocktail developed symptomatic Covid-19 after being exposed to the disease, compared to 17 cases in the placebo group. Although twice as many people received the antibody, the difference between the two groups was not considered statistically significant. Participants tolerated the cocktail well.

Even before it could be proven effective, the Astra medicine piqued people’s interest. The United States has ordered up to 700,000 doses of the medicine for delivery in 2021, while the United Kingdom is reconsidering an earlier order for one million. The trial results could have an impact on the US agreement, the value of which was contingent on the company obtaining an emergency use authorization to administer the drug for the prevention of Covid-19 in people who had already been exposed to the virus.

Astra said on Tuesday that discussions with the US government about next steps are “ongoing,” and that the company is waiting for the results of its Provent trial, which is testing whether the cocktail can prevent infection in people who are at high risk of contracting Covid-19 or who have compromised immune systems, “before determining our EUA plans.”

Astra shares were up 0.3 percent in London trading.

In the previous eight days, all trial participants had been exposed to someone infected with Covid-19. Further investigation revealed that the drug had some preventive effect.

According to the company, volunteers who became infected up to a week after taking the antibody were 51% less likely to develop symptoms. If the patient did not report infection for more than a week after the injection, the rate increased to 92 percent. To rule out prior infection, all participants had a negative antibody test after being dosed.

According to Myron Levin, the study’s lead researcher and a professor of medicine at the University of Colorado, the findings suggest the cocktail “may be useful in preventing symptomatic Covid-19 in individuals who are not already infected.” “For certain populations, there is still a significant need for prevention and treatment options.”

Antibody drugs are seen as a way to protect people like cancer patients, whose immune systems may not respond well to vaccines. They may also provide much-needed treatments as countries face new variants and outbreaks of infections due to varying vaccine rollout rates.

However, the products are difficult to use, and scale-up is limited. Unlike vaccines, which can produce billions of doses each year, antibody treatments can only produce a few million, according to Mark Esser, Astra’s head of microbial sciences, in a February interview.

Several antibody treatments are already available for purchase. GlaxoSmithKline and Vir Biotechnology Inc. received emergency-use authorization for their product in the United States last month after demonstrating that it could keep at-risk patients from worsening. In April, Eli Lilly & Co.’s approval for an antibody treatment was revoked due to concerns about its efficacy, but the company later had the product cleared for use in combination with another antibody.

The Storm Chaser trial is one of six advanced-stage studies that Astra is conducting to test its medicine. The Provent trial report is expected to be released soon.